[time-nuts] ADEV values of Thunderbolt PPS and OSC error estimates

[Mark Sims]

I just noticed that the ADEV values that I posted for the disciplined 
oscillator were for a unit fresh off the boat from China.  It had not been used 
for several years.  It had just completed the self survey right before the 
run/log was started.  Note the rather bad short term ADEVS that just started to 
get real at the 1 second area.  The undisciplined values were from this 
same unit after a couple weeks of continuous operation.  I am doing another 
disciplined run on this unit and will post the results.
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[time-nuts] ADEV values of Thunderbolt PPS and OSC error estimates

[Mark Sims]

Another try to see if I can get some stinkin' CR/LF spacing out of hotmail this 
week.


>
> Here is two sets of ADEV/OADEV data generated from the PPS and OSC error 
> estimates produced by the Thunderbolt. There is a set from a run where the 
> oscillator was disciplined by GPS and a set where the oscillator was 
> undisciplined. The PPS numbers are probably the best ones to use. Trimble's 
> documentation on exactly what the OSC error values are is rather poor and I 
> am just guessing what they mean and how to convert them into something that 
> can be fed into the ADEV code.
>
>
> #
> # PPS OADEV over 22000 points - sample period=1 secs - disciplined
> # 1 tau 1.4140e-006 (n=21998)
> # 2 tau 1.2248e-006 (n=21996)
> # 5 tau 4.8993e-007 (n=21990)
> # 10 tau 2.4503e-007 (n=21980)
> # 20 tau 1.2258e-007 (n=21960)
> # 50 tau 4.9104e-008 (n=21900)
> # 100 tau 2.4600e-008 (n=21800)
> # 200 tau 1.2359e-008 (n=21600)
> # 500 tau 5.0137e-009 (n=21000)
> # 1000 tau 2.5688e-009 (n=2)
> # 2000 tau 1.2359e-009 (n=18000)
> # 5000 tau 2.7077e-010 (n=12000)
> # 1 tau 5.4669e-013 (n=2000)
> #
> # PPS ADEV over 22000 points - sample period=1 secs - disciplined
> # 1 tau 1.4140e-006 (n=21998)
> # 2 tau 1.2235e-006 (n=10998)
> # 5 tau 7.7367e-007 (n=4398)
> # 10 tau 5.4722e-007 (n=2198)
> # 20 tau 2.6497e-010 (n=1098)
> # 50 tau 9.8233e-011 (n=438)
> # 100 tau 5.6151e-011 (n=218)
> # 200 tau 3.9024e-011 (n=108)
> # 500 tau 1.4521e-011 (n=42)
> # 1000 tau 6.3933e-012 (n=20)
> # 2000 tau 4.1976e-012 (n=9)
> #
>
>
>
> #
> # OSC OADEV over 22000 points - sample period=1 secs - disciplined
> # 1 tau 2.2969e-007 (n=21998)
> # 2 tau 1.6255e-007 (n=21996)
> # 5 tau 1.0351e-007 (n=21990)
> # 10 tau 7.4928e-008 (n=21980)
> # 20 tau 5.7408e-008 (n=21960)
> # 50 tau 4.6799e-008 (n=21900)
> # 100 tau 3.4445e-008 (n=21800)
> # 200 tau 1.7323e-008 (n=21600)
> # 500 tau 7.0268e-009 (n=21000)
> # 1000 tau 3.6004e-009 (n=2)
> # 2000 tau 1.7322e-009 (n=18000)
> # 5000 tau 3.7961e-010 (n=12000)
> # 1 tau 4.1514e-013 (n=2000)
> #
> # OSC ADEV over 22000 points - sample period=1 secs - disciplined
> # 1 tau 2.2969e-007 (n=21998)
> # 2 tau 1.6263e-007 (n=10998)
> # 5 tau 1.0336e-007 (n=4398)
> # 10 tau 7.4706e-008 (n=2198)
> # 20 tau 5.7006e-008 (n=1098)
> # 50 tau 4.5848e-008 (n=438)
> # 100 tau 3.9821e-008 (n=218)
> # 200 tau 2.8396e-008 (n=108)
> # 500 tau 1.0394e-010 (n=42)
> # 1000 tau 5.2360e-012 (n=20)
> # 2000 tau 2.4501e-012 (n=9)
>
>
> #
> # PPS OADEV over 22000 points - sample period=1 secs - undisciplined
> # 1 tau 7.5658e-010 (n=21998)
> # 2 tau 5.0045e-010 (n=21996)
> # 5 tau 2.8665e-010 (n=21990)
> # 10 tau 1.8127e-010 (n=21980)
> # 20 tau 1.1774e-010 (n=21960)
> # 50 tau 6.8339e-011 (n=21900)
> # 100 tau 4.6184e-011 (n=21800)
> # 200 tau 4.1377e-011 (n=21600)
> # 500 tau 7.4192e-011 (n=21000)
> # 1000 tau 1.0220e-010 (n=2)
> # 2000 tau 1.1842e-010 (n=18000)
> # 5000 tau 1.2073e-010 (n=12000)
> # 1 tau 3.7124e-011 (n=2000)
> #
> # PPS ADEV over 22000 points - sample period=1 secs - undisciplined
> # 1 tau 7.5658e-010 (n=21998)
> # 2 tau 5.1193e-010 (n=10998)
> # 5 tau 2.9314e-010 (n=4398)
> # 10 tau 1.8065e-010 (n=2198)
> # 20 tau 1.1872e-010 (n=1098)
> # 50 tau 7.2017e-011 (n=438)
> # 100 tau 4.6293e-011 (n=218)
> # 200 tau 3.9846e-011 (n=108)
> # 500 tau 7.2822e-011 (n=42)
> # 1000 tau 1.0120e-010 (n=20)
> # 2000 tau 1.1514e-010 (n=9)
> #
>
>
>
> # OSC OADEV over 22000 points - sample period=1 secs - undisciplined
> # 1 tau 3.9644e-009 (n=21998)
> # 2 tau 1.9671e-009 (n=21996)
> # 5 tau 7.8264e-010 (n=21990)
> # 10 tau 3.9313e-010 (n=21980)
> # 20 tau 1.9801e-010 (n=21960)
> # 50 tau 7.9931e-011 (n=21900)
> # 100 tau 4.1369e-011 (n=21800)
> # 200 tau 2.5506e-011 (n=21600)
> # 500 tau 2.2498e-011 (n=21000)
> # 1000 tau 2.0910e-011 (n=2)
> # 2000 tau 1.2765e-011 (n=18000)
> # 5000 tau 7.0420e-012 (n=12000)
> # 1 tau 1.5081e-012 (n=2000)
> #
> # OSC ADEV over 22000 points - sample period=1 secs - undisciplined
> # 1 tau 3.9644e-009 (n=21998)
> # 2 tau 1.9375e-009 (n=10998)
> # 5 tau 7.9203e-010 (n=4398)
> # 10 tau 3.8195e-010 (n=2198)
> # 20 tau 1.8606e-010 (n=1098)
> # 50 tau 7.7008e-011 (n=438)
> # 100 tau 3.5703e-011 (n=218)
> # 200 tau 2.0731e-011 (n=108)
> # 500 tau 2.1042e-011 (n=42)
> # 1000 tau 1.9238e-011 (n=20)
> # 2000 tau 1.4089e-011 (n=9)
>
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[time-nuts] ADEV values of Thunderbolt PPS and OSC error estimates

[Mark Sims]

Here is two sets of ADEV/OADEV data generated from the PPS and OSC error 
estimates produced by the Thunderbolt. There is a set from a run where the 
oscillator was disciplined by GPS and a set where the oscillator was 
undisciplined. The PPS numbers are probably the best ones to use. Trimble's 
documentation on exactly what the OSC error values are is rather poor and I am 
just guessing what they mean and how to convert them into something that can be 
fed into the ADEV code.##  PPS OADEV over 22000 points - sample period=1 secs - 
disciplined #1 tau  1.4140e-006 (n=21998)#2 tau  1.2248e-006 
(n=21996)#5 tau  4.8993e-007 (n=21990)#   10 tau  2.4503e-007 
(n=21980)#   20 tau  1.2258e-007 (n=21960)#   50 tau  4.9104e-008 
(n=21900)#  100 tau  2.4600e-008 (n=21800)#  200 tau  1.2359e-008 
(n=21600)#  500 tau  5.0137e-009 (n=21000)# 1000 tau  2.5688e-009 
(n=2)# 2000 tau  1.2359e-009 (n=18000)# 5000 tau  2.7077e-010 
(n=12000)#1 tau  5.4669e-013 (n=2000)##  PPS ADEV over 22000 points - 
sample period=1 secs - disciplined #1 tau  1.4140e-006 (n=21998)#   
 2 tau  1.2235e-006 (n=10998)#5 tau  7.7367e-007 (n=4398)#   10 tau 
 5.4722e-007 (n=2198)#   20 tau  2.6497e-010 (n=1098)#   50 tau  
9.8233e-011 (n=438)#  100 tau  5.6151e-011 (n=218)#  200 tau  
3.9024e-011 (n=108)#  500 tau  1.4521e-011 (n=42)# 1000 tau  
6.3933e-012 (n=20)# 2000 tau  4.1976e-012 (n=9)###  OSC OADEV over 22000 
points - sample period=1 secs - disciplined #1 tau  2.2969e-007 
(n=21998)#2 tau  1.6255e-007 (n=21996)#5 tau  1.0351e-007 
(n=21990)#   10 tau  7.4928e-008 (n=21980)#   20 tau  5.7408e-008 
(n=21960)#   50 tau  4.6799e-008 (n=21900)#  100 tau  3.4445e-008 
(n=21800)#  200 tau  1.7323e-008 (n=21600)#  500 tau  7.0268e-009 
(n=21000)# 1000 tau  3.6004e-009 (n=2)# 2000 tau  1.7322e-009 
(n=18000)# 5000 tau  3.7961e-010 (n=12000)#1 tau  4.1514e-013 
(n=2000)##  OSC ADEV over 22000 points - sample period=1 secs - disciplined #   
 1 tau  2.2969e-007 (n=21998)#2 tau  1.6263e-007 (n=10998)#
5 tau  1.0336e-007 (n=4398)#   10 tau  7.4706e-008 (n=2198)#   20 tau  
5.7006e-008 (n=1098)#   50 tau  4.5848e-008 (n=438)#  100 tau  
3.9821e-008 (n=218)#  200 tau  2.8396e-008 (n=108)#  500 tau  
1.0394e-010 (n=42)# 1000 tau  5.2360e-012 (n=20)# 2000 tau  2.4501e-012 
(n=9)##  PPS OADEV over 22000 points - sample period=1 secs  - undisciplined #  
  1 tau  7.5658e-010 (n=21998)#2 tau  5.0045e-010 (n=21996)#
5 tau  2.8665e-010 (n=21990)#   10 tau  1.8127e-010 (n=21980)#   20 tau 
 1.1774e-010 (n=21960)#   50 tau  6.8339e-011 (n=21900)#  100 tau  
4.6184e-011 (n=21800)#  200 tau  4.1377e-011 (n=21600)#  500 tau  
7.4192e-011 (n=21000)# 1000 tau  1.0220e-010 (n=2)# 2000 tau  
1.1842e-010 (n=18000)# 5000 tau  1.2073e-010 (n=12000)#1 tau  
3.7124e-011 (n=2000)##  PPS ADEV over 22000 points - sample period=1 secs - 
undisciplined#1 tau  7.5658e-010 (n=21998)#2 tau  5.1193e-010 
(n=10998)#5 tau  2.9314e-010 (n=4398)#   10 tau  1.8065e-010 
(n=2198)#   20 tau  1.1872e-010 (n=1098)#   50 tau  7.2017e-011 
(n=438)#  100 tau  4.6293e-011 (n=218)#  200 tau  3.9846e-011 (n=108)#  
500 tau  7.2822e-011 (n=42)# 1000 tau  1.0120e-010 (n=20)# 2000 tau 
 1.1514e-010 (n=9)##  OSC OADEV over 22000 points - sample period=1 secs - 
undisciplined #1 tau  3.9644e-009 (n=21998)#2 tau  1.9671e-009 
(n=21996)#5 tau  7.8264e-010 (n=21990)#   10 tau  3.9313e-010 
(n=21980)#   20 tau  1.9801e-010 (n=21960)#   50 tau  7.9931e-011 
(n=21900)#  100 tau  4.1369e-011 (n=21800)#  200 tau  2.5506e-011 
(n=21600)#  500 tau  2.2498e-011 (n=21000)# 1000 tau  2.0910e-011 
(n=2)# 2000 tau  1.2765e-011 (n=18000)# 5000 tau  7.0420e-012 
(n=12000)#1 tau  1.5081e-012 (n=2000)##  OSC ADEV over 22000 points - 
sample period=1 secs - undisciplined #1 tau  3.9644e-009 (n=21998)# 
   2 tau  1.9375e-009 (n=10998)#5 tau  7.9203e-010 (n=4398)#   10 
tau  3.8195e-010 (n=2198)#   20 tau  1.8606e-010 (n=1098)#   50 tau  
7.7008e-011 (n=438)#  100 tau  3.5703e-011 (n=218)#  200 tau  
2.0731e-011 (n=108)#  500 tau  2.1042e-011 (n=42)# 1000 tau  
1.9238e-011 (n=20)# 2000 tau  1.4089e-011 (n=9)
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Re: [time-nuts] ADEV test setup [was GPSDO TC & Damping]

[Bruce Griffiths]

Bill Janssen wrote:
> WarrenS wrote:
>   
>>> Bruce said:
>>> The critical requirement is that the 2 standards being compared are 
>>> statistically independent.
>>> Comparing a pair of Thunderbolts GPSDOs with similar time constants and
>>> damping will give optimistic results for Tau comparable with or greater 
>>> than the loop time constant.
>>> Its is even better is to use 3 or more similar standards simultaneously
>>> logging phase differences between the various pairs (0.5*N(N-1) pairs for N 
>>> standards).
>>> It is then possible to obtain estimates for ADEV, MDEV etc for each 
>>> standard.
>>> 
>>>   
>> The optimistic results at and above the loop time constant, that results 
>> even when 3 or more units are used, 
>> is because the noise is then mostly due to the GPS signal itself and NOT the 
>> local oscillators in the GPSDO.
>> In effect you are then using the same 1PPS signal into each unit, and any 
>> common noise on the 
>> GPS 1PPS signal will cancel and not be seen.
>> So I think what Bruce is saying is that you can not (or is it should not?) 
>> use the GPS signal to 
>> measure the GPS's noise.
>> But the point is, if you want to compare your GPSDO with different settings, 
>> or compare it to 
>> another OCXO, It can be done this way, if you do not have a better ref to 
>> use.
>> You could then add the noise of the GPS nose above the control loop time to 
>> your 
>> optimistic results if you want true results at high Tau values.
>>
>> Also note that having the GPS noise cancle is not necessary a bad thing,  It 
>> can be a good thing 
>> especially if the GPS noise is not what it is that you want to measure. 
>>
>>   
>> 
>>> Like all digital phase detectors its best to avoid, if possible, the 
>>> nonlinearity inherent at the ends of the range.
>>> 
>>>   
>> Using a phase detector near its end point (or at its crossover point if 
>> there is any deadband) 
>> is something that needs to be avoided. 
>> The two basic standard ways to insure that just the center of the phase 
>> detector's range is use:
>> 1) Divide the signals down just enough before sending them to the phase 
>> detector so that 
>> the end points is not an issue.  This works when both signals are from 
>> devices that are 
>> locked to a common signal such as the GPS.
>>
>> 2) When one of signals is from a non locked source such as a OCXO whose 
>> phase can drift 
>> any amount overtime, One of ways to limit phase detector issues, and use 
>> just the very accurate zero phase point, is to use the Phase detector's 
>> output to lock the OCXO in a fast control loop and then by knowing the gain 
>> of the EFC input, the filtered EFC voltage can be use as freq drift 
>> information to find the ADEV's.
>>
>> WarrenS
>>
>> *:
>> 
> What I am doing to ovoid the "end of range" problem is;
> First I divide the signal by two to get a 50% duty cycle. Then when the 
> phase difference gets to
> 10% or 90% of the full scale value I switch the phase detector (or 
> counter) to respond to
> to the opposite zero crossing. I keep track of those switches in 
> software. I use a computer to control things and to keep a log of the 
> phase difference.
>
> Bill K7NOM
>
>
>   
Bill

At this level of precision the waveform duty cycles are never precisely
50% so some correction for this also needs to be made.

Bruce

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Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Magnus Danielson wrote:
> Hej Bruce,
>
>   
>>> As the PLL filters the noise of the OCXO and passes noise from the input 
>>> side and the noise have several different components to it from either 
>>> source.
>>>
>>> You can't really extrapolate direction the results of an asynchronous 
>>> reciever such as the M12+T to that of a synchronous receiver such as the 
>>> Thunderbolt. The time-solution of thunderbolt is used in replacement of 
>>> the time-interval counter fluff slapped onto a PPS based receiver such 
>>> as the M12+T. Also, the Thunderbolt enjoys a much quiter and stable 
>>> reference than the M12+T which allows for narrower filters in the 
>>> sat-tracking as the phasenoise is lower. Notice how the Thunderbolt can 
>>> be configured for different uses, they are direct hints to what the 
>>> tracking loops may do as it reduces the physical dynamics of position as 
>>> well as inflicted G effects on the OCXO.
>>>
>>>   
>>>   
>> I wasn't attempting to do so.
>> However the phase noise of the GPS receiver will still dominate for
>> short tau whilst that of the OCXO is dominant for longer tau.
>> 
>
> Agreed. I'm just trying to keep various error sources separate here.
>
> The truncation noise isn't white noise one should recall.
>   
The GPS receiver's noise appears to be at least approximately white
phase noise noise after sawtooth correction.
>   
>>> With just two Thunderbolts and a reasonable TIC you can infact build a 
>>> three-cornered hat. You have three clocks: GPS, OCXO1 and OCXO2 and the 
>>> thunderbolts will measure GPS-OCXO1 and GPS-OCXO2 and the TIC will be 
>>> able to measure the OCXO1-OCXO2. An interesting aspect of this is that 
>>> when lockedup, the PPSes of the Thunderbolts will be confined into a 
>>> rather small area. This arrangement will, as any other, give not the 
>>> standalone OCXO noise when beeing steered, but it is not entierly lying 
>>> for those longer taus.
>>>
>>>   
>>>   
>> The 3 cornered hat technique only works well (even in the extended form
>> where finite correlations between sources are included) when the noise
>> of each of the 3 sources are comparable.
>> That is this technique will only work well in the vicinity of the point
>> where the GPS receiver and OCXO ADEVs crossover or equivalently near the
>> drift corrected minimum of the ADEV as measured by the Thunderbolt when
>> the OCXO is undisciplined. For shorter tau the GPS phase noise dominates.
>> 
>
> Yes. I think it would be usefull to provide confidence intervals etc. to 
> get a better "feel" of how good the measure is.
>
> As a reference measurement the thunderbolts could be driven into 
> holdover. I think the thunderbolts keep reporting timing errors.
>
>   
Yes, the Thunderbolt keeps measuring phase and frequency errors when the
OCXO is unlocked.
>>> True. However, I think there is still some more theoretical work to be 
>>> done to give us better tools. These does not remove the need for 
>>> measurements and I have never been foolish enougth to beleive so, but it 
>>> could guide us in the right direction for selecting and steering our 
>>> parameters.
>>>
>>>   
>>>   
>> It would be helpful if the ADEV (and MDEV) plots for several
>> Thunderbolts were plotted using the Thunderbolt's internal phase error
>> measures obtained when the OCXO is undisciplined.
>> This can easily be setup using the Trimble Thunderbolt Monitor program.
>> 
>
> Indeed. We should recall that the PLL locking fades over from the OCXO 
> to the GPS (as received) noise at about the BW frequency/tau of the PLL.
>
> Finding the optimum crossover properties involves both balancing PLL 
> bandwidth and damping.
>
>   
I suspect that when more divergent processes than flicker phase noise
dominate more damping is optimum.
 I had noted that your quoted damping factor was incorrect but I
 suspected that you would realise that.

 Actually according to Gardener critical damping factor is 1 ( minimum
 settling with no overshoot for a phase step).
 However a damping factor of 0.7071 is widely used.
 
 
>>> It is interesting to clear up why this difference exists. Could be 
>>> "critical" is judged different for different applications.
>>>
>>>   
>>>   
>> The usual meaning of critical damping in a second order differential
>> equation is for no overshoot to a step input.
>> Thus critical probably isn't the appropriate term when optimising for
>> other factors.
>> Optimum damping for a particular criterion is perhaps better description.
>> 
>
> Precisely. Critical damping is just a handy reference along the line, 
> but should not be incorrectly interprented as optimum in some generic 
> sense, there is several forms of optimum for different tasks.
>
> I think best ADEV or best TDEV might be more relevant here.
>
>   
>>> My point was that regardless of implementation, second order type II 
>>> loops seems to be the reference mark, which not ne

Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Joe Gwinn]

Magnus,

At 6:02 PM + 1/10/09, time-nuts-requ...@febo.com wrote:
>
>Message: 4
>Date: Sat, 10 Jan 2009 19:02:09 +0100
>From: Magnus Danielson 
>Subject: Re: [time-nuts] Standards sought for immunity of shielded
>   cable links to power-frequency ground loops
>To: Discussion of precise time and frequency measurement
>   
>Joe,
>
   For digital signals (1PPS, various triggers), it's RS422 over 100 ohm
   twinax (fancy shielded twisted pair).

   The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the
   signals 180 degrees out of phase.  This is acheived with a pair of hybrid
>>>   > transformers which convert from one-cable to two-cable and then back to
   one-cable, where all cables are 50 ohm coax.
>  >>
>>>  OUCH! The trouble with that arrangement is that the coax cables MUST be
>>>  twisted or else H-fields will induce differential mode current. It will
>>>  induce current into both directions which through the 180 degree will
>>>  not cancel but add up. The 0/180 degree arrangement will save you from
>>>  common mode problems. You would prefer a twisted cable over a twisted
>>>  cable pair, as the later allows for installation procedure errors to
>  >> have huge impact and the twisting properties will not be as good either
>>>  and thus compromising the quality. A single ended coax is not as
>>>  sensitive to H fields to induce diffrential currents, but can have some
>>>  other problems.
>>
>>  You are right about the twisting.  The cables are close and parallel,
>>  and ground offsets are the big problem, versus magnetic fields.
>
>I just want you to end up having that trouble instead. I think you
>should consider a shielded twisted pair instead. Use the transformer to
>go between 50 Ohm and 100-110 Ohm while also getting the common mode
>isolation. A double-transformer approach can be used in which the
>launch/receive-transformer has a center tap on the "inside" which is
>wired to local ground (needs to be very low impedance). This improves
>capacitive isolation for common mode currents. The inner transformers do
>impedance matching. This is really an alternative to getting isolation
>transformers, it might even be cheaper. Dual-shielded isolation
>transformers is better thought, as capacitive coupling as spread out
>over the coil is always terminated to each side own shield which reduces
>common-mode to diffrential mode conversion.

The engineer wanted to use catalog components, which means 
connectorized hybrid transformers, probably from Minicircuits or the 
like.   He did use real twinax elsewhere, and the hum pickup issue 
has occurred to him.

The connectors are Type N, and the cable will be some kind of robust 
double-shielded flexible type.  He may already be twisting the two 
cables, which are about 30 meters long.


>  > My worry was that the ground currents might be enough to saturate the
>>  tiny ferrite cores in the hybrid transformers.  The engineer's
>>  reaction to this was on the following day to say that if this turns
>>  out to be a problem, he will add DC blocks.  This would have to be
>>  the kind that blocks both center and shield paths.
>
>I have a bit hard [time] to realize how the common mode ground current would
>saturate the hybrid transformers unless the current is so high that the
>asymmetry in the transformers helps. Some form of DC blocker or LF
>current limiting may be wise thought.

The 60 Hz limit in MIL-STD-461 CS109 is one amp (120 dB over one 
microamp).  The EMI guy said that this limit was arrived at for 
submarines in the 1970s, and the currents were primarily due to 
charging currents from capacitor-input power supplies, and the like.

What saves us with the hybrids is that while the cores are small, the 
windings might have five turns, so it will take a very substantial 
current to have any effect, and the winding will blow out first.


>  > The problem is that the radar and the ship are not yet built, so we
>  > cannot yet make tests.
>
>So much better. You have a chance to get things right before it is too
>late and too expensive.

Yes and no.  The drawings are done long before, and change is 
painful.  But necessary.


>I am sure we can send a sub to sink it late if needed.

I'm not sure that solves the problem, but it certainly eliminates the problem.


>    energy straight through and allow for a higher resistive path for the
>   low frequency energy.
   The ground grid impedance between any two points is well less than one
   ohm, so 100 ohms will pretty much abolish all ground loops.  I've used 10
   ohms in like labs, with success.  I'll grant that this would 
not work with
   long wires outside.
>>>  Should be sufficient then. But remember that capacitive coupling helps
>>>  you in the RF area and impulse protection.
>>
>>  True.
>
>The reason I keep mentioning it is since that it is easy to focus and
>make a design "optimum" for one case and forgetting about other aspects

Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Lux, James P]

Or 1553, for that matter

-Original Message-
From: "David C. Partridge" 


Get 'em to use twin-ax (twisted pair inside screen) like the IBM AS/400
terminals (5250?) send differential signal down the cable.

Dave
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Re: [time-nuts] ADEV test setup [was GPSDO TC & Damping]

[Bill Janssen]

WarrenS wrote:
>> Bruce said:
>> The critical requirement is that the 2 standards being compared are 
>> statistically independent.
>> Comparing a pair of Thunderbolts GPSDOs with similar time constants and
>> damping will give optimistic results for Tau comparable with or greater than 
>> the loop time constant.
>> Its is even better is to use 3 or more similar standards simultaneously
>> logging phase differences between the various pairs (0.5*N(N-1) pairs for N 
>> standards).
>> It is then possible to obtain estimates for ADEV, MDEV etc for each standard.
>> 
>
> The optimistic results at and above the loop time constant, that results even 
> when 3 or more units are used, 
> is because the noise is then mostly due to the GPS signal itself and NOT the 
> local oscillators in the GPSDO.
> In effect you are then using the same 1PPS signal into each unit, and any 
> common noise on the 
> GPS 1PPS signal will cancel and not be seen.
> So I think what Bruce is saying is that you can not (or is it should not?) 
> use the GPS signal to 
> measure the GPS's noise.
> But the point is, if you want to compare your GPSDO with different settings, 
> or compare it to 
> another OCXO, It can be done this way, if you do not have a better ref to use.
> You could then add the noise of the GPS nose above the control loop time to 
> your 
> optimistic results if you want true results at high Tau values.
>
> Also note that having the GPS noise cancle is not necessary a bad thing,  It 
> can be a good thing 
> especially if the GPS noise is not what it is that you want to measure. 
>
>   
>> Like all digital phase detectors its best to avoid, if possible, the 
>> nonlinearity inherent at the ends of the range.
>> 
>
> Using a phase detector near its end point (or at its crossover point if there 
> is any deadband) 
> is something that needs to be avoided. 
> The two basic standard ways to insure that just the center of the phase 
> detector's range is use:
> 1) Divide the signals down just enough before sending them to the phase 
> detector so that 
> the end points is not an issue.  This works when both signals are from 
> devices that are 
> locked to a common signal such as the GPS.
>
> 2) When one of signals is from a non locked source such as a OCXO whose phase 
> can drift 
> any amount overtime, One of ways to limit phase detector issues, and use just 
> the very accurate zero phase point, is to use the Phase detector's output to 
> lock the OCXO in a fast control loop and then by knowing the gain of the EFC 
> input, the filtered EFC voltage can be use as freq drift information to find 
> the ADEV's.
>
> WarrenS
>
> *:
What I am doing to ovoid the "end of range" problem is;
First I divide the signal by two to get a 50% duty cycle. Then when the 
phase difference gets to
10% or 90% of the full scale value I switch the phase detector (or 
counter) to respond to
to the opposite zero crossing. I keep track of those switches in 
software. I use a computer to control things and to keep a log of the 
phase difference.

Bill K7NOM


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[time-nuts] Frequency Electronics FE-5602B

[EWKehren]

Does any one have pin out and or specs on the Frequency Electronics  FE-5062B 
Rubidium Standard. Any help would be greatly appreciated. Thanks Bert  Kehren 
WB5MZJ
**New year...new news.  Be the first to know what is making 
headlines. (http://www.aol.com/?ncid=emlcntaolcom0026)
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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[David C. Partridge]

Get 'em to use twin-ax (twisted pair inside screen) like the IBM AS/400
terminals (5250?) send differential signal down the cable. 

Dave

-Original Message-
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On
Behalf Of Joe Gwinn
Sent: 10 January 2009 15:23
To: time-nuts@febo.com
Subject: Re: [time-nuts] Standards sought for immunity of shielded cable
links to power-frequency ground loops

Magnus,

At 10:31 AM + 1/10/09, time-nuts-requ...@febo.com wrote:
>
>Message: 5
>Date: Sat, 10 Jan 2009 11:06:39 +0100
>From: Magnus Danielson 
>Subject: Re: [time-nuts] Standards sought for immunity of shielded
>   cable   links to power-frequency ground loops
>To: Discussion of precise time and frequency measurement
>   
>
>Joseph,
>
>>  time-nuts-boun...@febo.com wrote on 01/07/2009 10:47:46 PM:
>>
>>>  Joseph,
>>>
>>  Could be a differential TX and RX.  I recall that they send a 
>> RS422
  signal.
>  Depending on the speed, RS422 works fine with transformers.
  Yes.  It would be 10 MHz or 20 MHz, depending on coding.  Or 5 
 MHz, so
>>  the
  transitions are at 10 MHz.  I don't recall, or never knew.
>>>  RS422 does not imply any encoding as such so it would be 10 MHz but  
>>> naturally there is twice that many transitions, but it is the 
>>> frequency  of the signal you are interested in for this case.
>>
>>  I know that RS422 is not the encoding.  I cheated, and talked to the  
>> relevant engineer.
>
>That is to cheat! :)
>
>>  For digital signals (1PPS, various triggers), it's RS422 over 100 
>> ohm  twinax (fancy shielded twisted pair).
>>
>>  The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with 
>> the  signals 180 degrees out of phase.  This is acheived with a pair 
>> of hybrid
>  > transformers which convert from one-cable to two-cable and then 
> back to
>>  one-cable, where all cables are 50 ohm coax.
>
>OUCH! The trouble with that arrangement is that the coax cables MUST be 
>twisted or else H-fields will induce differential mode current. It will 
>induce current into both directions which through the 180 degree will 
>not cancel but add up. The 0/180 degree arrangement will save you from 
>common mode problems. You would prefer a twisted cable over a twisted 
>cable pair, as the later allows for installation procedure errors to 
>have huge impact and the twisting properties will not be as good either 
>and thus compromising the quality. A single ended coax is not as 
>sensitive to H fields to induce diffrential currents, but can have some 
>other problems.

You are right about the twisting.  The cables are close and parallel, and
ground offsets are the big problem, versus magnetic fields.

My worry was that the ground currents might be enough to saturate the tiny
ferrite cores in the hybrid transformers.  The engineer's reaction to this
was on the following day to say that if this turns out to be a problem, he
will add DC blocks.  This would have to be the kind that blocks both center
and shield paths.

The problem is that the radar and the ship are not yet built, so we cannot
yet make tests.


>   But you should never let the screen float in the far end, you should
>  terminate it with a 10M resistor and a sparkgap in parallel to the
>  local ground.
>
>  The resistor takes care of static electricity and the sparkgap will
>  do lightnings.
  I've done such things, but with a 100 ohm resistor (and a safety
>>  ground to
  ensure that the voltage doesn't get too large.  But this was
>>>  a lab lashup.
>>>
>>>  The trouble with 100 ohm is that still can be a little low in relation
>  >> to ground loop impedances, it still allow some fair current to roll
down
>  >> the cable. A capacitor in parallel would cut most of the transient
>>>  energy straight through and allow for a higher resistive path for the
>>>  low frequency energy.
>>
>>  The ground grid impedance between any two points is well less than one
>>  ohm, so 100 ohms will pretty much abolish all ground loops.  I've used
10
>>  ohms in like labs, with success.  I'll grant that this would not work
with
>>  long wires outside.
>
>Should be sufficient then. But remember that capacitive coupling helps
>you in the RF area and impulse protection.

True.


>  > By the way, I also finally talked to one of our most experienced
EMI/EMC
>>  engineers.  He suggested using MIL-STD-461 test CS109, even though CS109
>  > was developed for enclosures.  It turns out he was involved in
developing
>>  CS109 when he worked for the US Navy.
>
>Need to look it up. Never had to do any of the MIL-STD-461 stuff.

It's available for free on the web. 


Joe

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Magnus Danielson]

Joe,

>>>  For digital signals (1PPS, various triggers), it's RS422 over 100 ohm
>>>  twinax (fancy shielded twisted pair).
>>>
>>>  The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the
>>>  signals 180 degrees out of phase.  This is acheived with a pair of hybrid
>>  > transformers which convert from one-cable to two-cable and then back to
>>>  one-cable, where all cables are 50 ohm coax.
>> OUCH! The trouble with that arrangement is that the coax cables MUST be
>> twisted or else H-fields will induce differential mode current. It will
>> induce current into both directions which through the 180 degree will
>> not cancel but add up. The 0/180 degree arrangement will save you from
>> common mode problems. You would prefer a twisted cable over a twisted
>> cable pair, as the later allows for installation procedure errors to
>> have huge impact and the twisting properties will not be as good either
>> and thus compromising the quality. A single ended coax is not as
>> sensitive to H fields to induce diffrential currents, but can have some
>> other problems.
> 
> You are right about the twisting.  The cables are close and parallel, 
> and ground offsets are the big problem, versus magnetic fields.

I just want you to end up having that trouble instead. I think you 
should consider a shielded twisted pair instead. Use the transformer to 
go between 50 Ohm and 100-110 Ohm while also getting the common mode 
isolation. A double-transformer approach can be used in which the 
launch/receive-transformer has a center tap on the "inside" which is 
wired to local ground (needs to be very low impedance). This improves 
capacitive isolation for common mode currents. The inner transformers do 
impedance matching. This is really an alternative to getting isolation 
transformers, it might even be cheaper. Dual-shielded isolation 
transformers is better thought, as capacitive coupling as spread out 
over the coil is always terminated to each side own shield which reduces 
common-mode to diffrential mode conversion.

> My worry was that the ground currents might be enough to saturate the 
> tiny ferrite cores in the hybrid transformers.  The engineer's 
> reaction to this was on the following day to say that if this turns 
> out to be a problem, he will add DC blocks.  This would have to be 
> the kind that blocks both center and shield paths.

I have a bit hard to realize how the common mode ground current would 
saturate the hybrid transformers unless the current is so high that the 
asymmetry in the transformers helps. Some form of DC blocker or LF 
current limiting may be wise thought.

> The problem is that the radar and the ship are not yet built, so we 
> cannot yet make tests.

So much better. You have a chance to get things right before it is too 
late and too expensive.

I am sure we can send a sub to sink it late if needed.

  energy straight through and allow for a higher resistive path for the
  low frequency energy.
>>>  The ground grid impedance between any two points is well less than one
>>>  ohm, so 100 ohms will pretty much abolish all ground loops.  I've used 10
>>>  ohms in like labs, with success.  I'll grant that this would not work with
>>>  long wires outside.
>> Should be sufficient then. But remember that capacitive coupling helps
>> you in the RF area and impulse protection.
> 
> True.

The reason I keep mentioning it is since that it is easy to focus and 
make a design "optimum" for one case and forgetting about other aspects. 
Signal integrity, safety and EMC needs too be considered at the same time.

>>  > By the way, I also finally talked to one of our most experienced EMI/EMC
>>>  engineers.  He suggested using MIL-STD-461 test CS109, even though CS109
>>  > was developed for enclosures.  It turns out he was involved in developing
>>>  CS109 when he worked for the US Navy.
>> Need to look it up. Never had to do any of the MIL-STD-461 stuff.
> 
> It's available for free on the web. 
> 

Another site which can't keep their certs up-to-date.

By looking at it, it seems reasonably to use that or some suitable 
variant. Notice how the 10 MHz input/out wires is not included so some 
adaptation would be required. Essentially one where the 10 MHz generator 
is floating through isolation transformer and the current is induced on 
the generator ground.

Cheers,
Magnus

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Joe Gwinn]

Magnus,

At 10:31 AM + 1/10/09, time-nuts-requ...@febo.com wrote:
>
>Message: 5
>Date: Sat, 10 Jan 2009 11:06:39 +0100
>From: Magnus Danielson 
>Subject: Re: [time-nuts] Standards sought for immunity of shielded
>   cable   links to power-frequency ground loops
>To: Discussion of precise time and frequency measurement
>   
>
>Joseph,
>
>>  time-nuts-boun...@febo.com wrote on 01/07/2009 10:47:46 PM:
>>
>>>  Joseph,
>>>
>>  Could be a differential TX and RX.  I recall that they send a RS422
  signal.
>  Depending on the speed, RS422 works fine with transformers.
  Yes.  It would be 10 MHz or 20 MHz, depending on coding.  Or 5 MHz, so
>>  the
  transitions are at 10 MHz.  I don't recall, or never knew.
>>>  RS422 does not imply any encoding as such so it would be 10 MHz but
>>>  naturally there is twice that many transitions, but it is the frequency
>>>  of the signal you are interested in for this case.
>>
>>  I know that RS422 is not the encoding.  I cheated, and talked to the
>>  relevant engineer.
>
>That is to cheat! :)
>
>>  For digital signals (1PPS, various triggers), it's RS422 over 100 ohm
>>  twinax (fancy shielded twisted pair).
>>
>>  The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the
>>  signals 180 degrees out of phase.  This is acheived with a pair of hybrid
>  > transformers which convert from one-cable to two-cable and then back to
>>  one-cable, where all cables are 50 ohm coax.
>
>OUCH! The trouble with that arrangement is that the coax cables MUST be
>twisted or else H-fields will induce differential mode current. It will
>induce current into both directions which through the 180 degree will
>not cancel but add up. The 0/180 degree arrangement will save you from
>common mode problems. You would prefer a twisted cable over a twisted
>cable pair, as the later allows for installation procedure errors to
>have huge impact and the twisting properties will not be as good either
>and thus compromising the quality. A single ended coax is not as
>sensitive to H fields to induce diffrential currents, but can have some
>other problems.

You are right about the twisting.  The cables are close and parallel, 
and ground offsets are the big problem, versus magnetic fields.

My worry was that the ground currents might be enough to saturate the 
tiny ferrite cores in the hybrid transformers.  The engineer's 
reaction to this was on the following day to say that if this turns 
out to be a problem, he will add DC blocks.  This would have to be 
the kind that blocks both center and shield paths.

The problem is that the radar and the ship are not yet built, so we 
cannot yet make tests.


>   But you should never let the screen float in the far end, you should
>  terminate it with a 10M resistor and a sparkgap in parallel to the
>  local ground.
>
>  The resistor takes care of static electricity and the sparkgap will
>  do lightnings.
  I've done such things, but with a 100 ohm resistor (and a safety
>>  ground to
  ensure that the voltage doesn't get too large.  But this was
>>>  a lab lashup.
>>>
>>>  The trouble with 100 ohm is that still can be a little low in relation
>  >> to ground loop impedances, it still allow some fair current to roll down
>  >> the cable. A capacitor in parallel would cut most of the transient
>>>  energy straight through and allow for a higher resistive path for the
>>>  low frequency energy.
>>
>>  The ground grid impedance between any two points is well less than one
>>  ohm, so 100 ohms will pretty much abolish all ground loops.  I've used 10
>>  ohms in like labs, with success.  I'll grant that this would not work with
>>  long wires outside.
>
>Should be sufficient then. But remember that capacitive coupling helps
>you in the RF area and impulse protection.

True.


>  > By the way, I also finally talked to one of our most experienced EMI/EMC
>>  engineers.  He suggested using MIL-STD-461 test CS109, even though CS109
>  > was developed for enclosures.  It turns out he was involved in developing
>>  CS109 when he worked for the US Navy.
>
>Need to look it up. Never had to do any of the MIL-STD-461 stuff.

It's available for free on the web. 


Joe

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Re: [time-nuts] GPSDO TC

[Magnus Danielson]

Hej Bruce,

>> As the PLL filters the noise of the OCXO and passes noise from the input 
>> side and the noise have several different components to it from either 
>> source.
>>
>> You can't really extrapolate direction the results of an asynchronous 
>> reciever such as the M12+T to that of a synchronous receiver such as the 
>> Thunderbolt. The time-solution of thunderbolt is used in replacement of 
>> the time-interval counter fluff slapped onto a PPS based receiver such 
>> as the M12+T. Also, the Thunderbolt enjoys a much quiter and stable 
>> reference than the M12+T which allows for narrower filters in the 
>> sat-tracking as the phasenoise is lower. Notice how the Thunderbolt can 
>> be configured for different uses, they are direct hints to what the 
>> tracking loops may do as it reduces the physical dynamics of position as 
>> well as inflicted G effects on the OCXO.
>>
>>   
> I wasn't attempting to do so.
> However the phase noise of the GPS receiver will still dominate for
> short tau whilst that of the OCXO is dominant for longer tau.

Agreed. I'm just trying to keep various error sources separate here.

The truncation noise isn't white noise one should recall.

>> With just two Thunderbolts and a reasonable TIC you can infact build a 
>> three-cornered hat. You have three clocks: GPS, OCXO1 and OCXO2 and the 
>> thunderbolts will measure GPS-OCXO1 and GPS-OCXO2 and the TIC will be 
>> able to measure the OCXO1-OCXO2. An interesting aspect of this is that 
>> when lockedup, the PPSes of the Thunderbolts will be confined into a 
>> rather small area. This arrangement will, as any other, give not the 
>> standalone OCXO noise when beeing steered, but it is not entierly lying 
>> for those longer taus.
>>
>>   
> The 3 cornered hat technique only works well (even in the extended form
> where finite correlations between sources are included) when the noise
> of each of the 3 sources are comparable.
> That is this technique will only work well in the vicinity of the point
> where the GPS receiver and OCXO ADEVs crossover or equivalently near the
> drift corrected minimum of the ADEV as measured by the Thunderbolt when
> the OCXO is undisciplined. For shorter tau the GPS phase noise dominates.

Yes. I think it would be usefull to provide confidence intervals etc. to 
get a better "feel" of how good the measure is.

As a reference measurement the thunderbolts could be driven into 
holdover. I think the thunderbolts keep reporting timing errors.

>> True. However, I think there is still some more theoretical work to be 
>> done to give us better tools. These does not remove the need for 
>> measurements and I have never been foolish enougth to beleive so, but it 
>> could guide us in the right direction for selecting and steering our 
>> parameters.
>>
>>   
> It would be helpful if the ADEV (and MDEV) plots for several
> Thunderbolts were plotted using the Thunderbolt's internal phase error
> measures obtained when the OCXO is undisciplined.
> This can easily be setup using the Trimble Thunderbolt Monitor program.

Indeed. We should recall that the PLL locking fades over from the OCXO 
to the GPS (as received) noise at about the BW frequency/tau of the PLL.

Finding the optimum crossover properties involves both balancing PLL 
bandwidth and damping.

>>> I had noted that your quoted damping factor was incorrect but I
>>> suspected that you would realise that.
>>>
>>> Actually according to Gardener critical damping factor is 1 ( minimum
>>> settling with no overshoot for a phase step).
>>> However a damping factor of 0.7071 is widely used.
>>> 
>> It is interesting to clear up why this difference exists. Could be 
>> "critical" is judged different for different applications.
>>
>>   
> The usual meaning of critical damping in a second order differential
> equation is for no overshoot to a step input.
> Thus critical probably isn't the appropriate term when optimising for
> other factors.
> Optimum damping for a particular criterion is perhaps better description.

Precisely. Critical damping is just a handy reference along the line, 
but should not be incorrectly interprented as optimum in some generic 
sense, there is several forms of optimum for different tasks.

I think best ADEV or best TDEV might be more relevant here.

>> My point was that regardless of implementation, second order type II 
>> loops seems to be the reference mark, which not necessarilly is a good 
>> one. Third order loops should be considered as it removes or reduces a 
>> type of problem and allows a more freer setting of parameters with less 
>> things to compromise between. When doing the loop in digital processing 
>> it is not that more expensive. There are re-tunable architectures which 
>> is being used in for instance GPS receivers which is not hard at all to 
>> use for both PI and PID controllers.
>>
>> Cheers,
>> Magnus
>>
>>   
> In particular the ability of a third order loop to track linear frequency 
> drift

Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Hej Magnus

Magnus Danielson wrote:
> Bruce,
>
>   
>>> Yes, but the bump comes from the increase gain around the resonance and 
>>> spoils the OCXO/GPS cross-over. The simplified noise-bandwidth measure 
>>> does not really comply here since they usually build on a simplified 
>>> model of noise type (white noise - gaussian). A simple check in Gardiner 
>>> provides both the generic integrating formula, simplified results and a 
>>> graph showing the smae numbers that you give.
>>>   
>>>   
>> Whilst the phase noise of a sawtooth corrected M12+T GPS timing receiver
>> approximates white phase noise (at least for tau < 1 day), this may not
>> be so for the receiver used in the Thunderbolt.
>> The phase noise of the OCXO certainly cannot be accurately modeled as
>> white phase noise for large tau.
>> 
>
> As the PLL filters the noise of the OCXO and passes noise from the input 
> side and the noise have several different components to it from either 
> source.
>
> You can't really extrapolate direction the results of an asynchronous 
> reciever such as the M12+T to that of a synchronous receiver such as the 
> Thunderbolt. The time-solution of thunderbolt is used in replacement of 
> the time-interval counter fluff slapped onto a PPS based receiver such 
> as the M12+T. Also, the Thunderbolt enjoys a much quiter and stable 
> reference than the M12+T which allows for narrower filters in the 
> sat-tracking as the phasenoise is lower. Notice how the Thunderbolt can 
> be configured for different uses, they are direct hints to what the 
> tracking loops may do as it reduces the physical dynamics of position as 
> well as inflicted G effects on the OCXO.
>
>   
I wasn't attempting to do so.
However the phase noise of the GPS receiver will still dominate for
short tau whilst that of the OCXO is dominant for longer tau.
> With just two Thunderbolts and a reasonable TIC you can infact build a 
> three-cornered hat. You have three clocks: GPS, OCXO1 and OCXO2 and the 
> thunderbolts will measure GPS-OCXO1 and GPS-OCXO2 and the TIC will be 
> able to measure the OCXO1-OCXO2. An interesting aspect of this is that 
> when lockedup, the PPSes of the Thunderbolts will be confined into a 
> rather small area. This arrangement will, as any other, give not the 
> standalone OCXO noise when beeing steered, but it is not entierly lying 
> for those longer taus.
>
>   
The 3 cornered hat technique only works well (even in the extended form
where finite correlations between sources are included) when the noise
of each of the 3 sources are comparable.
That is this technique will only work well in the vicinity of the point
where the GPS receiver and OCXO ADEVs crossover or equivalently near the
drift corrected minimum of the ADEV as measured by the Thunderbolt when
the OCXO is undisciplined. For shorter tau the GPS phase noise dominates.
>>> I rather beleive what ADEV, MDEV and TDEV experience in this context.
>>>
>>>   
>>>   
>> Yes measurements are the key but if one doesnt have a suitable
>> statistically independent low noise frequency reference it isnt possible
>> to optimise the loop parameters for an individual GPSDO.
>> 
>
> True. However, I think there is still some more theoretical work to be 
> done to give us better tools. These does not remove the need for 
> measurements and I have never been foolish enougth to beleive so, but it 
> could guide us in the right direction for selecting and steering our 
> parameters.
>
>   
It would be helpful if the ADEV (and MDEV) plots for several
Thunderbolts were plotted using the Thunderbolt's internal phase error
measures obtained when the OCXO is undisciplined.
This can easily be setup using the Trimble Thunderbolt Monitor program.
>>> We could go back to the real integration formula, adapt it to various 
>>> powers of f^-n noises and analyse it for the same set of PLL loop 
>>> filters as analysed by Gardiner. Similarly we could cook up a simulation 
>>> and do the ADEV, MDEV and TDEV measures. Traditional noise bandwidth 
>>> measures can be calculated alongside.
>>>
>>> I am somewhat surprised that you missed the opportunity to correct me as 
>>> I was giving the incorrect value for damping factor of a critically 
>>> damped system. It is the square root of 1/2 and not 2, thus 0.7071 is 
>>> the appropriate damping factor for critically damped systems.
>>>
>>>   
>>>   
>> I had noted that your quoted damping factor was incorrect but I
>> suspected that you would realise that.
>>
>> Actually according to Gardener critical damping factor is 1 ( minimum
>> settling with no overshoot for a phase step).
>> However a damping factor of 0.7071 is widely used.
>> 
>
> It is interesting to clear up why this difference exists. Could be 
> "critical" is judged different for different applications.
>
>   
The usual meaning of critical damping in a second order differential
equation is for no overshoot to a step input.
Thus critical probably isn't the appropr

Re: [time-nuts] GPSDO TC

[Magnus Danielson]

Bruce,

>> Yes, but the bump comes from the increase gain around the resonance and 
>> spoils the OCXO/GPS cross-over. The simplified noise-bandwidth measure 
>> does not really comply here since they usually build on a simplified 
>> model of noise type (white noise - gaussian). A simple check in Gardiner 
>> provides both the generic integrating formula, simplified results and a 
>> graph showing the smae numbers that you give.
>>   
> Whilst the phase noise of a sawtooth corrected M12+T GPS timing receiver
> approximates white phase noise (at least for tau < 1 day), this may not
> be so for the receiver used in the Thunderbolt.
> The phase noise of the OCXO certainly cannot be accurately modeled as
> white phase noise for large tau.

As the PLL filters the noise of the OCXO and passes noise from the input 
side and the noise have several different components to it from either 
source.

You can't really extrapolate direction the results of an asynchronous 
reciever such as the M12+T to that of a synchronous receiver such as the 
Thunderbolt. The time-solution of thunderbolt is used in replacement of 
the time-interval counter fluff slapped onto a PPS based receiver such 
as the M12+T. Also, the Thunderbolt enjoys a much quiter and stable 
reference than the M12+T which allows for narrower filters in the 
sat-tracking as the phasenoise is lower. Notice how the Thunderbolt can 
be configured for different uses, they are direct hints to what the 
tracking loops may do as it reduces the physical dynamics of position as 
well as inflicted G effects on the OCXO.

With just two Thunderbolts and a reasonable TIC you can infact build a 
three-cornered hat. You have three clocks: GPS, OCXO1 and OCXO2 and the 
thunderbolts will measure GPS-OCXO1 and GPS-OCXO2 and the TIC will be 
able to measure the OCXO1-OCXO2. An interesting aspect of this is that 
when lockedup, the PPSes of the Thunderbolts will be confined into a 
rather small area. This arrangement will, as any other, give not the 
standalone OCXO noise when beeing steered, but it is not entierly lying 
for those longer taus.

>> I rather beleive what ADEV, MDEV and TDEV experience in this context.
>>
>>   
> Yes measurements are the key but if one doesnt have a suitable
> statistically independent low noise frequency reference it isnt possible
> to optimise the loop parameters for an individual GPSDO.

True. However, I think there is still some more theoretical work to be 
done to give us better tools. These does not remove the need for 
measurements and I have never been foolish enougth to beleive so, but it 
could guide us in the right direction for selecting and steering our 
parameters.

>> We could go back to the real integration formula, adapt it to various 
>> powers of f^-n noises and analyse it for the same set of PLL loop 
>> filters as analysed by Gardiner. Similarly we could cook up a simulation 
>> and do the ADEV, MDEV and TDEV measures. Traditional noise bandwidth 
>> measures can be calculated alongside.
>>
>> I am somewhat surprised that you missed the opportunity to correct me as 
>> I was giving the incorrect value for damping factor of a critically 
>> damped system. It is the square root of 1/2 and not 2, thus 0.7071 is 
>> the appropriate damping factor for critically damped systems.
>>
>>   
> I had noted that your quoted damping factor was incorrect but I
> suspected that you would realise that.
> 
> Actually according to Gardener critical damping factor is 1 ( minimum
> settling with no overshoot for a phase step).
> However a damping factor of 0.7071 is widely used.

It is interesting to clear up why this difference exists. Could be 
"critical" is judged different for different applications.

>> I am somewhat surprised that when we have been discussing the bandwidth 
>> of the PLLs and considering OCXOs being running with fairly high drift 
>> rate we have been assuming second degree loops. This form of 
>> acceleration requires third degree responses for proper handling, as 
>> being well documented in literature such as Gardiner. Going for third 
>> degree response the bandwidth of the loop can be (at least more freely) 
>> disconnected from tracking requirements due to drift rate.
>>   
> I only mentioned second order type II loops as the analysis is somewhat
> simpler and there is no indication from the number of tuning parameters
> for the Thunderbolt that a higher order loop is involved.

My point was that regardless of implementation, second order type II 
loops seems to be the reference mark, which not necessarilly is a good 
one. Third order loops should be considered as it removes or reduces a 
type of problem and allows a more freer setting of parameters with less 
things to compromise between. When doing the loop in digital processing 
it is not that more expensive. There are re-tunable architectures which 
is being used in for instance GPS receivers which is not hard at all to 
use for both PI and PID controllers.

Chee

Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Magnus Danielson]

Joseph,

> time-nuts-boun...@febo.com wrote on 01/07/2009 10:47:46 PM:
> 
>> Joseph,
>>
> Could be a differential TX and RX.  I recall that they send a RS422 
>>> signal.
 Depending on the speed, RS422 works fine with transformers.
>>> Yes.  It would be 10 MHz or 20 MHz, depending on coding.  Or 5 MHz, so 
> the 
>>> transitions are at 10 MHz.  I don't recall, or never knew.
>> RS422 does not imply any encoding as such so it would be 10 MHz but 
>> naturally there is twice that many transitions, but it is the frequency 
>> of the signal you are interested in for this case.
> 
> I know that RS422 is not the encoding.  I cheated, and talked to the 
> relevant engineer.

That is to cheat! :)

> For digital signals (1PPS, various triggers), it's RS422 over 100 ohm 
> twinax (fancy shielded twisted pair).
> 
> The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the 
> signals 180 degrees out of phase.  This is acheived with a pair of hybrid 
> transformers which convert from one-cable to two-cable and then back to 
> one-cable, where all cables are 50 ohm coax.

OUCH! The trouble with that arrangement is that the coax cables MUST be 
twisted or else H-fields will induce differential mode current. It will 
induce current into both directions which through the 180 degree will 
not cancel but add up. The 0/180 degree arrangement will save you from 
common mode problems. You would prefer a twisted cable over a twisted 
cable pair, as the later allows for installation procedure errors to 
have huge impact and the twisting properties will not be as good either 
and thus compromising the quality. A single ended coax is not as 
sensitive to H fields to induce diffrential currents, but can have some 
other problems.

> I imagine that the shield is grounded at both ends, if only for
> safety reasons.
 That is actually a very unsafe practice, unless there is another
 much thicker and reliable ground connection between the two domains.
>>> There is a very heavy grounding grid, and such systems almost always 
>>> ground the (outer) shields at every connector.
>> Which would imply that if the signal passes through a connector jack or 
>> through a wall, much of the current would be sent back to its EMF source 
> 
>> locally in the room. This does have its merits.
> 
> Yes, but that isn't the reason.  It's really a safety and EMC rationale.

As suspected, but this is really just another of these EMC rationales.

 But you should never let the screen float in the far end, you should
 terminate it with a 10M resistor and a sparkgap in parallel to the
 local ground.

 The resistor takes care of static electricity and the sparkgap will
 do lightnings.
>>> I've done such things, but with a 100 ohm resistor (and a safety 
> ground to 
>>> ensure that the voltage doesn't get too large.  But this was 
>> a lab lashup.
>>
>> The trouble with 100 ohm is that still can be a little low in relation 
>> to ground loop impedances, it still allow some fair current to roll down 
> 
>> the cable. A capacitor in parallel would cut most of the transient 
>> energy straight through and allow for a higher resistive path for the 
>> low frequency energy.
> 
> The ground grid impedance between any two points is well less than one 
> ohm, so 100 ohms will pretty much abolish all ground loops.  I've used 10 
> ohms in like labs, with success.  I'll grant that this would not work with 
> long wires outside.

Should be sufficient then. But remember that capacitive coupling helps 
you in the RF area and impulse protection.

> By the way, I also finally talked to one of our most experienced EMI/EMC 
> engineers.  He suggested using MIL-STD-461 test CS109, even though CS109 
> was developed for enclosures.  It turns out he was involved in developing 
> CS109 when he worked for the US Navy.

Need to look it up. Never had to do any of the MIL-STD-461 stuff.

Cheers,
Magnus

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Re: [time-nuts] GPSDO TC & Damping

[Magnus Danielson]

Dick,

> Sorry, Magnus, I didn't mean to put words in your mouth.

No worries, I just did not want it to be raised to "truth".

> I was remembering last Fall, when you suggested that people look at your  
> ADEV plot and to be mindful of the of the bounding slopes of the  
> curves. If I've mis-remembered the emphasis or the facts, I do  
> apologize. I thought your argument then, as I remember it, was strong  
> and valuable. Seems like it gave a good range of possible values to  
> use for Tau in the measurements.

Do not recall the particular discussion. A ref into the archives or date 
would be apprechiated.

> I probably won't go far beyond the capabilities of the TBolt, such as  
> implementing a PID controller with dynamic control of variables using  
> a microcontroller and LPGAys and writing my own software, but I love  
> it when you talk that way.

Sounds dangerous. :)

I have been looking further into the variance, ADEV, MDEV and TDEV with 
respect to the effect of various dampings. It is becomming clear to me 
that the gain effect which can be attributed to damping will ripple over 
to all those measures. I have not had the time to convert this to a 
complete analysis, but I think I can do that.

However, to give a partial answer to Bruce, I would like to point to a 
passage in Gardiner where the optimum damping for minimum flicker noise 
is being found, and that is for 1.14 rather than 0.5 which would be 
minimum noise bandwidth. Gardiner does not go into ADEV and friends, but 
does provide the refernces and gives an indication of convergance 
problems for flicker noise on variance, also indicating that this is a 
problem for further analysis. Using ADEV and friends in replacement 
analysis could be made analogously for other noise types and as it 
happends also work in cooperation with investigating the effect on 
various noisetypes.

Regardless, it is an interesting little problem and I get to exercise 
the little grey and refresh my always failing abilities in integration.

Cheers,
Magnus

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Re: [time-nuts] ADEV test setup [was GPSDO TC & Damping]

[Steve Rooke]

Bruce, thanks, I'm soaking it all up.

73, Steve

2009/1/10 Bruce Griffiths :
> Steve
>
> Steve Rooke wrote:
>> Bruce,
>>
>> Thanks for the detailed rundown. Looking at the picket-fence method,
>> this looks possible for me but I will have to get hold of the
>> reference standard. I have a Racal-Dana 1992 with IEEE488 but need to
>> get an interface card for the PC end. These are fairly cheap to buy.
>>
>> You spoke about some types of rubidium standards being suitable, would
>> you care to elaborate on that please? Would something like an Efratom
>> FRS be suitable?  Generating the picket-fence itself should not be
>> hard as long as care is taken not to introduce noise. Do you have any
>> links to articles on the design for the
>> mixer/zero-crossing/square-wave beat circuit? One question, assuming
>> that I have a 10MHz reference standard and I'm measuring a 10MHz dut,
>> how do I arrange for them to be about 1Hz apart, given that we are
>> measuring for accuracy here? 1HZ different would make the accuracy
>> 1E-7 out anyway, or am I missing something here?
>>
>>
> The best article I've come across on zero-crossing detector design is:
>
> The Design of Low Jitter Hard Limiters" Oliver Collins, IEEE
> transactions on Communications, Vol 44 No 5, May 1996 pp 601-608
>
> Unfortunately its not free, however you may be able to access it via a
> Library.
>
> However if you only want to use the technique described in the paper, I
> have a couple of spreadsheets that calculate the stage gains and low
> pass filter time constants both for the simplified analysis in the paper
> and the more general case where the input noise spectral density differs
> for each stage.
> Some pointers on what to include in the noise calculations for each
> stage can be found at:
>
> http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html
> 
>
> Some care is required, in that if the spreadsheet predicts a gain of
> less than unity for the input stage, it is in fact better to use a
> passive RC low pass filter in front of the first amplifier limiter stage
> (without a clamp as typically the IF signal amplitude at the mixer
> output is insufficient to cause the clamp diodes to conduct - more
> complex clamps are too noisy).
> The amplifier limiter chain is then redesigned to accommodate this change.
>
> Don't be taken in by those who would insist that everything should be
> linear as long as possible, the resultant deign is suboptimal.
> Such comments sprang from the fact that no one at that time had worked
> out how to include the effect of the clamps on the performance.
> Oliver Collins solved that problem, so there is no longer a valid excuse
> for such misguided recommendations.
>
>> So the real thing for the budget-conscious time-nut seems to be the
>> reference standard.  The ocxos you spoke about do seem to be on the
>> rare/expensive side and are an order of magnitude or two better than
>> the Option 4E I have in the 1992.
>>
>> 73, Steve
>>
> Bruce
>
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>



-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet

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Re: [time-nuts] ADEV test setup [was GPSDO TC & Damping]

[Magnus Danielson]

Tom Van Baak skrev:
>> I'd like to see a similar test conducted against a local Cs clock (and/or
>> maser), just to get everything on one graph.
>>
>> -- john, KE5FX
> 
> It's on the list.
> 
> One earlier idea was to measure tc=1 10 100 1000 10k
> simultaneously with 5 Thunderbolts, but I suspected that
> unit to unit variations among the GPSDO would partially
> cloud the results. So that's why I did back-to-back runs
> using the same TBolt, same reference, and same TIC for
> each run. All I changed was the TC in the GUI, figuring
> that was the safest thing to do.

You could do thia, but in a rotating scheme such that you end up having 
measured all thunderbolts for all TCs. That would help decorrelate 
individual differences among the thunderbolts.

Cheers,
Magnus

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