Re: [time-nuts] ACAM GP22 Chip

[Bob Camp]

Hi

If you head down to your local big box store, they will happily sell you a 
thousand foot spool 
of RG-6 coax for next to nothing. If their prices are still to high, the 
auction sites will sell it for 
even less. It has a 75 ohm impedance and a bandwidth of several GHz. The rather 
convent 
formula of RT = 0.35 / BW then comes in. A 3.5 GHz cable will limit you to a 
100 ps rise time. 
In all likelihood, you will be unable to generate a signal with this fast a 
rise time. 

You also will have some loss effects in the cable that are frequency dependent. 
The calculation above
assumes you have done a few tricks to take care of this. If not, to get a 10 ns 
rise time, you need to maintain 
a 35 MHz bandwidth. That works fine if you have a buffer every 500 feet. No 
tricks, just a CMOS buffer
chip.

As noted by others, it *is* coax. You need to drive it and terminate it with 75 
ohms. At 35 MHz, a cheap 
75 ohm resistor will do the trick just fine. At 3.5 GHz you may need to get a 
bit more careful. 

So is the 500’ limit an issue? I’d suggest that it’s not. Consider chopping up 
the spool in a binary series of
400, 200,100,50,25,12.5, 6.5, 3.25 feet.  You now have a set of buffered lines 
that can be arranged to give you
a nice set of 256 time steps. Yes, the delay of the buffers will get in the way 
a bit. The actual line lengths will 
be a bit shorter as the lengths drop. 

So how much delay do you get from a 400’ line? Velocity factor comes in here. 
Best guess is that
your foam RG-6 has a 0.78 velocity factor. The "speed of light” in the coax is 
78% of the speed of light
in vacuum. Your 400 foot coax has about a 520 ns delay. Your stack comes out 
just a bit over 1 us.

Bob 



> On Nov 24, 2015, at 9:04 AM, Thomas Allgeier  wrote:
> 
> Hello,
> 
> 
> 
> I have an ACAM GP22 TDC chip and evaluation board which I am looking at for 
> “work” purposes – I work for a company active in the weighing and force 
> measurement world.
> 
> 
> 
> I should say from the start that I am new to time and frequency measurements 
> and not even an electronics engineer – but then I have been exposed to 
> high-precision electronics for the last 25 years hence have picked up some 
> dangerous degree of half-knowledge.
> 
> 
> 
> We want to use this chip to measure the period of a square wave, of around 13 
> kHz i.e. in the 70 µs range. As the application is potentially high-accuracy 
> we need to know the period to within 1 ns or better.
> 
> 
> 
> In order to evaluate the chip I was planning to replicate John A’s experiment 
> with the coaxial delay line from the HP5370b – but as my interest is in 
> “measuring range 2” of the GP22 I need a delay of 500 ns or more (actually 1 
> µs sounds a better start). This is the equivalent of a 200 m length of cable. 
> I fear trouble with this: Am I not getting unwanted inductivities if I use a 
> coil of that size?
> 
> 
> 
> So, to come to the point: Am I pushing the concept of a coax delay too far 
> with 1 µs and are there other (simple/reliable) ways to achieve this kind of 
> delay? I have tried it with a shorter piece of cable (around 2 ns which is 
> measured in “range 1”), there I seem to get consistency virtually to within 
> 100 ps. But I need to know if the device sticks to this level of performance 
> when the periods are much longer, and thus measured in “range 2”.
> 
> 
> 
> Thanks and best regards,
> 
> Thomas.
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Re: [time-nuts] ACAM GP22 Chip

[Robert LaJeunesse]

If the goal is to create two signals consistently spaced near 70us apart why 
not use a good, fast 8-bit serial-in, parallel out shift register, clocked 
cleanly at 100kHz? Using the outputs from stages 1 and 8 would result in a 70us 
delay between signals. The data in would be fed 100KHz divided by 10 (or 16, or 
anything greater than 8) at whatever duty cycle is available. This allows the 
GP22 to see the combined instabilities of the clock and the shift register, 
which could be down in the nanosecond range, possibly less since the shift 
register delays would inherently cancel all but their differences.

Bob LaJeunesse

> Sent: Tuesday, November 24, 2015 at 9:04 AM
> From: "Thomas Allgeier" 
> To: time-nuts@febo.com
> Subject: [time-nuts] ACAM GP22 Chip
>
> Hello,
> 
> I have an ACAM GP22 TDC chip and evaluation board which I am looking at for 
> “work” purposes – I work for a company active in the weighing and force 
> measurement world. 

...

> We want to use this chip to measure the period of a square wave, of around 13 
> kHz i.e. in the 70 µs range. As the application is potentially high-accuracy 
> we need to know the period to within 1 ns or better.
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Re: [time-nuts] Trimble Thunderbolt E frequency pulling

[GandalfG8--- via time-nuts]

Al, Thanks for the good idea, that would certainly have saved me a bit  of 
work:-), but the switch box inserted in the coax line with the counter input 
 set to high impedance performs the same function, straight through 
connection  with the option to switch in a 50 ohm shunt resistor, and it's 
quite 
clear that  the frequency shift is virtually instantaneous.
 
Nigel
GM8PZR
 

Nigel,
 Set your counter to high impedance. Then install a T connector on the 
 input. Then you can install or remove a 50 ohm terminator on the T while 
 plotting the results. Then you can see how fast the frequency shift is 
 depending on the load.
 
 AL, k9si
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Re: [time-nuts] ACAM GP22 Chip

[Hal Murray]

th.allge...@gmail.com said:
> I should say from the start that I am new to time and frequency measurements
> and not even an electronics engineer – but then I have been exposed to
> high-precision electronics for the last 25 years hence have picked up some
> dangerous degree of half-knowledge. 

Do you have a copy of Art of Electronics by Horowitz and Hill?


> In order to evaluate the chip I was planning to replicate John A’s
> experiment with the coaxial delay line from the HP5370b – but as my interest
> is in “measuring range 2” of the GP22 I need a delay of 500 ns or more
> (actually 1 µs sounds a better start). This is the equivalent of a 200 m
> length of cable. I fear trouble with this: Am I not getting unwanted
> inductivities if I use a coil of that size? 

The trick is that your coil has 2 wires.  It's a transmission line.

Here is the handwaving explanation: The current on the return path is going 
in the other direction and cancels out the inductance.

Twisted pair will work almost as well as coax, maybe better than cheap coax.

AoE has an appendix on transmission lines.  You can find lots of info on the 
web.  You need to terminate it.  You will want a scope to check the 
termination.



-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Trimble Thunderbolt E frequency pulling

[Al Wolfe]

Nigel,
   Set your counter to high impedance. Then install a T connector on the 
input. Then you can install or remove a 50 ohm terminator on the T while 
plotting the results. Then you can see how fast the frequency shift is 
depending on the load.


AL, k9si


Switching the impedance at the counter itself resets any plot  that's in
progress, which does introduce some delay, so I put together a  small 
switch
box that could be inserted into the coax and confirmed it's a  rapid 
effect

that's unlikely to be thermal.


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Re: [time-nuts] ACAM GP22 Chip

[Attila Kinali]

Moin,

On Tue, 24 Nov 2015 14:04:32 -
"Thomas Allgeier"  wrote:

> We want to use this chip to measure the period of a square wave, of
> around 13 kHz i.e. in the 70 µs range. As the application is potentially
> high-accuracy we need to know the period to within 1 ns or better.

That's some modest requirement and should be doable with the GPS22
quite easily (or any other TDC for that matter). BTW: when specing
something like this, please make sure to mention whether 1ns is
1sigma, 3sigma or worst case/peak-to-peak. These 3 are quite different
requirements.


May I ask why you want to verify the specs of the GP22?
The specs say that it does something between 39ps and 70ps (1sigma),
which is probably way better than what you need. And as Acam is
a german company, I expect the datasheet to be accurate.

BTW: 1ns over 70us is approximately 14ppm. The GP22 uses the attached
crystal for absolute calibration. Please be aware that 14ppm will
require at least a TCXO to reach that level over the whole temperature
range, and depending on what TCXO you use, you might need to calibrate
the TCXO post-production and again after a couple of years of use.
Even if you don't need calibrate, I would add a TCXO frequency measurement
to the production test.

BTW2: the "we have a x ppm TCXO" value is usually misleading,
as that's the best-case, pre-soldering, pre-aging, pre-anything value.
The end-value can be 3 times as large... easily.
(unless you happen to choose one of the more honest manufacturers,
for example, like Abracon)

> In order to evaluate the chip I was planning to replicate John A’s
> experiment with the coaxial delay line from the HP5370b 

For those wondering: "John A" is John Ackermann and the experiment
in question is documented at http://www.febo.com/pages/hp5370b/

> – but as my interest
> is in “measuring range 2” of the GP22 I need a delay of 500 ns or more
> (actually 1 µs sounds a better start). This is the equivalent of a 200 m
> length of cable. I fear trouble with this: Am I not getting unwanted
> inductivities if I use a coil of that size?

The coax is a transmission line. Yes you have inductance and capacitance,
but it does not make that much sense to talk about that anymore,
the impedance is the right thing to talk about.
Your output will not be as sharp as your input due to dispersion,
but that can be easily recovered using some buffer gate.

Please make sure that your coil is reasonably temperature stabilized
and, if it's cheaper cable, also humidity stabilized, as both parameters
change your delay. (putting it into an isolated box should be good
enough for this kind of measurment).


Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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[time-nuts] ACAM GP22 Chip

[Thomas Allgeier]

Hello,



I have an ACAM GP22 TDC chip and evaluation board which I am looking at for 
“work” purposes – I work for a company active in the weighing and force 
measurement world.



I should say from the start that I am new to time and frequency measurements 
and not even an electronics engineer – but then I have been exposed to 
high-precision electronics for the last 25 years hence have picked up some 
dangerous degree of half-knowledge.



We want to use this chip to measure the period of a square wave, of around 13 
kHz i.e. in the 70 µs range. As the application is potentially high-accuracy we 
need to know the period to within 1 ns or better.



In order to evaluate the chip I was planning to replicate John A’s experiment 
with the coaxial delay line from the HP5370b – but as my interest is in 
“measuring range 2” of the GP22 I need a delay of 500 ns or more (actually 1 µs 
sounds a better start). This is the equivalent of a 200 m length of cable. I 
fear trouble with this: Am I not getting unwanted inductivities if I use a coil 
of that size?



So, to come to the point: Am I pushing the concept of a coax delay too far with 
1 µs and are there other (simple/reliable) ways to achieve this kind of delay? 
I have tried it with a shorter piece of cable (around 2 ns which is measured in 
“range 1”), there I seem to get consistency virtually to within 100 ps. But I 
need to know if the device sticks to this level of performance when the periods 
are much longer, and thus measured in “range 2”.



Thanks and best regards,

Thomas.
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Re: [time-nuts] PRS10 Heatsink Temperature

[Mike Cook]

> Le 24 nov. 2015 à 01:33, Charles Steinmetz  a écrit :
> 
> David wrote:
> 
>> I plan to add a small temperature controlled fan to limit the heatsink 
>> temperature. Can anyone recommend a suitable setpoint for the heatsink?
> 
> An SRS engineer once told me to keep the PRS10's baseplate temperature below 
> 40C.

  You think that they would know their stuff. But their heatsinks don’t quite 
do that. They get close. My 3 PRS10s are all bolted to heat sinks supplied by 
SRS. There are two versions of sinks that I have, a older thick ribbed type and 
the latest non ribbed U shaped type which I must admit looks terribly 
over-priced. The latest one is slightly better and keeps the measured temp at 
the baseplate surface to 42°C in a 25°C free air environment. The older one 
keeps it below 43°.  

> 
> Best regards,
> 
> Charles
> 
> 
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"The main function of a modern police force is filling in forms."
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Re: [time-nuts] Trimble Thunderbolt E frequency pulling.

[GandalfG8--- via time-nuts]

My bet is interaction of the load current and power supply with the EFC 
and/or OCXO

-
 
Well, I haven't started probing the hardware yet, I was hoping I might  get 
some confirmation first as to whether what I I'm seeing is an anomaly  for 
this one or just par for the course with the Tbolt E, but as far as Lady  
Heather indicates there's no step change in the EFC that matches the frequency 
 change, rather a gradual change after the event as the conditioning seeks 
to  correct the displacement.
 
Nigel
GM8PZR
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Re: [time-nuts] PRS10 Heatsink Temperature

[Neville Michie]

The LPRO data manual has a graph of MTBF and operating temperature.
The lower the temperature, the longer it lives.
I would choose a temperature so low that for maybe 5% of the time 
the fan control would not hold it down but let it run 1 or 2 degrees high.
It depends on how you value its life and how much harm escaping from 
thermostasis for an hour or two does to your task.
Cheers,
Neville Michie
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Re: [time-nuts] ACAM GP22 Chip

[paul swed]

Thomas
Welcome to the group. I am sure others will comment.
Many of us have a very wide range of experience and expertise so you should
feel comfortable with any question.
To the coax delay question. You are not pushing the limits.
But its important to understand the impacts of such long lines.
They need to be driven and terminated and the rise time will suffer from
the line capacitance. Essentially a fast rise time will become a slow
risetime on teh other end. There are lumped lc network delay lines. I have
experimented with them. They have the same effect. But you can cascade them
and use an inverter or buffer between each one.Each inverter also adds
delay. This helps the rise time issue. But the buffers add jitter and each
also adds delay thats temperature sensitive.
For cascaded delays of very short duration I have actually used 74LS244s
74HC244 line drivers cascaded and they work really well but only good for
each drivers delay.
Others will have better answers.
Regards
Paul
WB8TSL

On Tue, Nov 24, 2015 at 9:04 AM, Thomas Allgeier 
wrote:

> Hello,
>
>
>
> I have an ACAM GP22 TDC chip and evaluation board which I am looking at
> for “work” purposes – I work for a company active in the weighing and force
> measurement world.
>
>
>
> I should say from the start that I am new to time and frequency
> measurements and not even an electronics engineer – but then I have been
> exposed to high-precision electronics for the last 25 years hence have
> picked up some dangerous degree of half-knowledge.
>
>
>
> We want to use this chip to measure the period of a square wave, of around
> 13 kHz i.e. in the 70 µs range. As the application is potentially
> high-accuracy we need to know the period to within 1 ns or better.
>
>
>
> In order to evaluate the chip I was planning to replicate John A’s
> experiment with the coaxial delay line from the HP5370b – but as my
> interest is in “measuring range 2” of the GP22 I need a delay of 500 ns or
> more (actually 1 µs sounds a better start). This is the equivalent of a 200
> m length of cable. I fear trouble with this: Am I not getting unwanted
> inductivities if I use a coil of that size?
>
>
>
> So, to come to the point: Am I pushing the concept of a coax delay too far
> with 1 µs and are there other (simple/reliable) ways to achieve this kind
> of delay? I have tried it with a shorter piece of cable (around 2 ns which
> is measured in “range 1”), there I seem to get consistency virtually to
> within 100 ps. But I need to know if the device sticks to this level of
> performance when the periods are much longer, and thus measured in “range
> 2”.
>
>
>
> Thanks and best regards,
>
> Thomas.
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
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Re: [time-nuts] ACAM GP22 Chip

[Daniel Mendes]

my 2 cents:

1) hack a RC delay + comparator (isolate well the R and C for temp 
variations)

2) use a proper delay line (can be bought at digikey/mouser/etc)

Daniel

Em 24/11/2015 12:04, Thomas Allgeier escreveu:

Hello,



I have an ACAM GP22 TDC chip and evaluation board which I am looking at for 
“work” purposes – I work for a company active in the weighing and force 
measurement world.



I should say from the start that I am new to time and frequency measurements 
and not even an electronics engineer – but then I have been exposed to 
high-precision electronics for the last 25 years hence have picked up some 
dangerous degree of half-knowledge.



We want to use this chip to measure the period of a square wave, of around 13 
kHz i.e. in the 70 µs range. As the application is potentially high-accuracy we 
need to know the period to within 1 ns or better.



In order to evaluate the chip I was planning to replicate John A’s experiment 
with the coaxial delay line from the HP5370b – but as my interest is in 
“measuring range 2” of the GP22 I need a delay of 500 ns or more (actually 1 µs 
sounds a better start). This is the equivalent of a 200 m length of cable. I 
fear trouble with this: Am I not getting unwanted inductivities if I use a coil 
of that size?



So, to come to the point: Am I pushing the concept of a coax delay too far with 
1 µs and are there other (simple/reliable) ways to achieve this kind of delay? 
I have tried it with a shorter piece of cable (around 2 ns which is measured in 
“range 1”), there I seem to get consistency virtually to within 100 ps. But I 
need to know if the device sticks to this level of performance when the periods 
are much longer, and thus measured in “range 2”.



Thanks and best regards,

Thomas.
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