Re: [time-nuts] GPSDO - probably a stupid question.

[Charles Steinmetz]

Bob wrote:


The point is still looking at the noise characteristics of the oscillator and 
the reference.
It is best done in the frequency domain as phase noise. We substitute ADEV, but 
that
is not an ideal proxy.


Phase noise and xDEV measure the same thing -- the stability of an 
oscillator at different time scales.  They just express the result 
differently.  Phase noise expresses it as PM in the frequency domain, 
and xDEV expresses it as "parts per" in the time domain.  (Yes, this is 
a somewhat simplified view of it, but it captures the essential point 
without undue complexity.)


Conventionally, we switch from using PN to using xDEV at a time scale 
(reciprocal frequency scale) of around 1 second, but there is no 
mathematical reason why they both cannot be extended indefinitely in 
either direction.  The convention arose largely because the equipment 
and techniques we use[ed] to quantify them have traditionally been 
different at time scales (reciprocal frequency scales) greater than and 
less than about one second.  Now that we are in the era of "digitize 
everything, and let Laplace sort it out," we needn't view it as the 
rigid convention it once was.



Either way you want the loop to cross over from one to the other
somewhere in the vicinity of the “equal noise” point if it exists. If there is 
no equal noise
point, that makes you wonder a bit about why you are locking one to the other


Not really, if one has lower noise at all time (frequency) scales, just 
lock to that one at all scales.  (It may call into question why you're 
fiddling with two oscillators, rather than just using the output of the 
quiet one, if they are both at the same frequency -- but there are a 
number of reasons one might want to do that.)


Best regards,

Charles


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Re: [time-nuts] GPSDO - probably a stupid question.

[Magnus Danielson]

I use the frequency relationship ratio as an indication of how difficult 
the design is. Divide the oscillator frequency with the comparator 
frequency, and the number gives you a ratio, how many output cycles it 
goes between each comparison. Things like smoothing becomes harder when 
this number becomes large. Lock-in etc. also becomes harder.


1-100 is relatively trivial.
1000 is a little challenging and start to need care.

By increasing the comparator frequency, I made designs more trivial and 
that has helped a lot to get the job done without too much hassle.


I bring this up since there is more to the design than just the PLL 
bandwidth and damping factor.


Cheers,
Magnus

On 08/18/2016 03:17 AM, Didier Juges wrote:

Good point, and an example of how good digital filtering (helped with 
upsampling) can make the design of the analog filter much easier :)

Reference the digital audio battles of the past century when 1 bit D/As running 
very fast started replacing the expensive 16 bit audio DACs running at 44kHz.

Didier

On August 17, 2016 5:25:39 PM CDT, Magnus Danielson 
 wrote:

Hi,

I agree.

There is however a subtle detail, how they leak out over time.

At one time we had to lock an 155,52 MHz oscillator up to 8 kHz, this
for a 2,48832 Gb/s link, which needs to pass the SDH STM-16 jitter and
wander specifications. The first attempt at that PLL was using a 4046,
and the charge-pump was being used. The charge-pump has dead-time, and
well, they thought it was good to only push the EFC here and there.
What
this meant was that they created a triangle-waved frequency modulation
of low rate, which then created phase modulations as it went through
the
integration of the oscillator. The scale-up factor made this quite
noticeable at the actual bit-rate. It made the point that you need to
update often to keep deviations limited, and when doing it at a higher
frequency, they are easier to filter out.

In essence, you need to think what each comparison or update creates as

a step response and how it is averaged out over time.

In this regard a PWM is a really bad signal, as it can push the
strongest amplitude at the lowest frequency, which becomes hardest to
filter. For one design I needed to increase the resolution, so I made
an
interpolation but with inversed spectral density to that of PWM, to
push
the highest amplitude to the highest frequency so that filtering
becomes
easier. Turned out to be quite easy and work well.

High update rates can be very useful even if the bandwidth of the loop
is low. The bandwidth only limits how low the updaterate can be, but
the
phase-noise purity makes update rates and smoothing mechanisms
interesting.

Cheers,
Magnus

On 08/17/2016 11:53 PM, Bob kb8tq wrote:

Hi

You can update the EFC a billion times a second.  Update rate and

bandwidth are not the same thing. If you want good ADEV, the loop
better not have a bandwidth greater than 0.01 Hz. GPS ADEV is pretty
awful at 1 and 10 seconds. It is starts to be good past a few thousand
seconds. Yes, older modules are a bit worse than newer ones. Also
sawtooth correction can make things a bit better.


Bob

Sent from my iPad


On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts

 wrote:


Updating the EFC more quickly reduces the ADEV, though. I find that

the fiddly part of tuning a GPSDO design is balancing the ADEV against
phase control. If you want keep an iron fist on the phase, you can only
do so by constantly swatting around the frequency.


I won't say that getting more frequent phase feedback is a bad

thing, but if you're trying to get the PLL time constant to be longer
rather than shorter that it won't help a lot.


Sent from my iPhone


On Aug 17, 2016, at 9:57 AM, Peter Reilley

 wrote:


You can get crystal oscillators that have a frequency control

signal and are more

stable than the run of the mill oscillators.   Changing the GPS

oscillator would

require modifying a very tightly populated circuit board.   Perhaps

not possible.


What about some of the SDR (software defined radio) projects that

aim to

implement GPS functionality?   If you used the GPS chipping rate

(1.023 MHz)

to dicipline the 10 MHz oscillator then you are less sensitive to

crystal instabilities.

You are updating the crystal one million times a second rather than

once per second.

This is assuming that the chipping rate of the transmitter is just

as good as the

1 PPS signal.   This info from here;
https://www.e-education.psu.edu/geog862/node/1753
and here;
https://en.wikipedia.org/wiki/GPS_signals

Even using the 50 bits/sec data rate of the GPS signal would allow

updating the

GPSDO faster than the 1 PPS signal.

Pete.
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Re: [time-nuts] GPSDO - probably a stupid question.

[Bob Camp]

Hi

A mixer style phase detector running a GHz range oscillator is one example of a 
system that technically updated the EFC several billion times a second. There 
does not have to be a DAC involved. 

The point is still looking at the noise characteristics of the oscillator and 
the reference. 
It is best done in the frequency domain as phase noise. We substitute ADEV, but 
that
is not an ideal proxy. Either way you want the loop to cross over from one to 
the other 
somewhere in the vicinity of the “equal noise” point if it exists. If there is 
no equal noise
point, that makes you wonder a bit about why you are locking one to the other 
:) 

Taking ADEV, since that’s what we have the data on:

GPS starts out somewhere in the 20 ns to 0.2 ns range at 1 second depending on 
what 
you are looking at, which module you are using, and who you trust for your 
data. That 
compares to a good OCXO that should be in the 0.001 ns range at 1 second. There 
is a 
long way to go (larger time span) before the OCXO and GPS are anywhere near 
the same noise level. You will need to get to 200 seconds with the best GPS 
numbers above.
You will need to get to 100X that with the worst ones. Yes, there is a bit of 
hand waving
in all of that. 

Bob

> On Aug 17, 2016, at 6:14 PM, Nick Sayer via time-nuts  
> wrote:
> 
> Well, on a practical level, if you update the EFC that frequently then the 
> DAC change glitches will dominate the actual output even if you’re not 
> actually moving the needle much. 
> 
> 
>> On Aug 17, 2016, at 2:53 PM, Bob kb8tq  wrote:
>> 
>> Hi
>> 
>> You can update the EFC a billion times a second.  Update rate and bandwidth 
>> are not the same thing. If you want good ADEV, the loop better not have a 
>> bandwidth greater than 0.01 Hz. GPS ADEV is pretty awful at 1 and 10 
>> seconds. It is starts to be good past a few thousand seconds. Yes, older 
>> modules are a bit worse than newer ones. Also sawtooth correction can make 
>> things a bit better.
>> 
>> Bob
>> 
>> Sent from my iPad
>> 
>>> On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts  
>>> wrote:
>>> 
>>> Updating the EFC more quickly reduces the ADEV, though. I find that the 
>>> fiddly part of tuning a GPSDO design is balancing the ADEV against phase 
>>> control. If you want keep an iron fist on the phase, you can only do so by 
>>> constantly swatting around the frequency.
>>> 
>>> I won't say that getting more frequent phase feedback is a bad thing, but 
>>> if you're trying to get the PLL time constant to be longer rather than 
>>> shorter that it won't help a lot. 
>>> 
>>> Sent from my iPhone
>>> 
 On Aug 17, 2016, at 9:57 AM, Peter Reilley  
 wrote:
 
 You can get crystal oscillators that have a frequency control signal and 
 are more
 stable than the run of the mill oscillators.   Changing the GPS oscillator 
 would
 require modifying a very tightly populated circuit board.   Perhaps not 
 possible.
 
 What about some of the SDR (software defined radio) projects that aim to
 implement GPS functionality?   If you used the GPS chipping rate (1.023 
 MHz)
 to dicipline the 10 MHz oscillator then you are less sensitive to crystal 
 instabilities.
 You are updating the crystal one million times a second rather than once 
 per second.
 This is assuming that the chipping rate of the transmitter is just as good 
 as the
 1 PPS signal.   This info from here;
 https://www.e-education.psu.edu/geog862/node/1753
 and here;
 https://en.wikipedia.org/wiki/GPS_signals
 
 Even using the 50 bits/sec data rate of the GPS signal would allow 
 updating the
 GPSDO faster than the 1 PPS signal.
 
 Pete.
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Re: [time-nuts] GPSDO - probably a stupid question.

[KA2WEU--- via time-nuts]

There are no (rarely maybe ) stupid questions, mostly silly answers 
 
 
In a message dated 8/17/2016 5:03:07 P.M. Eastern Daylight Time,  
elfchief-timen...@lupine.org writes:

Wouldn't  you also not be able to actually sync to the individual chips, 
since you  can't really see the start of any given chip so much as you 
just see the  correlation over larger sections of the stream? Plus you'd 
have to track  only one SV at a time (right? Since I doubt the edges of 
every chip are  perfectly aligned across all SVs even under the best 
conditions), so  things like brief multipath excursions or even 
atmospheric/ionospheric  fluctuations would push you off by a bit as well...

(which is why, of  course, you have the long control loop that GPSDOs  use)

-j


On 2016-08-17 11:41 , Didier Juges wrote:
> In  fact, you do not want to "update the crystal one million  
times/second".
> The whole point of a GPSDO is to combine the excellent  short term 
stability
> of the crystal with the excellent long term  stability of the GPS signal. 
If
> you update the crystal in real time  from the GPS data, you do not need 
the
> crystal...
> The control  loop of GPSDOs usually have an effective bandwidth measured 
in
> minutes  or even hours in the case of rubidium oscillators.
>
>
> On  Wed, Aug 17, 2016 at 11:57 AM, Peter Reilley  

> wrote:
>
>> You can  get crystal oscillators that have a frequency control signal and
>>  are more
>> stable than the run of the mill oscillators.Changing the GPS 
oscillator
>> would
>> require modifying a  very tightly populated circuit board.   Perhaps not
>>  possible.
>>
>> What about some of the SDR (software defined  radio) projects that aim to
>> implement GPS  functionality?   If you used the GPS chipping rate  (1.023
>> MHz)
>> to dicipline the 10 MHz oscillator then  you are less sensitive to 
crystal
>> instabilities.
>> You  are updating the crystal one million times a second rather than  
once
>> per second.
>> This is assuming that the chipping  rate of the transmitter is just as 
good
>> as the
>> 1 PPS  signal.   This info from here;
>>  https://www.e-education.psu.edu/geog862/node/1753
>> and  here;
>>  https://en.wikipedia.org/wiki/GPS_signals
>>
>> Even using  the 50 bits/sec data rate of the GPS signal would allow
>> updating  the
>> GPSDO faster than the 1 PPS signal.
>>
>>  Pete.
>>
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Re: [time-nuts] GPSDO - probably a stupid question.

[Magnus Danielson]

Hi,

I agree.

There is however a subtle detail, how they leak out over time.

At one time we had to lock an 155,52 MHz oscillator up to 8 kHz, this 
for a 2,48832 Gb/s link, which needs to pass the SDH STM-16 jitter and 
wander specifications. The first attempt at that PLL was using a 4046, 
and the charge-pump was being used. The charge-pump has dead-time, and 
well, they thought it was good to only push the EFC here and there. What 
this meant was that they created a triangle-waved frequency modulation 
of low rate, which then created phase modulations as it went through the 
integration of the oscillator. The scale-up factor made this quite 
noticeable at the actual bit-rate. It made the point that you need to 
update often to keep deviations limited, and when doing it at a higher 
frequency, they are easier to filter out.


In essence, you need to think what each comparison or update creates as 
a step response and how it is averaged out over time.


In this regard a PWM is a really bad signal, as it can push the 
strongest amplitude at the lowest frequency, which becomes hardest to 
filter. For one design I needed to increase the resolution, so I made an 
interpolation but with inversed spectral density to that of PWM, to push 
the highest amplitude to the highest frequency so that filtering becomes 
easier. Turned out to be quite easy and work well.


High update rates can be very useful even if the bandwidth of the loop 
is low. The bandwidth only limits how low the updaterate can be, but the 
phase-noise purity makes update rates and smoothing mechanisms interesting.


Cheers,
Magnus

On 08/17/2016 11:53 PM, Bob kb8tq wrote:

Hi

You can update the EFC a billion times a second.  Update rate and bandwidth are 
not the same thing. If you want good ADEV, the loop better not have a bandwidth 
greater than 0.01 Hz. GPS ADEV is pretty awful at 1 and 10 seconds. It is 
starts to be good past a few thousand seconds. Yes, older modules are a bit 
worse than newer ones. Also sawtooth correction can make things a bit better.

Bob

Sent from my iPad


On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts  
wrote:

Updating the EFC more quickly reduces the ADEV, though. I find that the fiddly 
part of tuning a GPSDO design is balancing the ADEV against phase control. If 
you want keep an iron fist on the phase, you can only do so by constantly 
swatting around the frequency.

I won't say that getting more frequent phase feedback is a bad thing, but if 
you're trying to get the PLL time constant to be longer rather than shorter 
that it won't help a lot.

Sent from my iPhone


On Aug 17, 2016, at 9:57 AM, Peter Reilley  wrote:

You can get crystal oscillators that have a frequency control signal and are 
more
stable than the run of the mill oscillators.   Changing the GPS oscillator would
require modifying a very tightly populated circuit board.   Perhaps not 
possible.

What about some of the SDR (software defined radio) projects that aim to
implement GPS functionality?   If you used the GPS chipping rate (1.023 MHz)
to dicipline the 10 MHz oscillator then you are less sensitive to crystal 
instabilities.
You are updating the crystal one million times a second rather than once per 
second.
This is assuming that the chipping rate of the transmitter is just as good as 
the
1 PPS signal.   This info from here;
https://www.e-education.psu.edu/geog862/node/1753
and here;
https://en.wikipedia.org/wiki/GPS_signals

Even using the 50 bits/sec data rate of the GPS signal would allow updating the
GPSDO faster than the 1 PPS signal.

Pete.
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Re: [time-nuts] GPSDO - probably a stupid question.

[Clint Jay]

Interested to know if anyone has done this with a ublox receiver,  I
spotted the option in some of the technical documents and went as far as
finding a stockist for the external DAC I think it'd need.

On 17 Aug 2016 22:02, "Mark Sims"  wrote:

> The Ublox modules (at least some of them) can support an external
> oscillator and have  messages for controlling oscillator parameters and
> disciplining.
>
> 
>
> > This is actually done.  But you need to design the GPS receiver from the
> ground up to use a very high quality oscillator.   The Thunderbolt is an
> example of this.  It has a 10MHz OCXO used as the internal clock.
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Re: [time-nuts] GPSDO - probably a stupid question.

[Nick Sayer via time-nuts]

Well, on a practical level, if you update the EFC that frequently then the DAC 
change glitches will dominate the actual output even if you’re not actually 
moving the needle much. 


> On Aug 17, 2016, at 2:53 PM, Bob kb8tq  wrote:
> 
> Hi
> 
> You can update the EFC a billion times a second.  Update rate and bandwidth 
> are not the same thing. If you want good ADEV, the loop better not have a 
> bandwidth greater than 0.01 Hz. GPS ADEV is pretty awful at 1 and 10 seconds. 
> It is starts to be good past a few thousand seconds. Yes, older modules are a 
> bit worse than newer ones. Also sawtooth correction can make things a bit 
> better.
> 
> Bob
> 
> Sent from my iPad
> 
>> On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts  
>> wrote:
>> 
>> Updating the EFC more quickly reduces the ADEV, though. I find that the 
>> fiddly part of tuning a GPSDO design is balancing the ADEV against phase 
>> control. If you want keep an iron fist on the phase, you can only do so by 
>> constantly swatting around the frequency.
>> 
>> I won't say that getting more frequent phase feedback is a bad thing, but if 
>> you're trying to get the PLL time constant to be longer rather than shorter 
>> that it won't help a lot. 
>> 
>> Sent from my iPhone
>> 
>>> On Aug 17, 2016, at 9:57 AM, Peter Reilley  wrote:
>>> 
>>> You can get crystal oscillators that have a frequency control signal and 
>>> are more
>>> stable than the run of the mill oscillators.   Changing the GPS oscillator 
>>> would
>>> require modifying a very tightly populated circuit board.   Perhaps not 
>>> possible.
>>> 
>>> What about some of the SDR (software defined radio) projects that aim to
>>> implement GPS functionality?   If you used the GPS chipping rate (1.023 MHz)
>>> to dicipline the 10 MHz oscillator then you are less sensitive to crystal 
>>> instabilities.
>>> You are updating the crystal one million times a second rather than once 
>>> per second.
>>> This is assuming that the chipping rate of the transmitter is just as good 
>>> as the
>>> 1 PPS signal.   This info from here;
>>> https://www.e-education.psu.edu/geog862/node/1753
>>> and here;
>>> https://en.wikipedia.org/wiki/GPS_signals
>>> 
>>> Even using the 50 bits/sec data rate of the GPS signal would allow updating 
>>> the
>>> GPSDO faster than the 1 PPS signal.
>>> 
>>> Pete.
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Re: [time-nuts] GPSDO - probably a stupid question.

[Bob kb8tq]

Hi

You can update the EFC a billion times a second.  Update rate and bandwidth are 
not the same thing. If you want good ADEV, the loop better not have a bandwidth 
greater than 0.01 Hz. GPS ADEV is pretty awful at 1 and 10 seconds. It is 
starts to be good past a few thousand seconds. Yes, older modules are a bit 
worse than newer ones. Also sawtooth correction can make things a bit better.

Bob

Sent from my iPad

> On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts  
> wrote:
> 
> Updating the EFC more quickly reduces the ADEV, though. I find that the 
> fiddly part of tuning a GPSDO design is balancing the ADEV against phase 
> control. If you want keep an iron fist on the phase, you can only do so by 
> constantly swatting around the frequency.
> 
> I won't say that getting more frequent phase feedback is a bad thing, but if 
> you're trying to get the PLL time constant to be longer rather than shorter 
> that it won't help a lot. 
> 
> Sent from my iPhone
> 
>> On Aug 17, 2016, at 9:57 AM, Peter Reilley  wrote:
>> 
>> You can get crystal oscillators that have a frequency control signal and are 
>> more
>> stable than the run of the mill oscillators.   Changing the GPS oscillator 
>> would
>> require modifying a very tightly populated circuit board.   Perhaps not 
>> possible.
>> 
>> What about some of the SDR (software defined radio) projects that aim to
>> implement GPS functionality?   If you used the GPS chipping rate (1.023 MHz)
>> to dicipline the 10 MHz oscillator then you are less sensitive to crystal 
>> instabilities.
>> You are updating the crystal one million times a second rather than once per 
>> second.
>> This is assuming that the chipping rate of the transmitter is just as good 
>> as the
>> 1 PPS signal.   This info from here;
>> https://www.e-education.psu.edu/geog862/node/1753
>> and here;
>> https://en.wikipedia.org/wiki/GPS_signals
>> 
>> Even using the 50 bits/sec data rate of the GPS signal would allow updating 
>> the
>> GPSDO faster than the 1 PPS signal.
>> 
>> Pete.
>> ___
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[time-nuts] GPSDO - probably a stupid question.

[Mark Sims]

The Ublox modules (at least some of them) can support an external oscillator 
and have  messages for controlling oscillator parameters and disciplining.



> This is actually done.  But you need to design the GPS receiver from the
ground up to use a very high quality oscillator.   The Thunderbolt is an
example of this.  It has a 10MHz OCXO used as the internal clock.
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Re: [time-nuts] GPSDO - probably a stupid question.

[Didier Juges]

In fact, you do not want to "update the crystal one million times/second".
The whole point of a GPSDO is to combine the excellent short term stability
of the crystal with the excellent long term stability of the GPS signal. If
you update the crystal in real time from the GPS data, you do not need the
crystal...
The control loop of GPSDOs usually have an effective bandwidth measured in
minutes or even hours in the case of rubidium oscillators.


On Wed, Aug 17, 2016 at 11:57 AM, Peter Reilley 
wrote:

> You can get crystal oscillators that have a frequency control signal and
> are more
> stable than the run of the mill oscillators.   Changing the GPS oscillator
> would
> require modifying a very tightly populated circuit board.   Perhaps not
> possible.
>
> What about some of the SDR (software defined radio) projects that aim to
> implement GPS functionality?   If you used the GPS chipping rate (1.023
> MHz)
> to dicipline the 10 MHz oscillator then you are less sensitive to crystal
> instabilities.
> You are updating the crystal one million times a second rather than once
> per second.
> This is assuming that the chipping rate of the transmitter is just as good
> as the
> 1 PPS signal.   This info from here;
> https://www.e-education.psu.edu/geog862/node/1753
> and here;
> https://en.wikipedia.org/wiki/GPS_signals
>
> Even using the 50 bits/sec data rate of the GPS signal would allow
> updating the
> GPSDO faster than the 1 PPS signal.
>
> Pete.
>
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Re: [time-nuts] GPSDO - probably a stupid question.

[Peter Reilley]

You can get crystal oscillators that have a frequency control signal and 
are more
stable than the run of the mill oscillators.   Changing the GPS 
oscillator would
require modifying a very tightly populated circuit board.   Perhaps not 
possible.


What about some of the SDR (software defined radio) projects that aim to
implement GPS functionality?   If you used the GPS chipping rate (1.023 MHz)
to dicipline the 10 MHz oscillator then you are less sensitive to 
crystal instabilities.
You are updating the crystal one million times a second rather than once 
per second.
This is assuming that the chipping rate of the transmitter is just as 
good as the

1 PPS signal.   This info from here;
https://www.e-education.psu.edu/geog862/node/1753
and here;
https://en.wikipedia.org/wiki/GPS_signals

Even using the 50 bits/sec data rate of the GPS signal would allow 
updating the

GPSDO faster than the 1 PPS signal.

Pete.
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Re: [time-nuts] GPSDO - probably a stupid question.

[Chris Albertson]

On Tue, Aug 16, 2016 at 6:02 PM, Peter Reilley 
wrote:

> As a neophyte, I was wondering: rather that trying to discipline an
> external
> oscillator to create a GPSDO and produce a precise 10 MHz why not
> discipline the oscillator
> of the GPS receiver itself?
>
>
This is actually done.  But you need to design the GPS receiver from the
ground up to use a very high quality oscillator.   The Thunderbolt is an
example of this.  It has a 10MHz OCXO used as the internal clock.

I've seen the same thing done with NTP servers.  Rather then adjust the
system time using software, why not discipline the mother board's XO?   Yes
but it is a large amount of work to modify an existing product.



-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] GPSDO - probably a stupid question.

[Björn Gabrielsson]

Some (all?) Novatel receivers have an option to sync their internal TCXO
or let it freewheel.

--

 Björn

> The stability of the typical GPS receiver oscillator is usually inadequate
> to be useful as a GPSDO. An OCXO (as in the Trimble Thunderbolt for
> example) or equivalent is usually required. One can't usually just add a
> varicap to adjust the frequency of a packaged oscillator. If an external
> crystal is used the varicap should be placed in series with the crystal.
> However to do this reliably one needs to know the crystal parameters and
> those of the oscillator circuit. If you need to adjust the frequency both
> up and down (due to crystal parmeter tolerances) then an inductor in
> series with the crystal and varicap will also be required.
> Bruce
>
>
> On Wednesday, 17 August 2016 7:05 PM, Peter Reilley
>  wrote:
>
>
>  As a neophyte, I was wondering: rather that trying to discipline an
> external
> oscillator to create a GPSDO and produce a precise 10 MHz why not
> discipline the oscillator
> of the GPS receiver itself?  This could be done with a varactor diode
> across crystal of the
> receiver's oscillator.  Of course there are the same problems with
> trying to servo this
> oscillator as there are trying to servo an external oscillator but there
> are fewer parts.
>
> Being a beginner I assume that I am missing something, but what?
>
> Thanks,
> Pete.
>
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Re: [time-nuts] GPSDO - probably a stupid question.

[Bruce Griffiths]

The stability of the typical GPS receiver oscillator is usually inadequate to 
be useful as a GPSDO. An OCXO (as in the Trimble Thunderbolt for example) or 
equivalent is usually required. One can't usually just add a varicap to adjust 
the frequency of a packaged oscillator. If an external crystal is used the 
varicap should be placed in series with the crystal. However to do this 
reliably one needs to know the crystal parameters and those of the oscillator 
circuit. If you need to adjust the frequency both up and down (due to crystal 
parmeter tolerances) then an inductor in series with the crystal and varicap 
will also be required.
Bruce
 

On Wednesday, 17 August 2016 7:05 PM, Peter Reilley 
 wrote:
 

 As a neophyte, I was wondering: rather that trying to discipline an external
oscillator to create a GPSDO and produce a precise 10 MHz why not 
discipline the oscillator
of the GPS receiver itself?  This could be done with a varactor diode 
across crystal of the
receiver's oscillator.  Of course there are the same problems with 
trying to servo this
oscillator as there are trying to servo an external oscillator but there 
are fewer parts.

Being a beginner I assume that I am missing something, but what?

Thanks,
Pete.

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Re: [time-nuts] GPSDO - probably a stupid question.

[Pete Stephenson]

On Aug 17, 2016 09:04, "Peter Reilley"  wrote:
>
> As a neophyte, I was wondering: rather that trying to discipline an
external
> oscillator to create a GPSDO and produce a precise 10 MHz why not
discipline the oscillator
> of the GPS receiver itself?   This could be done with a varactor diode
across crystal of the
> receiver's oscillator.   Of course there are the same problems with
trying to servo this
> oscillator as there are trying to servo an external oscillator but there
are fewer parts.

It's a very good idea, and is precisely what the Trimble Thunderbolt does
with its 10MHz OCXO that it uses both as a system clock and a 10MHz output.

Cheers!
-Pete
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[time-nuts] GPSDO - probably a stupid question.

[Peter Reilley]

As a neophyte, I was wondering: rather that trying to discipline an external
oscillator to create a GPSDO and produce a precise 10 MHz why not 
discipline the oscillator
of the GPS receiver itself?   This could be done with a varactor diode 
across crystal of the
receiver's oscillator.   Of course there are the same problems with 
trying to servo this
oscillator as there are trying to servo an external oscillator but there 
are fewer parts.


Being a beginner I assume that I am missing something, but what?

Thanks,
Pete.

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