[time-nuts] GPSDO Design

[WarrenS]

Yep

Everything is a trade off. If you want to make a simple, cheap, good GPSDO, 
including some manual tuning has several benefits.
Shorter time constants and smaller caps for the same response TC is the one 
I was pointing out.
And yes, there are all kinds of ways to do it wrong.

IF YOU have an Osc that drifts more than 5 or 10 mv in a few hrs, You should 
do use a divide by a thousand,
and you likely have other problems that should be taken care of first.
   At a divide by 100 with 10 volt out, that gives a 100 mv EFC tuning 
range.
Show me a OSC that drifts that much short term and it is likley from an 
Oscillator that should not be use for GPSDO.
The typical "good" 10811 Oscillators I've seen don't drift 100 mv or even 10 
mv once they are run in a bit.
Of course depends on their sensitivity, If an OCXO has low EFC sensitivity 
that is because it has ALREADY been divided down by a lot.
(and therefore would already provide some of the reduced loop again I was 
talking about)

But If you did want to ALSO take care of low sensitivity or high drift OSC 
then you can add an automatic offset adjustment to the circuit. (some call 
it an integrator).

moving on to something else I read:

>In the short term the GPS receiver noise will dominate.
>In the long term the 4046 phase detector noise and drift together with the 
>OCXO noise and drift will dominate.

That definition and mine seem to be reversed somewhere.

For SHORT term GPSDO noise,  the GPS noise does not matter,
Just the OCXO and what ever noise you may be silly enough to add to it's EFC 
input.
But most know that, so I don't understand what the point was.

For Long term >> 1000 Sec, with standard GPSDO control loops <<1000 sec
The OCXO, opamp, 4046 have very little effect on long term average freq 
error.
It is really hard to make a phase detector whose noise would dominate at 
LONG Times,
if average freq is what is being talked about.
Even a 100 ns per day phase drift is only about 1e-12

ws

****************
----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] GPSDO Design

> If one attenuates the amplifier output by a factor of 1000 (effective EFC 
> range is then ~1E-10) or so then a 10811A with an aging rate of 5E-10/day 
> will require manual retuning every few hours.
> Typical aging may be better than this but this would still require manual 
> tuning every few days.
>
> Bruce
>
*************
***************
Bruce wrote:
There are 4 principal sources of noise

1) The GPS receiver

2) The 4046 Phase detector

3) The opamp

4) The OCXO

In the short term the GPS receiver noise will dominate.
In the long term the 4046 phase detector noise and drift together with
the OCXO noise and drift will dominate.

Unless you make an extremely poor choice of opamp the 4046 phase
detector noise and drift will be much larger than that of the opamp.

Bruce

****************
> WarrenS wrote:
>>
>> John ask
>>
>>> Translating nV/sqrt(Hz) to something
>>> practical is basically the assistance I'm looking for here.
>>> I would appreciate anyone being able to teach me a bit more about this.
>>>
>> If that is ALL you want to know, That's easy and quick.
>> For this application sounds like you already know ALL you need to know 
>> about that,  nothing.
>> Putting a 1 sec or so RC filter at the EFC input, takes care of all that 
>> AND if you want it even better,
>> and to get the long Control loop time constants needed, JUST reduce the 
>> (loop) gain, don't need no BIG caps.
>> That is attenuate the output of the control amp by typically a hundred to 
>> a thousand instead of multiplying by 1.6 and add a fixed, adjustable, 
>> stable, offset source. (electrical or mechanical)
>> The Buffer amp is not going to be your problem.
>>
>> ws
>>
>> ************
>>
>> [time-nuts] GPSDO Design
>> John Foege john.foege at gmail.com
>>
>> Hi All,
>>
>> Quick question for the more experienced members here with GPSDO
>> design/operation. Let's assume I'm using a 4096 phase comparator chip
>> followed by some kind of long time constant lowpass loop filter,
>> whether it be analog or digital, is not of concern for the following
>> question.
>>
>> Obviously using a 74HCT4096 would mean that my EFC voltage range would
>> be approx. 0-5V. If I wanted to use an OCXO with say a 0-8V EFC
>> voltage range, then I would be inclined to simply use an op-amp
>> amplifier with a gain of 1.6 to scale the EFC voltage accordingly.
>>
>> But not just any op-amp would do I take it? High-speed would of course
>> be of no concern. Also low-offset would be of little concern, as the
>> PLL would work to correct this, and it therefore seems to be
>> negligible. However, the part that's got me thinking is noise.
>> Obviously any noise at the ouput of the amp would adversely affect the
>> frequency stability of the OCXO.
>>
>> I thought the best way to control this would be to use an extremely
>> low noise op-amp employing a rather large compensation cap to give me
>> a rather small bandwidth, perhaps only a few hundred hertz.
>>
>> Anyone have experience with this? Assuming I have an OXCO with a max.
>> pulling range of 1ppm or 1e-6 over a 10V range, then I effectively can
>> pull 1e-7 per volt. This translates to 1e-10 per millivolt and 1e-13
>> per microvolt. Assuming that is a logical conclusion, then for a good
>> OCXO, in which I can at best hope for 5e-12 stability for tau=1s (e.g.
>> HP10811A), I would strive to to keep the noise at such a level that it
>> is an order of magnitude better than the best short term stability
>> figure. Accordingly, then I should shoot to keep any noise under 1
>> microvolt?
>>
>> I don't have much experience with noise calculations. I know it is
>> specified in nV/sqrt(Hz) generally. Translating this to something
>> practical is basically the assistance I'm looking for here.
>>
>> I would appreciate anyone being able to teach me a bit more about this.
>>
>> Thank you in all in advance.
>>
>> Sincerely,
>>
>> John Foege
>> **************
>
>