[time-nuts] GPSDO Design

[Bruce Griffiths]

Warren

WarrenS wrote:
> Bruce
>
>> If one attenuates the amplifier output by a factor of 1000 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.
>
> You are not taking into account that not all OXCO are as insensitive 
> to their EFC voltage.
> On a 5Hz/V Tbolt OXCO, 10mv is 0.05Hz = 5e-9 and 100mv is ... (well, 
> it is ALMOST never going to drift that much once it is run in, unless 
> you break it).
>
I left the simple exercise of substituting the relevant parameters for 
the OCXO one has in mind to calculate the corresponding tuning range and 
associated tradeoffs.

One potential disadvantage of having a high EFC sensitivity is increased 
phase noise and sensitivity to extraneous external signals such as 
ground loop noise etc.

It is very important to monitor/log the EFC voltage (as you do) 
particularly when one choses to use only a small portion of the 
available range.
Some simple GPSDO designs omit this feature.

AND
> I do agree one should be aware of worse case errors especially if 
> designing a care free commercial product with NO access to adjustments,
> BUT sometimes you seem to miss the practical simple solutions.
> You don't always need a 24 bit DAC to make a great performing unit.
> Sometimes a bit of original thinking can be just as useful.
> For the home nut, real values of their actual unit is more important 
> if they are just building a one of a kind.
>
>
> The EFC voltage ageing slope of My dual oven 10811 that is disciplined 
> by a tbolt is now a near linear function at 200 uv /day  (-2e-11/ day)
> (also has small temp dependence of a couple 1e-12 /C)  (Dac gain = 
> 0.909Hz/volt, More range than a single 10811 because there is no 
> manual adjustment)
>
> and on another "standard" Tbolt unit the EFC voltage variation is a 
> non linear, non accumulative, 100uv max peak to peak variation over 3 
> days.
> <<1e-11 /day ageing, It's EFC change is mostly due to temp variation 
> of about 1e-11 / C)    (Dac gain = -2.85Hz/volt)
>
> So having a max EFC range of 10 to 100mv would work OK on these two 
> not so typical OXCO.
>
> ws
> ******************
>
None of the solutions mentioned are particularly useful if one has a 
stable and quiet OCXO that has drifted a little more than the EFC or 
manual adjustment range.
If one has a stable and quiet OCXO or other reference that has no tuning 
adjustment one doesnt even have the simple option of locking it to the 
closest harmonic of 1Hz.
As long as the actual frequency is known a source that is a few Hz 
different from 5MHz (or 10MHz etc) can be just as useful as one that is 
locked to a nominal 5MHz or 10MHz.


Bruce
> ----- Original Message ----- From: "Bruce Griffiths"
>>
>> 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
>> ******************
>> 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
>>> **************
>
>