[time-nuts] GPSDO Design

[WarrenS]

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).

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
******************

----- 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
>> **************