[time-nuts] GPSDO Design
bruce.griffiths at xtra.co.nz
Thu Jan 14 22:12:00 UTC 2010
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.
> 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.
> [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
> 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
> 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.
> John Foege
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