[time-nuts] GPSDO Design

[Bob Camp]

Hi

If you are doing a GPSDO, your input to the 4096 is at 1 pps / 1 Hz. The PLL loop filter would be part of the circuitry around the op-amp you are talking about. That filter is going to have time constants around 300 to 10,000 seconds associated with it. What ever noise you have will be attenuated by the filter. More or less, you only are worried about the noise in a bandwidth << 1 Hz.

Another way of looking at it: 

You have an OCXO moves 120 ppb total for 10 volts (I'm lazy ...). That's:

 12 ppb / V (1.2x10-8 / V)
12 ppt / mv

A good op amp is going to run below 100 nV / Hz. That's .1 uV or 1x10-7 V. A very good op amp might be 100X better than that. A poor one could be 10X worse.

1x10-7V x 1.2x10-8 per volt = 1.2x10-15

Even with your more sensitive OCXO, you are at ~ 1.2x10-14.

Unless you are running a hydrogen maser, that's not going to be an issue.

Now, thermal offsets, that's another question entirely ....

Bob

On Jan 14, 2010, at 6:57 AM, John Foege wrote:

> 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
> 
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