[time-nuts] GPSDO design

[Dick Moore]

John, I wouldn't bother with amplitude scaling, just unity gain with a DC offset to center the control range to the 10811A to zero volts or thereabout, and choose inverting or non-inverting to suit the characteristics of the control voltage slope vis-avis the 10811A's slope, which is negative if I remember right. About a 1Hz upper band limit works pretty well, so amplifier noise really isn't a problem as you'll LP the output pretty severely anyway.... my 2 cents.

Dick Moore


On Jan 14, 2010, at 4:00 AM, time-nuts-request at febo.com wrote:

> Message: 2
> Date: Thu, 14 Jan 2010 06:57:39 -0500
> From: John Foege <john.foege at gmail.com>
> Subject: [time-nuts] GPSDO Design
> To: time-nuts at febo.com
> Message-ID:
> 	<888d55281001140357s6021cfa9i4227741eedb2f633 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> 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