[time-nuts] GPSDO Design

[Bruce Griffiths]

OCXO aging may actually prevent a lock ever occurring with too small a 
tuning range, unless some form of acquisition aid is used.

One simple method of reducing the initial phase error is to initially 
reset the PPS divider using the PPS output of the GPS receiver.
This will reduce the initial phase error to 100ns or so with a 10MHz 
divider input.
This is one advantage of using a fully sychronous divider, as a 
synchronous reset is easy to arrange whereas with an asynchronous 
divider reliable phase alignment via a reset is more difficult to arrange.

Bruce

Bob Camp wrote:
> Hi
>
> If you fire up a conventional 1 pps loop with 10 ppb tune range, it's going
> to take it a while to track out any initial error. The most it can move by
> is +/-5 ns per second. You probably don't want to wait a year for it to
> catch up with a 158 ms initial error.
>
> Lots of fun little details to worry about managing....
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of WarrenS
> Sent: Thursday, January 14, 2010 4:52 PM
> To: Discussion of precise time and frequency
> Subject: [time-nuts] GPSDO Design
>
>
>
> 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
> **************
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