[time-nuts] Comparing Reference Accuracy

Stanley Reynolds stanley_reynolds at yahoo.com
Thu May 22 17:53:08 EDT 2008

----- Original Message ----
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Thursday, May 22, 2008 4:26:43 PM
Subject: Re: [time-nuts] Comparing Reference Accuracy

Stanley Reynolds wrote:
> Block diagram of using a modified version of the DO controllers to compare several disciplined oscillators. The single logging PC could be several PCs as a way to have enough serial ports.  
> http://i307.photobucket.com/albums/nn306/stanleyreynolds/planB.jpg
This scheme is fatally flawed.

1) The various GPSDOs will only be statistically independent for short 
averaging times.

2) For such short averaging times the monitoring scheme has inadequate 
resolution unless the short term stability of the oscillators is very poor.

Without an independent standard of accurately known frequency such 
intercomparisons can only measure the relative short term instabilities 
of the various oscillators.
The errors of the various GPSDOs cannot be determined without access to 
a primary standard.

Common View and All in View GPS code and GPS carrier phase measurements 
may be used to compare the long term performance of the GPSDOs with a 
primary standard if someone with a primary standard is logging the same 
data using their primary standard and making it available.

For short term relative stability measurements a much higher resolution 
scheme is required.
The SNR is likely to be sufficiently high to permit picosecond or even 
subpicosecond measurement of the relative phases of the various 
oscillator outputs.

Yes such a measurement is not much better than the log produced by the DO itself, but the idea was to pick the bad apple from a bunch of DOs. Adding a primary standard in place or in addition to one of the DOs would be nice. A separate run with a gate of 2 sec, 4 sec would produce larger counts and a little better comparison provided the firmware in the PIC doesn't overflow or overflow wrong. 


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