[time-nuts] Help with Novatel OEM6 external oscillator

[Bob kb8tq]

Hi

Simple answer is: 

1) (assuming you just want GPS) to try each of the settings and see what gives 
you the best signal to noise. I’d bet they have very little impact if you have a good 
oscillator. 

2) If you are building a GPSDO, the only useful setting is USER. There it’s a matter
of measuring the noise on the composite GPS + OCXO (or whatever) and tweaking 
to get the best result. Their presets might or might not be reasonable starting points
for the type of oscillator you have. 

I’ll let others write the 200 pages that it would take to go through the other stuff completely :)
Part of it gets into why we have more than just ADEV out there ….

Bob



> On Jan 21, 2019, at 4:51 PM, Skip Withrow <skip.withrow at gmail.com> wrote:
> 
> Hello Time-Nuts,
> Hopefully, this doesn't get too long.  What I desire to do is hook up
> an external oscillator to a Novatel OEM6 series dual-band GPS
> receiver.
> 
> Sounds easy. But, Novatel has four fixed (TCXO, OCXO, rubidium,
> cesium) plus a USER specifications to model the external clock
> performance.  Each model specifies three coefficients of the power law
> spectral density (h-2, h-1, h0).  If you google 'novatel
> externalclock' you can see the command. My question is, how do I
> translate an oscillator ADEV plot into the three coefficients?
> 
> I'm not an oscillator noise guru by any means (in fact, I'm probably a
> noise dummy).  It took me a while to figure out that the number range
> that Novatel is looking for is Allen Variance.  So taking the square
> root of their model numbers would be Allen Deviation (hope this is
> correct).  I believe the h-2 coefficient represents Random Walk FM
> (Would this be slope=-2 on ADEV plot?), the h-1 would be Flicker FM
> (slope =-1?), and the h0 coefficient would be White FM (slope =1 (or
> is it 0)?).
> 
> If I look at oscillator ADEV plots I often don't see anything as steep
> as -2 slope.  Is this because it is below the lowest tau of the plot
> and/or below the noise floor of the measurement system?
> 
> The real question is how to I translate a typical ADEV plot into the
> three coefficients that the Novatel receiver wants to see.  An
> illustrative example would be most helpful.  From looking at the
> numbers in the Novatel documentation it appears that the h-2 number
> represents short taus, h-1 intermediate taus, and h0 long taus.
> 
> For extra credit - Google ' Novatel clockmodel', and explain (simply)
> what the covariance matrix is.  Again, if an illustrative example
> could be shown it would be most helpful.
> 
> That's all for now before my brain explodes.  Thanks in advance for any help.
> 
> Regards,
> Skip Withrow
> 
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