[time-nuts] Z3801A performance plots

[John Miles]

That looks like a classical underdamped PLL response.  You usually see this
sort of hump in a phase-noise plot when your PLL is about to break into
oscillation. :)

Either someone at HP was careless with the loop-filter constants and came
close to running out of phase margin, the Z3801A is not configured properly,
or (who knows?) something fundamental about the disciplining process changed
when selective availability was turned off.

It looks like the disciplining loop bandwidth should be reduced, and/or the
phase margin should be increased.  It's at least an order of magnitude too
wide.

-- john, KE5FX


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Hal Murray
> Sent: Friday, January 25, 2008 3:34 PM
> To: Tom Van Baak; Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Z3801A performance plots
>
>
> > Here I put the two runs in color on the same plot:
> > http://www.leapsecond.com/museum/z3801a/z3801a-free-lock.gif
>
> That's an interesting graph.
>
> I find it strange that the free-running system is better than the locked
> system in the range from 50 to 5000 seconds.
>
> Does that mean the filter needs a longer time constant?  Or is there
> something fundamental about systems like this that makes graphs with that
> sort of pattern?
>
> How much did they understand about things like this graph when
> the Z3801A was
> shipped?
>
>
>
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
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