[time-nuts] PLL question

David t_list_1_only at braw.co.uk
Sat Aug 8 14:30:31 UTC 2009


As discussed the CW12 10MHz output has short term noise/jitter that need
removal for many purposes. The GPSDO I built with one (CW25, same thing for
this ...) locked a Temex double oven 10MHz Osc. The CW12 jitter drove me to
a relatively narrow loop, I ended up using divide by 512 so about 20kHz ref
frequency. Much of the design was dominated by what was in the junk box, if
the attachments work you should find the theoretical closed loop transfer
function with the attempt to suppress the jitter transfer. The other plot
shows phase noise on the 10MHz signals measured with an HP8560E and KE5FX's
excellent software. The pink trace is PN on the CW25's 10MHz, the blue trace
is the output seen on OCXO 10MHz which is dominated by the noise floor of
the analyser rather than the signal.

David
GM4HJQ


> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Ed Palmer wrote:
> > Navsync doesn't really call it a synthesizer, they call it a 
> > Numerically
> > Controlled Oscillator (NCO).  My tests suggest that they're 
> doing what 
> > others have done on the 1 PPS output - change the state of 
> the 10 MHz 
> > output (high to low or vice versa) on a transition of their 
> internal 120 
> > MHz clock.  This means that you occasionally get a pulse 
> that's ~8 ns 
> > shorter or (presumably) longer than normal.  My unit puts 
> out a short 
> > pulse about 200 times per second.  Navsync has an app note 
> where they 
> > say that for some applications a phase-locked cleanup 
> oscillator will be 
> > required.
> 
> This 200 Hz tone (and overtones) needs to be suppressed. The 
> size of it 
> is 8/100 = 80 mUI. A 10 Hz BW with PI regulation would suppress this 
> with about 3 + 6 + 20 = 29 dB. This brings it from 8 ns to 
> 280 ps. I assume a -6 dB/Oct slope as it forms a 2'nd degree loop.
> 
> Cheers,
> Magnus
> 
>
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