[time-nuts] PLL question

[Ed Palmer]

I should also have mentioned that the 10 MHz output also has all the 
typical GPS jitter.  So suppressing it will also suppress the 200 Hz 
tone.  I'm assuming that this correction rate is unique to each unit.  
Depending on the exact frequency of your unit's oscillator, it will 
correct the signal X times per second.  For my unit, X happens to be 200.

Ed

Magnus Danielson wrote:
> 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
>