[time-nuts] PLL question

[gonzo moto]

> Date: Fri, 07 Aug 2009 18:35:12 +0200
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> 
>~
> If possible, keep the comparator frequency fairly high and avoid 
> charge-pump detectors (according to my experience with cheaper-tronic 
> onces where dead-band created low rate wanderings while simple designs 
> excelled in stability).
> 
> Using a low phase-noise oscillator for cleanup is a wise idea, even if 
> no frequency multiplication occurs. Active loop PI-regulation of 
> sufficient bandwidth suppresses most of frequency trackings of the 
> oscillator, so long-term stability is less of an issue where as 
> phase-noise plots are.
> 
> Cheers,
> Magnus

Hi Magnus,
playing with my prototype (using a 74AC86 XOR) showed that the lock-up time varied from 30sec to almost 5min.
Because of this variation, I decided to use a 74HC7046 because of it's 'lock detect'. I must admit, the tri-state PC
did look good too until you pointed out the dead zone. I had assumed this would be insignificant, but of course it can't be.
I'll still try the 74HC7046, but use the XOR PC instead.



>------------------------------
>
>
>
>Date: Fri, 07 Aug 2009 10:44:42 -0600
>From: Ed Palmer <ed_palmer at sasktel.net>
>
>
>Which software version do you have, NMEA or Motorola?  If you have the 
>
>NMEA version another question pops up:  Should you do the division 
>
>externally, or program the unit to directly put out the lower frequency?
>
>
>
>Ed
>

Hi Ed,
I have the NMEA version.
If the decision was to run the PLL (PC) at less than 10MHz, (as per my first build),
I've run each signal through half a 74HC390. I hopped this would avoid any
device-to-device variation and any device introduced error would be consistent in both signals.

>------------------------------
> Date: Fri, 07 Aug 2009 10:52:17 -0600
> From: Ed Palmer <ed_palmer at sasktel.net>
> 
> 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.
> 
> Ed

Thanks again Ed, I was unaware of the occasional distorted pulse.


In case anyone has noticed, there is an ebay seller in Calgary listing some CW12s.
I do not believe these are the NMEA version and his price (for used units) is 30% over retail.

73
ian

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