[time-nuts] Z3805 initial behaviour after power up

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jun 17 18:09:49 EDT 2009


Hi Francesco,

Francesco Ledda skrev:
> A third order PPL has better sideband suppression.  The tracking capability
> depends upon the phase range of the phase detector.  If 1:1 phase comparison
> and a XOR phase detector is used, the range is 180 degrees. Dividers on the
> feedback portion of the PLL increase the phase range of the phase detector.
> For example, if the expected jitter is 10UI, a divider larger than 10 must
> be used, to maintain lock under all conditions. A monotonically decreasing
> phase delta, on the phase detector, still means that the PLL is locked.

True, but regardless of how large you have made your phase detectors 
range, if you have a loop unable to track to the dynamics you put into 
it, you eventually reach the wrapping or limiting point. By back out 
into quicker response, the PLL is able to quicker reduce the phase error 
and track it in, as it quicker adjust its state to match the difference 
between the source and the oscillator.

> The phase/frequency detector, avoids the need for a frequency aquisition
> aid.  Once a phase reversal is detected by the flip flops in the phase/freq
> detector, it goes back to phase detector mode.

Sure, but it does not totally remove the benefits of shifting PLL 
bandwidth according to need. I'm all for phase/frequency detectors... 
and have designed several.

> My experience is that most fancy syncrhonizer for telecom application
> (Stratum, LORAN and GPS) use start-stop phase detector with averager in
> front of the AC-DC gain circuits.  Changing the closed loop bandwidth of the
> PLL on the fly is not easy, due to secondary effects (done that many times).

Such a detector isn't particularly useful for most GPSDOs as they get 
have 1 Hz comparator frequency with the PPS pulse. The analog 
time-constants would be a terrible mess to design. For GPSDOs using PPS 
updates a digital loop filter can be assumed (unless you use say the 
Jupiter receiver and it's 10 kHz output, see performance on TvB pages!).

Being able to scale loop parameters without having to re-scale state is 
indeed an issue, but there exists topologies that can achieve this 
without the need for rescaling. These can be realized both in the 
analogue world using say OTAs and or through multipliers at the right 
positions when done in the digital world. It is used with great success 
in the GPS code and carrier tracking channels. You can look it up in the 
Kaplan GPS book for instance... which only summarize what's found in 
other places.

Cheers,
Magnus




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