[time-nuts] ? phase comparison or other device
magnus at rubidium.dyndns.org
Fri Jun 29 09:37:43 EDT 2007
From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] ? phase comparison or other device
Date: Fri, 29 Jun 2007 19:52:22 +1200
Message-ID: <4684BA36.3040401 at xtra.co.nz>
> Ulrich Bangert wrote:
> > Enrico,
> > you are right: Both of these articles should be read with Collins's
> > perhaps the better (and newer!) one.
> > There is however one question remaining for me: When I learned
> > electronics it was generally considered bad design to let an amplifier
> > run into limiting due to supply limitations. If limiting was needed, so
> > was the rule, then it should be accomplished by planned feedback, say an
> > pair of antiparallel diodes in the feedback path. Can you give some
> > comments on whether this also applies to ZCDs or if really supply based
> > limiting is necessary?
> > Best regards
> > Ulrich Bangert
> Diode clamps are usually essential particularly with opamps as in most
> cases their recovery from input overdrive is otherwise too slow to be
> useful in such circuits.
> The Collins paper also indicates, as I suspected, that the -5v and +5V
> clamp levels used in the JPL ZCD are somewhat arbitrary and lower clamp
> levels can be used.
The clamp levels is gain-wise not very relevant since in the next gainstage
they will be clamped out anyways. The lowpass filtering in each stage will
integrate the clamp levels such that the rise and fall positions will be
shifted from their "true" positions by the clamp level and these shifts will
not have an zero mean. However, if the clamp levels are stable they are less of
a problem. The main clamp level effect will be time-shift and as long as the
levels are stable this time-shift will be stable.
I must have a propper reading of the Collins paper, but I will do that
tomorrow. Refreshing new info from the JPL ZCD paper which confirmed my
initial thoughts while introducing the noise aspect. It did bug me that the
noise may not be continous so the Collins findings in that respect was fairly
obvious. However, it seems like he is not caring about the noise level during
the clamping time, which would make his noise estimates somewhat
over-optimistic. But then again, I haven't given the paper a propper read-
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