[time-nuts] ? phase comparison or other device
magnus at rubidium.dyndns.org
Sat Jun 30 10:33:41 EDT 2007
From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] ? phase comparison or other device
Date: Sun, 01 Jul 2007 02:15:55 +1200
Message-ID: <4686659B.8000205 at xtra.co.nz>
> Magnus Danielson wrote:
> > Ulrich,
> >> 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?
> > It is actually a specific design-trick on Collins behalf to saturate the
> > op-amp since this infact acts like a noise-gate. By having the output stage
> > saturated rather than operating linearly the noise as seen by the output RC
> > filter is that of only the saturated transistor and not that of the input
> > gained up. A diode limiter in the feedback path will maintain the op-amp in
> > the linear operating range and thus cause the noise to continue to polute the
> > output filter. What you can possibly acheive is the steer how deeply you run
> > into output saturation tought.
> > So, in this case op-amp saturation is a key trick to increased performance.
> Not true, there's nothing magic about amplifier saturation, any means
> that limits the amplifier output whilst dropping the small signal gain
> to a low value will have exactly the same effect.
So you don't think the input-to-output gain is greatly affected when in
saturation? That is usually what happneds IMHO.
The gain in saturation will be less than 1. Thus, early noise sources will be
dampend rather than gained. In a diode clamp they will have unity gain rather
than gained. There's the difference.
> In most cases recovery from saturation will be too slow for the later
> stages of the ZCD.
> Those amplifiers that have fast recovery from saturation usually employ
> internal diode clamps.
> A diode clamp in the feedback path will cut the noise gain to 1 when
> either diode turns on. The following diode clamp across the filter
> capacitor will reduce the noise gain to a very small value when it turns on.
> Both diode clamps and internal saturation will still produce some output
> noise although not from the amplifier input stages.
Well, this is true. But it again shifts the noise levels from the ideal zero.
The full gain is when the output saturates, but this has the drawback in
recovery time. I think the output saturation may be better for the first stage
where as a clamped variant is better for the following stages. For the first
stage you also have the noise out of the mixer to consider.
I might buy your argument better if you'd show me that the noise of the
saturated output stage is worse than the gain-of-1 alternative. If the noise
is just slightlty worse with the clamping, then its benefits outperforms the
loss in noise margin.
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