[time-nuts] ? phase comparison or other device

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jun 30 10:15:55 EDT 2007

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.
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.
> Cheers,
> Magnus


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