[time-nuts] Zero-Crossing Detector Design?

[Didier Juges]

Narrow filters have high tempco on their group delay, so that's no good either.

Didier KO4BB

Bill Fuqua <wlfuqu00 at uky.edu> wrote:

>Wow, I have not checked this list for some time. But there is a lot
>said 
>about zero crossing detectors.
>Lots and lots of replies, so many that I have not looked at all of
>them.
>1. Do not use CMOS inverters. Even though so much has been published on
>
>using these in linear mode by
>adding a feedback resistor, they can be a nightmare. The fast ones
>(74HC, 
>74AC, etc)  have so much high  frequency gain they are
>likely to take off into oscillation on their own.
>2. The first thing you can do to get  a good clean zero crossing is to 
>reduce the noise. This means to pass it
>thru a narrow band pass filter such as a crystal filter. The narrower
>this 
>filter is the closer to a pure sinewave it becomes
>and the less noise you have.
> 3. In research when we want a precise trigger we use what is called a 
>constant fraction discriminator.
>This may not be needed if you have a very clean signal and its
>amplitude 
>does not vary and you are wanting to
>trigger exactly at zero. But a constant fraction discriminator triggers
>on 
>a point that is a constant fraction of the
>amplitude of the signal. They require a delay so that a fraction of the
>
>peak of the cycle can be compared with the rising edge
>of that cycle.  This is mostly used with triggering on pulses of
>varying 
>heights and when subnanosecond
>timing is required.
>
>My suggestion is to clean up your signal as much as possible and reduce
>
>noise bandwidth using a bandpass filter and
>then use a low noise amplifier for the front end of your zero-crossing 
>detector.
>
>73
>Bill wa4lav
>
>
>
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