[time-nuts] Zero-Crossing Detector Design?

[Bob Camp]

Hi

The feedback inverter is indeed a problem with fast logic, just bias it to mid point off the supply instead.

Narrow filters can be both a good and a bad thing at the same time. They clean up the signal, but the also have delay. If they are narrow enough they have lots of delay. That would be ok, except it drifts with temperature. It also changes with input frequency. You can do a delay correction stage to help with the frequency variation part, but the delay correction adds more delay that's also temperature dependent.

No easy solutions ….

Bob

On Jul 22, 2012, at 4:32 PM, Bill Fuqua 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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