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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