Narrow filters have high tempco on their group delay, so that's no good either.
Didier KO4BB Bill Fuqua <wlfuq...@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 > > > >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Motorola Droid Razr phone with K-9 Mail. Please excuse my brevity. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.