Electronic – High Frequency (16 Mhz-48 MHz) Peak Hold/Envelope Detector Circuit

detectorfrequencysample and hold

I'm trying to make a peak hold circuit for a very high frequency bandpass filter that needs to be able to operate at 16 Mhz, 40 Mhz, and 48 Mhz.

I've singled out the peak hold part of my overall circuit to try to get it to work. Here's the one I'm currently trying, which isn't working:

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Here(below) is the rest of the circuit with one of the other peak hold circuits I tried using (also doesn't work.) The circuit is supposed to take in a frequency and determine whether or not it is within ~500 kHz of 16 Mhz. I'm using a 2nd order Chebshev resonant frequency filter as a bandpass for the incomming signal, then I'd like to use a peak hold circuit to get the amplitude of it before pushing it to a comparator, which will determine if it is within the desired frequency. If there is a easier way to approach this that doesn't involve and FPGA, that will work as well.

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I was mainly wondering if there are other peak hold circuits that will work at high frequencies (16, 40 and 48 MHz.) I've tried other circuits with BJTs and they haven't worked either. Do the OP amps I have selected have anything to do with these problems?

Thanks

Best Answer

Opamp + diode based peak detectors are useful for narrow band and low frequencies only. This is mainly due to the reverse recovery time of the diodes and the opamp bandwidth + slew rate considerations. Such peak detectors might be good at giving you a relative measure of signal strengths at various frequencies, but will not give you true absolute accurate dynamic range.

Furthermore,as you need a bleed resistor on your peak detect capacitor, this essentially limits the bandwidth you can detect accurately because you have a fixed RC time constant to work with. You need to look into other ways of detecting a signals magnitude, power etc, depending on the dynamic range, accuracy and frequency you need.

Your choice of diode is also not ideal, you need to use schottky types

Check out: AD8307, this is a broadband Log detector, it uses a completely different topology to achieve good dynamic range and bandwidth!

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