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

Question

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:

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.

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

Answer 1

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!

Answer 2

If you can live with a signal range limited to between 0V and 5V use a MAX999 comparator and a BAS16\$^1\$ diode with a zero ohm feedback after the cathode to the inverting input. Using a 1uF output capacitor is asking for trouble - something more like 100 pF is more suitable along with a bleed resistor of a few kohm.

You might also wish to improve things with a feedback diode for the "unused polarity" directly from the output to the inverting input. Doing this last thing also ensures the device doesn't enter saturation when the wrong signal polarity is being processed. You can also employ this "other" diode as a negative peak detector (but you will need feedback resistors and not zero ohms feedback resistors). Don't do what you have done with D3 and D1 - that won't work; you need to use this topology: -

In your circuit R2 would be zero ohms.

I've used this type of peak hold circuit on a variable amplitude data signal toggling at 80 Mbps and it works.

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\$^1\$ Reverse recovery time for the BAS16 is 4 ns and about the same as a 1N4148.

Answer 3

You might be able to extend the range of a standard diode peak detector with the circuit below. It uses an op-amp to DC-compensate the diode detector's forward voltage. Note that the op-amp is dealing with near-DC voltages, not RF. Germanium diodes might work best, if you can match D1 & D2. The low-amplitude response is still compressed, but not as much as a bare diode peak detector.

^{simulate this circuit – Schematic created using CircuitLab} Adil Malik's suggestion of using an AD8307 is a good one too - it gives output voltage proportional to dBm and has very wide dynamic range. Its input impedance is about 1k ohm.

Answer 4

1) You are really asking for trouble using an LT1812 at this frequency. With a 100 MHz GBW, you only have an open-loop gain of 6 at 16 MHz. Combined with the non-linear feedback network your circuit is likely in trouble.

2) As Adi Malik has pointed out, your output capacitor is far too large, and you need a bleed resistor on it..

3) You also need a load resistor on D1. As configured, you are attempting to look at the reverse characteristics of a 1N4148 under a load current of a few microamps, and this is not remotely a good idea.

4) Even ignoring the current issues with D1, you are almost certainly having issues with parasitic capacitance of D1 feeding a distorted signal to the - input. What this does to the loop behavior can't be good, particularly since any attempt to acquire a new peak will provide a slew rate limited transient as the output of the op amp comes out of saturation and tries to reach a signal level of some volts. This large transient will overwhelm the op amp input. Andy_aka's use of a limiting diode is appropriate, but I'm not at all certain how well the idea will work out with a feedback path consisting entirely of diodes.