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# simple peak RF detector (peak RF to DC) using discrete electronics?

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#### neazoi

Hi, I am trying to find a suitable simple peak RF detector (peak RF to DC) using discrete electronics.
It will be used to drive the base of an NPN which will in turn drive an analogue meter.
I want this meter to show the peaks of RF (HF frequencies to 30MHZ max).

I could simply connect a shunt capacitor to the base of the NPN and have the average RF, but there are 2 problems in it.
1. This will show the average and not the peak
2. This will attenuate the RF, as RF power will be shunted to the ground.

Any ideas?

Regards, Dana.

### neazoi

Points: 2
Thanks Dana. I am looking for something that can be built with discrete electronics, else the AD chip is simple and great. A simple RF peak detector instead of average (that most detectors are) implemented in just a few transistors if possible. Accuracy or linearity is not that much of concern, this is for a hobbyists circuit.
There is a circuit I have found but I am not sure it will do.
plus I do not like the -/+ voltage
it seems that it might be able to hold the peak for some time?

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• Peak-Voltmeter.jpg
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A simple diode detector and capacitor will do it. If the stage driving the meter has a high input impedance it won't discharge the capacitor so it will show peak voltage.

Brian.

### neazoi

Points: 2
A simple diode detector and capacitor will do it. If the stage driving the meter has a high input impedance it won't discharge the capacitor so it will show peak voltage.

Brian.
That may be the problem. Here is the input of my circuit, driving the BJT. Maybe it is loading down the capacitor too much, or maybe this 1k in the input is doing this. (I have also tried a diode at the input to form an envelope detector).
The attached schematic is a variation of this circuit http://qrp.gr/mecheye/ of mine

The circuit is a variation proposed by Dana a long time ago, which can measure down to 100mV. I would like to avoid an input diode which would bring the minimum level to 600mV or so...

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Thanks Dana. I am looking for something that can be built with discrete electronics, else the AD chip is simple and great. A simple RF peak detector instead of average (that most detectors are) implemented in just a few transistors if possible. Accuracy or linearity is not that much of concern, this is for a hobbyists circuit.
There is a circuit I have found but I am not sure it will do.
plus I do not like the -/+ voltage
it seems that it might be able to hold the peak for some time?

The amatuer radio handbooks have variations you can consider.

Are you trying to measure power or just simply envelope pk-pk ? Dynamic range needed ?

Regards, Dana.

### neazoi

Points: 2
The left side of the schematic is missing but I guess it is just a saturated transistor of the same type as the one using its B-C junction as the detector. Basically, offsetting the diode drop to overcome the turn-on voltage.

At its simplest, remove the capacitor at the input and connect it across the C-E of the final transistor instead.
A better solution is to make a 'precision rectifier' by using an amplifier followed by an ordinary diode detector but using an identical diode in the bias feedback path. Do a search on 'precision rectifiers' to see the principle but to use discrete devices you would have to be careful designing the amplifier.

Brian.

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