syler

Hi, I am trying to make an envelope detector for 915MHz AM modulated signal. Has anybody here a tested design? I actually need the specifications for diode used. And want to see how the impedance mismatch things are tackled..

would appreciate any comments

Syler

RadioRon

Such a thing intended for demodulating information from a 915MHz carrier is quite rare, but a peak power detector, which is essentially just an AM detector, is quite commonly used as a power sensor to control transmitter output level of cellular phones.

A good diode for this would be the SMS7621 series, perhaps the -079 version for example. For best sensitivity, you should put some DC bias through the diode, say about 30 microamps or so. This biases the diode closer to its voltage knee. You should follow your envelope detector with a high impedance amp so that you don't load the detector very much.

Beware that such a detector's output is sensitive to temperature variation. If you are only making something simple, then don't worry about it, but if you are making a precision detector that is coupled down to DC, and that will be replicated, then you must create a circuit that eliminates this temperature sensitivity.

Microstrip layout procedures must be followed, and surface mount components are essential in this kind of circuit.

In the ones that I have seen, the input impedance of the diode circuit is typically highly variable, so the mismatch is dealt with by inserting an attenuator at the input, to buffer the 50 ohms source from the variations in the detector. You can choose the amount of isolation from this attenuator, but a good value might be 6 or perhaps 10 dB. This would give you 12 dB or better return loss, which isn't bad.

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RadioRon