I took a physical lab course where I worked on the microwave PASCO set-up WA-9316.

Having a little background on electronics, I wondered how their microwave receiver works and found a more or less dated schematic. Here's the interesting part:

enter image description here

According to the documentation:

"A microwave horn [...] collects the microwave signal and channels it to a Schottky diode in a 10.525 GHz resonant cavity. The diode responds only to the component of a microwave signal that is polarized along the diode axis, producing a DC voltage that varies with the magnitude of the microwave signal."

So my questions are:

  1. I would use a Schottky diode as an incoherent demodulator as usually done in microwave front-ends (see e.g. this link). Why is the diode used with the anode as the positive electrode?
  2. Do you have other references to this alleged (to me) configuration?
  • \$\begingroup\$ I think you meant something other than "alleged," but I'm not sure what. \$\endgroup\$
    – JRE
    Jun 14, 2022 at 14:12
  • \$\begingroup\$ What do you find strange about the setup or the circuit? It's a simple AM demodulator circuit. The only "odd" thing is that it is used at 10GHz. \$\endgroup\$
    – JRE
    Jun 14, 2022 at 14:17
  • \$\begingroup\$ @JRE, isn't the diode flipped with respect to a classical AM incoherent demodulator? I.e., the cathode in the PASCO drawing is connected to GND and we expect the voltage on C1 to be positive. Sorry, I'm a novice in RF... please explain \$\endgroup\$
    – simonstar
    Jun 14, 2022 at 17:40
  • \$\begingroup\$ The polarity doesn't matter to the RF. The polarity of the demodulated signal only matters to the following low frequency circuitry. \$\endgroup\$
    – JRE
    Jun 14, 2022 at 19:05
  • \$\begingroup\$ @JRE, I know that the output of the amplifier is positive by direct measurement on the instrument, so the voltage on the capacitor is positive. The question becomes: does the diode placed as in the schematics, with anode connected to C1 and cathode to ground (i.e. the DC signal ground) produce a positive (DC) voltage io C1 itself? I think the answer can be "yes" or "no". \$\endgroup\$
    – simonstar
    Jun 14, 2022 at 20:25

1 Answer 1


It's a crystal receiver with a low frequency amplifier. The horn antenna is assumed to be good enough bandpass filter (=to keep out lower frequency signals) and of course it can catch enough power from a near or strong enough transmitter to generate a DC voltage by rectifying the microwaves with a diode. The diode is a Schottky diode designed to rectify at several GHz, no common Si-PN diode would work.

The amp is an ordinary opamp connected to a non-inverting amplifier. The gain is adjustable.

If the diode rectifies a pulsed microwave signal the amp output DC pulses, which can be heard as a sound, if one feeds them into a speaker. The speaker should be active one, an opamp cannot feed a speaker properly. Audible sound needs that the microwave signal is pulsed in an audible pulse repetition frequency.

A continuous microwave signal generates non-zero DC output. There seems to be an offset zeroing potentiometer intended for calibration the output to 0V when there's no DC input.

This receiver is not especially sensitive, but it can detect signals in many experiments. The linked system very likely allows experiments where moving target causes doppler shifts.

When Doppler shifted and the original signal from a continuous sinewave transmitter meet in this receiver at the same time, one can see how the DC output voltage varies just in the Doppler shift frequency. That's effectively the "frequency difference mixing result". A speaker may produce a sound of the Doppler shift. In the past there were land surveillance radars based on this effect. A well trained radar operator could hear what kind of target moved in the radar beam.

Communication radio links must have much more advanced receiver designs.

  • \$\begingroup\$ It feels like the diode should be forward biased for this to work, at least below some volts/m field strength. Is that right? Is the diode biased by the input leakage current of the op-amp? \$\endgroup\$
    – tomnexus
    Jun 14, 2022 at 15:00
  • \$\begingroup\$ @user287001, from the schematic symbol I suppose they use a shunt-diode. In this configuration I always saw the anode referred to the DC ground, so that the voltage on cathode will be positive. In the PASCO schematics it's the cathode that is referred to ground, and wouldn't this imply that the voltage on capacitor is negative? Please explain me why this is not the case. \$\endgroup\$
    – simonstar
    Jun 15, 2022 at 14:51
  • \$\begingroup\$ I actually decided it backwards. The meter at the opamp output expects +voltage. The amp is non-inverting. I guess the diode is drawn so that the drawing only shows there's a diode and DC path for it. I wouldn't believe it shows the actual circuit connections inside the microwave parts. The diode may well be placed so that it shorts the negative half cycle of the microwave. Or it's actually in series - impossible to decide without seeing the actual waveguide structure. \$\endgroup\$
    – user136077
    Jun 15, 2022 at 17:35

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