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Diode detector (Read 171 times)
Horror Vacui
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Diode detector
Apr 03rd, 2020, 5:57pm
 
Hi everybody,

I am a little confused about the working principle of RF diode detectors. They supposed to be extremely simple circuits, which are being used for more than a hundred years. They deliver a small-signal current, which is proportional to the square of the voltage applied to them, and therefore they deliver a signal proportional to the energy of the signal to be detected. The circuit schematic* together with the derivation from Pozar's book is attached. The input biasing is not shown, but a bias-tee is used to set the operation point of the diode.

The problem to understand: the diode's voltage bias depends on the output voltage. This will reduce the linearity of the detector, which is an important parameter of the circuit. The output voltage also supposed to be meaningfully high for SNR reasons.

So, how does the schematic of a diode power detector look like? Is the attached schematic good? If the schematic is good, how is the issue of output dependent operation point is worked around? AC coupling the output is not an option at low IF frequencies**.

* I was not able to find any detailed schematic, so a drawn one based on my understanding. Usually just a diode is shown in the publications.
** Even in power detectors we can think of the output as a demodulated AM signal. The modulation frequency of the envelope is the IF in the figure.
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Ken Kundert
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Re: Diode detector
Reply #1 - Apr 4th, 2020, 12:34am
 
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Horror Vacui
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Re: Diode detector
Reply #2 - Apr 4th, 2020, 7:38am
 
I am interested in the operation of diode power detectors used in precision power measurements rather than in demodulation in a receiver. I've read in multiple dozens of articles (micro- and mm-wave), in which the authors just stated that they have used the diode power detector with square low characteristic, but I was not able to understand how does it work. Any web search resulted in AM detector circuits.

The linked pages describe the operation for AM demodulation, where the input voltage envelope is detected. These circuits are maximum detectors, and does not follow square law characteristic. These circuits use a largish RL whose only purpose is the discharge the cap for following a falling envelope. Otherwise the circuits are identical for the two different application. This duality was one of the things which awake my curiosity about this circuit.
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