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I'm trying my hand at making a radio transmitter and receiver and I'm not sure exactly how this receiver works. I understand its a continuous wave receiver which is good for on/off transmission. I'm not exactly sure how the circuit doesn't have any continuous oscillations.

enter image description here

Taken from: Simple Pyro RF Receiver (27 MHz).

From what I understand it is a regenerative receiver which means the signal is amplified by passing the amplified signal back into the amplifier. I found a similar question asked here: Can someone break down how this receiver works? From what I understand in that question, L2, C2 and C1 act to filter a specific filter which gets amplified again.

My confusion comes when I was looking at an FM transmitter schematic. enter image description here Taken from: How to make FM transmitter.

From what I understand about FM transmitters they are voltage controlled oscillators and essentially the mic in the circuit affects the voltage which then modulates the frequency. The circuit however is still continuously oscillating.

The only real difference I see between the regenerative receiver and the transmitter circuit is that R1 is introduced in the receiver circuit (the other question doesn't talk about it either). How does this cause the circuit to not oscillate on its own and only when a radio signal is picked up?

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1 Answer 1

up vote 6 down vote accepted

It intentionally DOES oscillate.

It goes beyond a regenerative receiver and is called a super-regenerative receiver, invented by Edwin Armstrong in 1922.

The values of R3 and C3 are arranged so that after oscillation starts the voltage builds up on the emitter due to rectification of the oscillations. After some period (10's of microseconds typically) the bias will be such that the oscillations stop. The voltage will then decay with the time constant of R3/C3 until oscillations start again. This typically occurs at a frequency in the range of 10's to hundreds of kilohertz.

Normally in an oscillator the oscillations build up from noise however if any signal at the oscillation frequency is present the oscillations will build up more quickly. This is the signal being received. The average supply current will depend upon how quickly the oscillation starts which depends upon the signal present. Thus the supply current is dependent on the received signal, the output from R1 depends on the supply current.

Usually the transmitter is amplitude modulated at an audio frequency and this can be recovered form C7 with suitable filtering and amplification.

A super-regenerative receiver can get extremely high gain out of a single stage but when no signal is present the output will be very noisy. Because the build up of oscillations is exponential it usually has a logarithmic response.

They are used less these days because they tend to radiate and also have a very broad bandwidth. A common application is for garage door openers.

kevin

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This was very helpful. Thanks! –  Saad Ahmad Jul 6 at 14:27

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