Electronic – Understanding this simple FM receiver

Your FM radio works because it receives a band of frequencies around the frequency it shows or that you tune it at. As you say, the nature of FM means that the frequency will vary. However, the extent of these variations is well defined, and the radio is designed to "find" the carrier anywhere in this range.

This may surprise you, but AM also requires the receiver to pick up a range. I'm not going to go into Fourier analisys right now, but basically changing amplitude is adding frequencies. Put another way, a true pure single frequency can't ever change in amplitude and can't carry any information.

The way AM works, there is a frequency band on either side of the carrier that is the width of the highest frequency that can be transmitted. For example, if the signal being modulated onto the AM carrier can be up to 10 kHz, then there is a 10 kHz band of stuff on either side of the carrier. In fact, the carrier is constant and the actual information is in these side bands. Yes, I know that may be unintuitive, but at the level you are asking and what I have time to explain here you'll just have to trust me on this. Look up Fourier analisys if you want to learn more.

For example a AM radio station a 1 MHz carrying up to 10 kHz content will have a signal spread out over the range of 990 kHz to 1.01 MHz.

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 from 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