RF reciever circuit doubt

What do you think about the next RF receiver circuit? I think that it's works because there is a feedback which makes the 2n2222 oscilate and there is an demodulation and power step.

It is a horrible "super-regen" radio (look in Google). It actually detects AM but can detect FM if it is tuned to one side of the station's frequency so the FM produces amplitude changes as the station's frequency becomes closer or farther from the tuned LC frequency. But then it also detects amplitude interference the same as an AM radio. It has only one tuned LC so it probably picks up a few FM stations at the same time.

A real FM radio circuit has many tuned LC circuits for good selectivity and lots of gain for good sensitivity.
A real FM radio circuit is not overloaded by strong local stations like this one is.
A real FM radio circuit does not detect AM so has very low audible interference.

Do you have a good fm receiver?

A good FM receiver circuit is VERY complicated. Where would you buy parts for it?

In electronic store. But I want to do a regenerative circuit.

Audioguru said:

It is a horrible "super-regen" radio (look in Google). It actually detects AM but can detect FM if it is tuned to one side of the station's frequency so the FM produces amplitude changes as the station's frequency becomes closer or farther from the tuned LC frequency.

Click to expand...

Please excuse my ignorance, but I thought FM is supposed to be detected that way! Where I can learn more?

A good FM receiver first isolates the wanted signal by filtering then recovers the modulation by observing the frequency shift.

A super-regenerative receive like the one shown works on the principle that the incoming signal pre-empts the start of each "squegging" oscillation cycle so it is amplitude sensitive only. The change in loading on the tuned circuit gives it a broad bandwidth so although it is sensitive, it's selectivity is poor. To recover FM, it has to use slope demodulation, offsetting the received tuned frequency so the FM signal lies on one or the other bandwidth edges. Having poor selectivity means it might be difficult to offset the tuning point without picking up adjacent stations as well. It's AM rejection is almost zero so as already pointed out, it is prone to interference.

Brian.

A very cheap child's radio controlled toy car and some garage door opener circuits use a regenerative receiver circuit.
AM= the amplitude of the radio signal or interference swings up and down by the modulation.
FM= the frequency of the radio signal swings up and down by the modulation.

A proper FM detector detects only the frequency changes of an FM broadcast ignoring any amplitude changes which are probably caused by interference. Ratio Detector or Foster Seeley Discriminator circuits are good FM detectors but cheap radios and TVs use a more simple Quadrature Detector that has poor performance.

I was seeing that same circuits take the antenna potential from an inductor's loop likes this:



Why the antenna goes to an inductor's loop? why it can not go to Vcc potential?

VCC is a DC potential but ground as far as RF is concerned because C1 and C6 ensure no signal can be there.

Brian.

What happen if I take out the capacitors C1 and C6? there would not be short circuit for RF signal. I think C1 is a Vcc filter but C6 maybe it's necesary.

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I means why the antenna can not be between the tank circuit and R5? and it's has to be in ainductor's loop

R5, C1 and C6 are there to filter out any noise from the supply line. C6 is needed because elcos aren't very good at RF. The purpose of C6 is to ground the RF currents in the tank circuit, necessary for the feedback that makes the circuit oscillate.

If you don't want to make a tap in the tank coil you can use the capacitive coupling in the first circuit. The problem with that is that it loads down the tank circuit, altering both sensitivity and frequency. Any movement of the antenna will change everything. A short, rigid antenna and reducing the coupling cap may mitigate the problem.

You can also connect the antenna to a 1 or 2 turns coil, grounded at the other end and place it close to the tank coil.

Note: this is for fun and experimentation. Nobody expects it to be a high faluting hi-fi receiver.