Hi Alan!

I would like to ask something. I want to build a simple trf fm receiver. I found this:

http://www.mikroe.com/old/books/rrbook/chapter3/41b.gif

(right side of the picture)

Will this receiver work? The headphene lead is placed properly, in series with the battery?

All the best

Richard

how this special quenching takes place

Alex,

9 Turns on an FT23-43 ferrite core. That is a small ~ 6mm toroidal type-43 ferrite core. If you can't get the core a few turns on a ferrite bead should do nicely as a replacement. It is not all that critical, it is just an RFC, you could use an aircoil, say 20-30 turns on a drinking straw.

Regards,

Alan

how is the coil (9t ft23-43)?

Good day sir Alan. Can I use this circuit as a wireless microphone receiver? or if not, can you

teach me how to create one? Thanks in advance.

Bert,

I try to reply to everything eventually, but work has been crazy of late. Getting behind on everything.

Regards,

Alan

Hi Alan,

Thanks for your comprehensive explanation on the working principle of self-quenching super-regens.Waiting for so long, I thought I will never get your reply !

All the best,

Bert.

Bert,

The gain of a bipolar transistor varies with its collector-emitter voltage, generally the larger the voltage the more gain it will have. When a BJT is saturated the Vce drops to a minimum value usually < 0.5 volts or so and the transistor has no significant gain.

That 10 nF from the resistor in the emitter to the resistor in the collector is the key. Imagine it starting discharged with the circuit turned off. When you switch on the circuit, the capacitor has zero volts across it and it won't let it rise immediately, it has to charge up through the resistances in the collector and emitter. This keeps the gain of the transistor below that required to oscillate. Eventually a Vce is established that allows RF gain and the oscillator starts, but as it does so the transistor pulls more current and the building oscillations shift the operating point of the transistor, shifting the emitter voltage up, tending to reduce the Vce and shutting it down again. Eventually the oscillation collapses and the capacitor discharges again, allowing the Vce to rise and the oscillator to start once more.

It is a form of relaxation oscillator. The equilibrium is disturbed by the RF oscillation, and the non-linearity of the circuit is important is providing the voltage shift at the emitter. It is actually quite complicated, and most circuit analysis programs have great difficulty simulating the self-quenching super-regenerative detector.

Because how long is takes to start oscillating once it is ramping up gain depends on the signal being received the quench frequency is a function of the signal strength. This is how my 555 super-regen spectrum analyser works. It takes the quench waveform and runs it through a monostable to extract a voltage proportional to the frequency of the quench - and hence the log signal strength.

Regards,

Alan

Bert,

Mine only hisses strongly when the signal is pretty weak. That said super-regen detection of WBFM with stereo sub-carriers is pretty crappy to listen to, it can sound pretty awful, but generally it is only hissy when the signal is weak.

You might just be in a weak signal area compared to where I live. The easiest way to improve it is to add an antenna of some kind, even a fairly short one should help.

Regards,

Alan

Hi Alan,

Finally I successfully made this receiver, receiving FM broadcast stations.

But, the signal is very noisy; hissing sound is strong and nearly blanket out the audio signal, and it is impossible for me to discriminate a radio station without connecting an antenna to the emitter, as said in your article. I wonder why the hissing sound is so loud?

I turned the 2k pot, adjusted a preset capacitor taking the place of your 10pF, turning the main capacitor and also squeezed and pulled the coil, but nothing works to reduce the hissing. Is yours behave in this way too?

Hi Alan,

I am very interested to know how the self-quenching works in this circuit. Would you mind explain which components play the part and how to determine the quench frequency?

All the best,

Bert.

Richard,

Hissing is a good sign, especially if it starts fairly abruptly at some setting of the pot and/or supply voltage. That almost certainly means it is oscillating/super-regenerating.

The problem is likely that it isn't oscillating on the right frequency.,. What test equipment do you have? If you have a CRO you can confirm it is oscillating properly by probing the emitter circuit at the pot and hopefully seeing the quench sawtooth waveform. Getting it to super-regenerate is 90% of the battle, once you know it is working it is then a matter of aligning it to receive the band of interest.

If you have a VHF signal generator, sweep an AM modulated signal from 50-150 MHz near the receiver, you should find it pretty easily. Once you know roughly where you radio is receiving you can adjust the coil or capacitors to put it on the broadcast band (or anywhere else you like).

Without a signal generator you are going to have to make-do with off-air signals to try and work out where it is oscillating (or perhaps a frequency counter or communications receiver to try and pick up its oscillations). To count its oscillator you'll probably need to stop it super-regenerating, disconnect the 10 nF capacitor at the emitter choke for a while, it should oscillate cleanly then (and the hissing should drop dramatically). Mine tunes a very broad range and can hear TV signals, pagers, aircraft, all kinds of things, but the tuning is fairly critical because of the huge range tuned over the 180 degrees of capacitor motion that map to the ~100 MHz range (very roughly - because the tuning is not linear - about 1.8 degrees per MHz or 22 minutes of arc per FM radio station!).

If you have no RF test equipment consider building a diode probe, or my RF sniffer project... With those you can tell if it is oscillating, in fact the sniffer is more reliable as the probe often overloads oscillators and stops them.

Regards,

Alan

Hi Alan,

How can I adjust the receiver with the pot?

It doesn't work. There is only strong hiss.

Thanks and Regards,

Richard