2008-12-07
Elektor FM Broadcast Super-Regenerative Receiver
In my continuing procrastination in not playing with levelling loops I finally got around to building the FM broadcast receiver from September 2007's Elektor magazine. Peter VK2TPM saw this article and suggested it to me because of my well-known love/hate relationship with super-regenerative receivers.
The circuit appears in the "Mini Projects" section of the magazine and is credited to Burkhard Kainka. The circuit is fairly conventional in all but one way; it returns the emitter/source capacitor to the collector circuit to partially suppress the quench waveform from the AF output. The effect isn't perfect, but it does work fairly well.
The audio output into 64 ohms isn't very loud, as the article says, it works better with a higher impedance load. A small 1K:8R audio transformer was tried, it worked quite well and is highly recommended, but I stripped it out of the final version to save space. The audio output is enough for listening in a quiet room. RF Performance is reasonable, but nothing special. My micropower FM receiver works better IMHO.
Google Translate is useful for looking at Mr Kainka's fine website. The receiver is mentioned here. Note also the regenerative receiver similar to his KW-Audion. It is very similar to my Noisy Regen, as is the KW Audion, except it uses an "infinite impedance" detector of sorts instead of the emitter current to recover the AF - something I must try myself.
Build Notes
I did have some trouble convincing the circuit to super-regenerate properly, it was extremely fiddly to get a good quench oscillation going, and would take-off in UHF oscillations if I wasn't careful. The picture in the article shows - well - an abysmal layout, perhaps that is why the author's circuit worked better than mine - I guess too much RF hygiene can be a bad thing?!
Looking up with BF494 I found its Ft is only 200 MHz or so... This explains a lot, I used a BF199 which has a *much* higher transition frequency. I probably would have made it a lot easier on myself if I had used a "worse" transistor. I am still suspicious about my current batch of 10 nF caps. I have thousands of them and they are axial-leaded so I rather like building with them, but they have caused me trouble before... I tried swapping them out with 10 nF disk ceramics, but it didn't seem to help, so perhaps the collector circuit strays are responsible, I did use fairly long wires on the resonator coil. Anyway, I replaced the emitter resistor with a multi-turn cermet trimmer and was able to adjust the circuit with good usability without resorting to a different transistor.
The RFC suggested in the article killed oscillations above 90 MHz with the other component values suggested, but oscillation could be achieved by varying the emitter resistor and/or power supply voltage. I replaced it with 9 turns on an FT23-43 which works great to beyond 150 MHz in all configurations.
As built I used the following circuit:
It tunes from the VHF-Low channel 0 audio carrier (about 51 MHz) to about 150 MHz, covering the FM broadcast band, air band, 2 metres and the pagers just above it. More bandspread would be preferable, especially if dedicated to FM broadcast use, this is a major reason why I favour my micropower unit who's tuning is carefully arranged to cover just the 3 metre broadcast band.
The antenna may be applied at the emitter, but isn't really needed as the unshielded unit just sitting on the bench works fine with the local stations, even airband signals, Sydney approach being heard fairly easily... The AF stage had a tendency to take-off during the minima of the quench cycles without the 10 nF in its collector and the rail decoupling 150 nF (it might be preferable to add collector-base feedback instead of the collector cap as it might cause instability itself with the inductive collector load, but with my headphones it didn't). The frequency of oscillation was quite high, in the HF region, and was pulled by the headphone leads. Again a "poor" transistor would probably avoid this problem, but the BC559C specified is actually quite likely to take off itself without the capacitor. Using a BC559C is probably overkill for the quality of audio recovered by the regenerative detector.
Using a trimmer instead of the fixed 10 pF emitter/collector feedback capacitor is helpful if your unit misbehaves. It allows more precise adjustment of the feedback. In the past I've had some transistors refuse to oscillate except in very narrow ranges of feedback capacitance. This particular circuit was fairly well behaved, but I did at one point use a trimmer. 10 pF was near the optimal, so I replaced it with a fixed value.
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Attachments
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| Circuit Diagram Source | application/postscript | 15.441 kbytes |