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It's been quite a while since I built a super-regenerative receiver for the FM broadcast band and I've been meaning to play with a different source feedback topology, so this little radio was thrown together on the Saturday of the APEC long weekend.
The topology is your classic grounded-gate FET VHF Hartley oscillator. The drain resonator inductance is centre-tapped with feedback to the source through a small capacitance. By tapping down towards the cold-end of the coil the feedback isn't as critical as your usual source-drain capacitor feedback and it tends to be far less difficult to get to work across a broad range of frequencies. The RFC to an RC source circuit to implement self-quenching is very traditional for super-regenerative detectors. The quench gets frequency-modulated somewhat by the drain current, so it varies with signal strength and the recovered modulation, this is typical for self-quenched circuits (simplicity has its price).
The detector alone provides sufficient audio to drive a crystal ear piece in a very quiet room, giving a true "single transistor" FM receiver. A largish resistor (~10 k) prevents the source circuit from seeing too much of the fairly large capacitance of the piezo element (about 14 nF) and pulling the quench well down into the audio range. Some additional audio volume can be achieved by redesigning the quench circuit to utilize the piezo capacitance directly, but the source resistance has to be dropped quite a lot to achieve a viable quench frequency and the gain in sensitivity isn't as fantastic as one might hope. Still, give it a try, a single active device FM radio, pulling < 100 uA is mighty impressive!
The detector can operate with the source resistance approaching 1 M, even at extremely feeble currents it is still very sensitive. Best over-all performance was achieved with 10 K and 6.8 nF in the source circuit.
I decided to add a stage of audio gain, retaining the use of a high impedance ear piece to keep the current consumption as small as possible. I picked a super-beta transistor, the MPSA18, for the audio amplifier, and used a simple self-bias topology. This was all to keep the total receiver current consumption very small and maximise the battery life. The audio quality is quite acceptable (the usual super-regen' slope-detection distortion and quench inter-modulation with stereo sub-carrier, etc). There is no volume control, the super-regenerative receiver has an AGC-like quality because of its physics. The audio power available is on the low side, it is for quiet environment listening only; not exactly library-quiet, but not the local pub on Friday evening either!
The complete receiver pulls around 500 uA from 6 volts. Four of your average bargain-store dry cells should run the receiver for at least a month continuously. Band-name alkaline cells might run it for a very long time indeed.
Tuning is achieved with a small alignment screwdriver, or similar insulated tool. The trimmer rotor is "grounded", but hand-capacitance is still slightly present because of the very high frequencies and gains involved (i.e. minor circuit layout strays).
Some effort was put into setting up the trimmer bandspread to cover the FM broadcast band (i.e. picking C5 and C6 to make C7 tune 88-108 MHz). I spent a lot of time doing the algebra to try to come up with a way to calculate the circuit stray capacitance and the actual tank inductance by trial frequency measurements with different fixed capacitances. The solution is truly horrible; involving finding a parabola that fits three points, which means solving a determinate of a 4x4 matrix equated to zero... I gave up after a few hours of wading through my sign and subscript mistakes, the whole experience leaving me feeling somewhat defeated!
I really wanted to achieve a result I could use to write a calculator, not unlike the VFO helper one which I did the "hard way" with pencil and paper as well. It would be extremely useful to be able to determine stray tank parameters just by measuring the frequency produced after a few capacitor swaps. I'll revisit this I think. Anyway, the geometry of the coil (7 mm diameter and length) gives about 120 nH using the Wheeler formula, and my inductance meter agrees. Some empirical capacitor swapping and trimmer jig twiddling later I arrived at a bandset (C5) of 10 pF and bandspread (C6) of 22 pF, giving a tuning range of 86-110 MHz, give or take. The stray capacitance that fits this is around 4.5 pF if I've done the math right. For comparison, my capacitance meter says the detector drain looks like 21.8 pF, but that is without a drain current, having the inductance disconnected, being measured at AF, etc... I'm happy, it tunes the whole band well.
Notes
Component substitution: The J310 is obsolete, I just happen to have a lot of them. Any RF FET should be a suitable replacement. The MPF102 is quite suitable. The MPSA18 could be replaced by any NPN signal transistor if you don't mind burning a bit more current. I'd recommend a low-noise device with good gain like the BC549C or BC550C. You'll obviously need to experiment (calculate) new resistor values for the audio stage if you change the transistor, the circuit is not particularly Β independent.
You might like to play with the quench frequency by altering R1 and C1. The selectivity is at a minimum 4 times the quench frequency. Lower quench frequencies become audible and will mix down higher signal components. If you want to place the quench below 15 kHz you'll need to add much better filtering, perhaps a Sallen Key filter or two. Higher quench frequencies reduce the gain somewhat, so pushing it too high is a bad idea. The FM stereo MPX signal has energy to around 56 kHz, more if there are SCA services. Typically the quench is set around 30 kHz (8 kHz into the lower L-R sideband), but as discussed it will vary with signal strength and the modulation. The quench will tend to mix down the L-R sidebands and/or beat with the pilot tone at 19kHz. The result can be absolutely horrible to listen to, especially when the quench is getting pulled around by the modulation or the L-R sidebands are especially intense (lots of stereo difference content). For purely mono signals the recovered audio can be reasonably high fidelity if you position the slope properly. For AM signals (i.e. The Airband) the receiver is especially affective.
L2 is not especially critical, it is just an RFC to isolate the RF signal at the source from being shunted by the quench oscillation capacitor. Anything that gives > j1 kΩ of reactance should be fine, so 1.6 uH or more is sufficient, perhaps a little less would still work. The 10 pF feedback capacitor is about -j160 Ω at 100 MHz, anything at least 5-10 times larger in magnitude than that should be fine. The RFC specified has about j15 kΩ of reactance. A few turns on a ferrite bead will work, as will an RFC wound on a high-value resistor. Just make sure the inductor's self-resonant frequency is far above the frequency of interest so it is still inductive. It is difficult to make an inductor too large at VHF that would upset the circuit that isn't already looking very capacitive.
L1 and the associated C5,C6,C7 capacitors can be changed to put the receiver anywhere you like from high-HF to low-UHF. My particular receiver topped-out at 235 MHz with the 120 nH coil (indicating a stray capacitance of around 4 pF which is in reasonable agreement with the bandspread capacitor calculations), but could go much higher with smaller inductances.
Putting the radio on 10 metres is an interesting idea, it isn't especially difficult to build a miniature AM transceiver using this as the receiver, if you had enough poles on your TR switch/relay you could use the same transistor for the TX and RX, even the same tank. Similar ideas were explored years ago when frequency stability standards weren't what they are now. I've seen articles describing construction of 2 metre HTs using pairs of nuvistors or acorn tubes with free-running LC oscillators on TX and RX, switching around the cathode circuit to achieve either super-regeneration for RX or plate-modulated smooth oscillation for TX.
Direct (TRF) FM Receivers
Frequency modulation is used in radio broadcast in the bandwidth range from 88 MHz til 108 MHz. This range is being marked as “FM” on the band scales of the radio receivers, and the devices that are able to receive such signals are called the FM receivers.
Radio broadcast transmitters are using the amplitude modulation on LW, MW and SW bandwidths. According to international treaties, each of the transmitters has a 9 kHz wide broadcasting channel, therefore making maximum frequency of the information being transferred fNFmax=4.5 kHz, according to the characteristics of the AM signal. To put it more simple, the highest frequency of the sound that can be heard from the loudspeaker of an AM receiver is 4.5 kHz, all above it will be simply truncated in the circuitry. Considering the speech itself, this isn’t so important since the most important components are located below these 4.5 kHz (during the telephone transfer, all the components above 3.2 kHz are being cut, and nobody is complaining). Things stand different, however, for the transfer of music. Music has much more sound components, with their frequencies spreading up to 15 kHz, so truncating them above 4.5 kHz does deteriorate the transmission quality.
The radio-broadcast FM transmitter has a 250 kHz wide channel on its disposal, therefore allowing for the maximum frequency of the information (acc. to the characteristics of the FM signal) to be fNFmax=15 kHz. That means that music is being fully transferred and its quality is significantly better than in the case of the AM transfer. The FM transfer has some other advantages, perhaps the most significant of them being the possibility of eliminating various disturbances that are manifesting themselves as snapping, squeaking etc. The main disadvantage, however, is not the result of the frequency modulation itself, but rather of the fact that this method is being used on high frequencies, and that high-frequency electromagnetic waves behave themself as light, spreading themselves in straight line, not reflecting from the ionosphere etc. This is why obtaining this kind of radio-link requires optical visibility between the transmission and reception antennas, which is not the case for the links obtained on frequencies which are less than 40 MHz. In practical terms, it is possible to receive the SW signal from anywhere on Earth, whilst the range of an UHF link is limited to the horizon. Or, as Hamlet would say: “The quality or the range, that is the question!”
Can we have it both, somehow? Yes we can, and it is already being done, over the satellite links, using the same equipment as for the TV signal receipt and an audio amplifier connected to the audio output of the satellite receiver. For now, in the earthly conditions, those that are interested in the worldwide news will make and use the AM receivers, and music lovers will stick to the FM’s. And what can those interested in both do? Well, they make AM-FM receivers :)
The direct-type (TRF) FM receivers have never been produced, the industry started right away with the superheterodynes, made acc. to the block diagram on Pic.4.6, which will later be discussed. In amateur life, however, the direct FM receivers do exist, having very simple electronic diagrams and being easy to manufacture. These receivers have very strong positive feedback, making the intermittent oscillations in it, and are therefore being called the super-reaction receivers.
3.15.1. The Simplest FM Receiver
On Pic.3.43 you can se the electronic circuit of an extremely simple direct FM receiver. The T2 transistor together with the R1 resistor, the coil L the variable capacitor C and internal capacitances of the T1 transistor, comprises the so-called Kolpitz oscillator. The resonance frequency of this oscillator is being set by C to correspond to the one of the station that we wish to hear (meaning it has to be altered between 88 and 108 MHz). The signal, i.e. the information being used in the transmitter to perform the modulation, is extracted on the R1 resistor, and being led from it to the high-resistance headphones, over the coupling capacitor C1.
* The capacitance of the variable capacitor should be able to change from a couple of pF (Cmin) to app. 20 pF. During the testing off this device, we were using the capacitor from Pic.3.8. The legs marked as FO and G were used, the G leg being connected to the ground. When all the trimmers from the circuit on the Pic.3.8 are set to minimum capacitance, the capacitance between the FO and G legs should be adjustable between 7 and 27 pF.
* The coil L has 4 quirks of lacquer-isolated copper wire (CuL), bended to have a 4 mm internal diameter. The practical realization of this coil is explained in text connected with Pic.3.45. During the setup of the bandwidth, the inductance of the coil can be altered by changing the distance between the quirks. If the coil is stretched the inductance decreases, and vice versa. If this cannot give the desired results, new coil must be made.
* The telescopic antenna taken from a disused device can be used. If you can’t find one, you obtain very good results with a piece of isolated copper wire, about 60 cm long (the optimum length to be found experimentally).
The Simplest FM Receiver with Audio Amplifier
The radio-broadcast FM transmitters operate with output power that is much smaller than that of the AM transmitters. That is why the LF signal coming from the device on Pic.3.43 is rather small, urging the use of very sensitive headphones. They are much more expensive than the “ordinary” ones, making it better to use the cheap headphones in connection with audio amplifier. One such solution where TDA7050 IC is used is given on the Pic.3.44. The R3 resistor and capacitors C5 and C6 are to be added only if the operation of the device is unstable. There optimum values are to be found experimentally, starting with those shown in the picture.
For loudspeaker reproduction any of the previously described amplifiers can be used, e.g. that from Pic.3.21 (which we have been using, very successfully), or one of the devices described in P.E.4 and P.E.5. Since in these amplifiers a battery with voltage bigger than 3 V is used, using of R3 and C5 is obligatory. The R3 is counted from the formula
 where UBAT is battery voltage, and 0.235 mA is
the current through R1, that supplies T1 and T2. E.g.
if UBAT=9 V, it is then and the nearest existing resistor is used.
 Capacitors C5 and C6 comprise, together with R3, a pass-filter for very low frequencies, which is used to separate the HF and LF parts of the receiver. The battery itself acts as a short-circuit for the AC currents. But when it ages its resistance increases and there is no more short-circuit. That is why C3 and C4 are added, to accomplish it.
FM Receiver with one Transistor and Audio Amplifier
We have made this receiver on the experimental plate, and it was playing for days in our lab. Its electronic diagram is given on Pic.3.46. Regretfully we had to disassemble it, since we needed the plate for one of the devices described later in this book. This, too, is a reaction-type receiver, where the BF256 transistor, coil L and capacitors C, C* and C2 form the Hartley oscillator. Its frequency is being adjusted by means of the variable capacitor C to be equal to the frequency of the station that we wish to listen to. The LF signal is being taken from the R1 resistor, and led into the audio amplifier.  * The coil L is self-supporting (doesn't have the body), made of 5 quirks of CuL wire, its diameter being from 0.8 to 1 mm. It is spooled on some cylindrical object (pencil, pen etc., the best thing is the round part of a 9 mm drill), in one layer, quirks put tight to each other, as shown in the left, framed part of the picture. When the coil is finished, it is taken off the cylinder and stretched a little, so that the quirks do not touch each other. Its final length should be about 10 mm. The mid coil leg, which is to be connected to the left end of the C3 capacitor, is made by taking off couple of millimetres of the lacquer from the wire, approximately in the middle of the coil. This place is then tinned and a piece of thin wire is soldered to it. The other end of this wire is soldered onto the PCB, on its place, to be connected to the left end of C2.
* For the variable capacitor C the one from the Pic.3.8 (legs FO and G, G goes to Gnd). If you are using some other capacitor, that has bigger capacitance, and you cannot achieve the reception of the full FM bandwidth (88 til 108 MHz), try changing the value of the C*. Its capacitance is to be determined experimentally, usually being about a dozen pF.
* HFC is the high-frequency choke. Together with C2, it makes a filter that prevents the HF current to flow through the R1, simultaneously allowing for DC and LF current to go through. The muffler is, in fact, a coil that has 16 quirks of 0.6 mm CuL wire, spooled on a round part of a 3 mm drill.
* This receiver works well even without the external antenna. It can, of course, be connected to it, as shown in dashed line. Instead of antenna, a 50 mm piece of wire can also be used.
FM Receiver with (just) one Transistor
On the left side of the Pic.3.46 you can see the diagram of another very simple FM receiver, that has only one transistor as the active element. That is, as one can see, the HF part of the receiver from Pic.3.45, where the reproduction is being accomplished over the high-resistance headphones. But, as previously noticed, they are pretty expensive, therefore making it better to use the "regular" headphones and a simple amplifier, as shown on the right side of the Pic.3.46.
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