What is super-regenerative receiver, how it works, what are its advantages and disadvantages, in what designs it can be used? This article answers all this questions.
A super-regenerator - it is a special type of amplifier or amplifying-detecting circuit with amplifying ratio 105...106, i.e. it's almost a million! But at the same time the circuit is extremely simple! The high amplifying ratio means, that input signals with a level of 1 μV and less can be amplified to the level of 1 Volt. Of course, single-stage circuit can not reach this amplifying ratio, but super-regenerator uses different method of amplification.
Common amplifying process is a non-discrete process, input and output of an amplifier is separated. But regenerator works in different way. Its amplifying process occurs in the same resonant tank, that connected with an input, but this is also a non-discrete process.
A super-regenerator works with samples of input signal, obtained in some specific moments of time. After that, a sample will be amplified in time, and a moment later an amplified signal can be taken from the output, sometimes it can be taken from the same terminals that are connected with input signals. While amplification process is occurring, the super-regenerator doesn't react for input signals, and the next sample is obtained only after all processes are completed. This amplification method allows us to get enormous amplification ratio, inputs and outputs don't need to be separated or screened - because input and output signals are separated in time, so they cannot interact or interfere with each other.
There is an disadvantage in the super-regenerative process. By the Nyquist-Shannon sampling theorem, to transmit an undistorted envelope, it takes sampling frequency not less than double maximum modulating frequency. For broadcast AM signal with maximum modulating frequency is 10 kHz, for FM signal - 15 kHz, so the sampling frequency have to be not less than 20...30 kHz (for monophonic signals). In this case, the bandwidth of the super-regenerative receiver would be much wider, it would be 200...300 kHz. This disadvantage cannot be avoided when receiving AM signals. So, this disadvantage is one of main reasons that super-regenerative receivers were replaced with more advanced, but more complex superheterodyne receivers. In superheterodyne receivers the bandwidth is equal to maximum modulating frequency.
This disadvantage doesn't affect so much for FM receiving. FM demodulation using slope detection of super-regenerator, so it converts FM signal into AM signal and then detecting occurring. In this case a width of the resonance curve have to be not less than double deviation frequency (100...150 kHz), so it results in better matching between bandwidth and signal spectrum bandwidth.
In the past, super-regenerative receivers were built based on valves, this designs were very popular in the middle of 20th century. In that time there were not many FM broadcast radio stations, so the wide bandwidth wasn't a big disadvantage, sometimes it makes tuning easier and it helps to find widely spaced radio stations. After that, transistors were used in super-regenerative receivers. In modern time this circuits are used in RC models, in security systems, and very rare in radio broadcast receivers.
Circuit diagrams of super-regenerative receivers almost looks like circuit diagrams of regenerative receivers - they are not much different from each other. If we take a regenerative receiver and will periodically increase a feedback amount till oscillation starts, and then reduce the feedback amount till oscillation stops, so we get a super-regenerative receiver. This type of circuits uses an additional lower frequency oscillation with frequency of about 20...50 kHz, it periodically changes a feedback amount. But in most cases there is no additional frequency oscillation, the super-regenerative receiver circuit itself produces lower frequency oscillation (it is so called "self-quenched" circuit).
To better understand processes in a super-regenerator, let's see the circuit diagram, shown in Figure 1. This circuit can work as a regenerative or super-regenerative receiver - it depends on value of network R1C2.
This circuit diagram has been developed as results of experiments, and as it seems, the circuit diagram is very simple and it easy to adjust, it's providing good results.
Transistor VT1 is used as an oscillator, this is a Hartley oscillator circuit. A single tapped inductor L1 and a capacitor C1 forms a resonant tank circuit. The coil L1 has a tap, connected to the ground, the tap is closer to the bottom of the coil. It allows to match impedance between high impedance of the collector VT1 and low impedance of its base.
The way a power is applied to the transistor is kinda unusual - the DC voltage at the transistor base is the same, as DC voltage at the transistor collector. A transistor, especially a silicon transistor, can work in this mode, because it begins to conduct current since its base-emitter voltage is about 0.5 volts, and a saturation voltage is about 0.2...0.4 volts (it depends on transistor type). In this circuit diagram (see Fig. 1) the base and the collector are connected to the ground (by DC path). A supply voltage is applied to the emitter of VT1 through the resistor R1.
At the same time, a DC voltage at the emitter will be stabilize automatically at 0.5 volts - it means that the transistor works as a Zener diode with a breakdown voltage of 0.5 volts. Indeed, if the voltage at the emitter will drop down, then the transistor closes, a current through the emitter will drop down, and after that the voltage across the resistor R1 will drop down - as a result, the voltage across the emitter will be increased. But if this voltage is higher, then the transistor will conduct more current, and the voltage across resistor R1 will rise, so it will compensate the voltage across the emitter of the transistor. There is only one requirement to make this circuit works right - the power supply voltage needs to be at least 1.2 volts and more. Then the current of the transistor can be adjusted by matching the value of the resistor R1.
Let's consider how this circuit works at high frequency. An AC voltage at the lower end of the coil L1 is applied to the base-emitter junction of the transistor VT1. This AC voltage is amplified by the transistor VT1. A capacitor C2 is a bypass capacitor, it has zero resistance for high frequency signals. An impedance of the resonant tank L1C1 is a load of the collector of the transistor VT1.
The transistor inverts the phase of the signal, and then the phase is inverted again by the coil L1 (the coils works as an inverting transformer), so the whole phase shift is therefore 360°, it means that a balance of phases is achieved.
If the transistor VT1 has enough gain, then a balance of amplitudes is achieved too. The gain of the transistor depends on the emitter current, this current can be easily adjusted by changing value of resistor R1 (replace it with a potentiometer, connected in serial with a resistor).
The circuit has some advantages - it is simple, it easy to adjust, and it has very small current consumption - the transistor takes only as much current as it needs to amplify the signal. The circuit has very smooth regeneration control, it allows to go into and out of oscillation smoothly. By the way, the regeneration control is operating in low frequency part of the circuit, therefore, it can be placed in any suitable place. The regeneration control does not affect much on the frequency of the resonant tank L1C1, because the voltage across transistor VT1 remains the same (about 0.5 volts), so the inter-junction capacitances are almost not changed.
Let's get back to the super-regenerator. Let's see what happen if the power supply (one long pulse) is applied to the circuit in a moment of time t0, as shown in Figure 2, above. Even if the transistor VT1 has enough gain and the feedback value is optimal, oscillations cannot start immediately, it will building up by the exponential law for a time Ts. The fading of oscillations will happen by the same law for a time Te.
The process of building up and fading of oscillations can be described by formula
Utank = U0*exp(-rt/2L),
where U0 - an initial voltage across the resonant tank L1C1;
r - equivalent loss resistance of the resonant tank;
L - inductance of the coil L1;
t - time.
It's easy to describe the process when oscillations fading, in this case r = rloss (a resistance of losses of the resonant tank), see Fig. 3. But when oscillations building up, it is much complicated to describe: the transistor adds some negative resistance to the resonant tank, - rnr (the feedback network compensates the losses), so the equivalent loss resistance of the resonant tank become negative. The "minus" sign will go, and the law for building up the oscillations can be described as following:
Utank = Us*exp(rt/2L),
where r = rnr - rloss
From this formula we can find the time for building up the oscillations, it starts at amplitude Us and it builds up till amplitude U0, after that the transistor VT1 begins to work as a voltage limiter, the gain of the transistor getting lower, and amplitude of oscillations stabilizes:
Ts = (2L/r)*Ln(U0/Uc).
As we can see, the building up time is proportional to the logarithm of a value, that is inversely proportional to the level of a signal in the resonant tank. The stronger signal, the less build up time.
If pulses of power supply are periodically applied to the super-regenerator, with a quench frequency of about 20...50 kHz, then groups of oscillation will occur in the resonant tank (see Fig .4). The width of any group depends on the signal amplitude - the less build up time, the longer a group. If this groups of oscillation will be detected, then we get a demodulated signal with an amplitude, proportional to the average value of envelope of oscillation groups.
The transistor can have a small gain (units or tens) that is only enough to start and sustain oscillations, but in the same time, the whole gain of the super-regenerative receiver is very high - it is equal to the amplitude of output signal divided by the amplitude of input signal.
The above described working mode of the super-regenerative receiver is called nonlinear or logarithmic mode, because the output signal is proportional to the logarithm of the input signal. It adds some distortion to the output signal, but it does a good job at the same time - because the super-regenerator is more sensitive to weak signals, and is less sensitive to strong signals, this effect works as an automatic gain control.
The super-regenerative receiver can work in a linear mode if the pulse of power supply (see fig. 2) is less than build up time of oscillations. Oscillations have not enough time to get the maximum amplitude, and the transistor will work in a linear region. Then an amplitude of a group will be directly proportional to the signal amplitude. But this mode is very unstable, due to any changes of the transistor gain or equivalent resistance r of the resonant tank will result in reduced of groups amplitude - it means that the gain of the super-regenerative receiver will drop, or the super-regenerative receiver will operate in nonlinear mode. Because of this, the linear mode is used very rare.
It is not necessary to switch the power supply to get groups of oscillations. The same result can be obtained by applying the quench frequency to the grid of a valve, the base or gate of a transistor - the gain will be modulated, so the feedback will be modulated too. Square pulses of the quench frequency is not optimal, it is better use the sine wave, but it is much better to use the sawtooth wave with the smooth ramp and the sharp drop. The last one allows to go into oscillations very smoothly, the bandwidth will be narrowed. The regeneration gives the gain. Oscillations builds up slowly and then faster. Oscillations are fading very fast.
Super-regenerative receivers with automatic quench is mostly widely used, there is no any additional quench oscillator in this circuits. They are operating only in the nonlinear mode. The automatic quench, or self quench can be easily obtained in the circuit from Figure 1, it takes only the time constant of the network R1C2 should be more than the time of build up of oscillations.
After that, next events will take place: the current through the transistor will rise up because of oscillations, but the charge of the capacitor C2 will support oscillations for a while. When the capacitor discharges, the voltage at the emitter of the transistor VT1 will decrease, the transistor stops conducting, and oscillations stop. The capacitor C2 slowly starts to charge through the resistor R1 till the transistor VT1 starts conducting, and then a new group of oscillations appears.
Oscillograms of the voltage at the emitter is shown in Figure 4 as they can be seen with a wide-bandwidth oscilloscope. Levels of voltage 0.5 and 0.4 volts are absolutely arbitrary - they are depends on transistor type and its mode of operation.
The width of a group of oscillation is now depends on the charge of the capacitor C2, it means that the group width is constant. What would happen when a signal from the antenna is applied to the resonant tank? When the amplitude of the signal grows, the build up time is getting shorter, so groups follow more often (the quench frequency is growing). We can use a separate envelope detector to detect groups of oscillations, in this case the average level of the output signal is proportional to the logarithm of the input signal. But instead of the separate detector, the transistor VT1 can detect the envelope of the signal (see Figure 1) - an average voltage at the emitter is getting down while the amplitude of the signal grows.
And last, what would happen if there is no input signal? It would be all the same, but the amplitude of oscillations in any group will start at random amplitude, that is depends on the noise level in the resonant tank of the super-regenerator. The quench frequency is minimum, but it is very unstable - the period of repetition is changing chaotically. The gain of the super-regenerator is maximum, and we can hear very loud noise in headphones. If we tune to a station, the noise sharply drops off. Therefore, the sensitivity of the super-regenerative receiver is extremely high - it depends only on its internal noise level. The additional theory about super-regenerative receivers can be found in references [1, 2].
Let's now consider real circuit designs of super-regenerative receivers. There is lots of them on the Internet and in the literature. For example, a simple circuit diagram was published in the magazine "Popular Electronics" of March, 1968 year. This circuit has a quite high supply voltage of 9 V, it provides a high level of oscillation in the resonant tank, as a result, it gives high gain to the circuit. But here is the disadvantage - the circuit radiates high level of interference at the resonant frequency, because the antenna is directly coupled with the resonant tank. It is better use this design somewhere far from town where is no other radio receivers.
The circuit diagram of the simple FM radio receiver with low voltage power supply is shown in Figure 5. This design is based on the circuit of Figure 1. The induction coil L1 works an antenna, the coil is made of copper wire 1.5 mm (and more) in diameter, it formed in shape of a loop. The diameter of the loop is 90 mm. The variable capacitor C1 is used for frequency tuning. Because it is hard to make a tap of the loop, the Colpitts oscillator is used in this circuit. The feedback signal is applied to the emitter of the transistor VT1 through the capacitive divider formed by C2 and C3.
The quench frequency is determined by the total resistance of resistors R1, R2, R3 in series and by the capacitor C4. If the value of the capacitor C4 will be reduced to some hundred picofarads, the circuit will work as a regenerative receiver, because the quench oscillation will stop. We can add a switch to the circuit, and replace the capacitor C4 with two capacitors in parallel - 0.047 μF and 470 pF. Then the circuit could work in two modes - as a regenerative or super-regenerative receiver. The regenerative mode provides better audio quality with less noise, but it takes stronger radio signals. The amount of feedback can be adjusted by the potentiometer R2.
This circuit in super-regenerative mode radiates very small level of interference, because the amplitude of groups oscillations in the resonant tank is about 0.1 volts, and the small loop antenna is very inefficient for transmission.
The two-stage audio amplifier is based on transistors VT2 and VT3 of different conductivity type. The headphones with the impedance of 50...200 Ω are connected to the collector of the output transistor VT3.
Fig. 5. Low-voltage power supply FM Radio for 66...108 MHz
VT1, VT3 - KT315A (n-p-n silicon transistors with HFE = 20...90, ft = 250 MHz)
VT2 - KT361A (p-n-p silicon transistor with HFE = 20...90, ft = 250 MHz)
C1 - 2..7 pF; C2 - 4.3 pF; C3 - 15 pF; C4 - 0.05 μF; C5 - 0.1 μF; C6 - 20 μF x 6V;
R1 - 1K; R2 - potentiometer 6.8K; R3 - 1.8K; R4 - 47K; R5 - 15K;
VD1 - D18 (the germanium diode);
BF1 - headphones with the impedance of 50...200 Ω
L1 - a loop with diameter of 90 mm, made of thick copper wire 1.5 mm and more in diameter.
The bias voltage for the transistor VT2 is derived across the resistor R4 from the emitter circuit. As it was mentioned before, the stable voltage at the emitter of the transistor VT1 is about 0.5 volts. The capacitor C5 passes the audio signal to the base of the transistor VT2.
Pulses of quench frequency (30...60 KHz) is not filtered out, so the audio amplifier operates in impulse mode - the output transistor opens and closes completely. The quench frequency can't be hearing in the headphones, but the pulse sequence includes audio pulses, which we can hear. The diode VD1 is used to short the extra current across the headphones in the moment of closing the transistor VT3. This diode removes voltage spikes, it provides a little better quality and increases the volume of the sound.
The receiver uses a battery of 1.5 V or an accumulator of 1.2 volts, the current consumption is about 3 mA. If necessary, it can be adjusted by matching the resistor R4.
To adjust the circuit, check for oscillations by turning the potentiometer R2. If you hear very loud noise, then it is OK. Another way to check for oscillations - using an oscilloscope to watch the signal across C4 - it should looks like a sawtooth signal. The quench frequency depends on value of the capacitor C4 and the potentiometer R2. Change the value of C4, if necessary. The quench frequency should not be set around 19 kHz, 38 kHz and 54 kHz (in case of the polar-modulation system avoid frequencies of 31.25 kHz and 62.5 kHz - this is the subcarrier frequency and its 2nd harmonic) - this is a pilot signal and its harmonics. It helps to avoid frequency beating that hinder reception.
Now we need to adjust the the frequency range, do it by change the size of the loop antenna - increase its diameter to reduce the frequency, and vise versa. Use a tiny wire to increase the frequency.
The disadvantage of this circuit is that it slightly changes the frequency as the hand is moved near the antenna. But it is the common disadvantage of all these super-regenerative circuits where the antenna is directly connected to the resonant tank.
This disadvantage can be eliminated by using an RF amplifier that will "isolate" the resonant tank from the antenna. Another good side of this RF amplifier is that it totally blocks the radiation from the antenna - it will not create interference for other receivers operating nearby. The gain of the RF amplifier can be quite low, because the super-regenerative receiver has very high sensitivity. A one-transistor RF amplifier circuit can be used here, in the configuration with common base or common gate (in case of a FET transistor - see the reference ).
The circuit diagram, shown in Figure 6, is the super-regenerative receiver, it was developed with the intention to get low battery consumption - its current consumption is about 3 mA from 3 V batteries. The current consumption can be reduced to 0.16 mA by removing the RF amplifier from the circuit. The sensitivity of this circuit is about 1 μV.
A signal from the antenna is applied to the emitter of the transistor VT1 which is connected in the common-base configuration. Because the common-base configuration has a low input impedance, and take into account the value of the resistor R1, we get the input impedance of the receiver is about 75 Ω. This impedance matches with the impedance of coaxial cable, this cable can be used to connect an external antenna to the receiver. The external antenna can be used for better reception from greater distances (≥ 100 km). In good reception conditions can be used any suitable antenna. The capacitor C1 is used as a simple high-pass filter, it blocks SW signals.
Fig. 6. FM receiver with low battery consumption
VT1, VT2 - KT3109V (p-n-p silicon transistors with HFE ≥ 15, ft = 600 MHz)
VT3..VT5 - KT315V (n-p-n silicon transistors with HFE = 20...90, ft = 250 MHz)
VT6 - MP37B (an old n-p-n germanium transistor with HFE = 25...50 , ft = 1 MHz)
VT7 - MP39B (an old p-n-p germanium transistor with HFE = 20...60 , ft = 0.5 MHz)
C1 - 27 pF; C2, C8 - 0.1 μF; C3 - 24 pF; C4 - 2..7 pF; C5...C7 - 10nF; C9 - 0.68 μF; C10, C11 - 68 μF x 6 V;
R1 - 100 Ω; R2 - 4.7 K; R3 - 1.8 K; R4 - potentiometer 33 K; R5 - 18 K; R6, R7 - 15K;
R8 - 3.9 K; R9 - 240 K; R10 - 130 K; R11 - 68 K; R12 - 2.2 Meg;
R13 - 4.3 Meg (match it value); R14 - potentiometer 4.7 Meg;
L1 - 3 turns of 0.25 mm (AWG 30) wire;
L2 - 5.75 turns (the tap at the 2nd turn from the bottom) of 0.6 mm (AWG 23) wire;
L1 and L1 are wound on the same frame of 5.5 mm in diameter, the distance between them is 2 mm;
Headphones with impedance 1000 Ω and higher.
The transistor VT1 operates with the same collector voltage as the voltage at the base - it is about 0.5 volts. It stabilizes the operating point of the transistor and it makes unnecessary any adjustment. The coupling coil L1 is connected in the collector circuit, the coil L1 is wound on the same frame as the coil L2. The coil L1 has 3 turns of copper enameled wire with a diameter of 0.25 mm (AWG 30), the coil L2 has 5.75 turns of copper enameled wire with a diameter of 0.6 mm (AWG 23), it tapped at 2nd turn from the bottom. The diameter of the frame is 5.5 mm, the distance between coils is 2 mm. A brass or ferrite slug is inserted in the frame, use it to adjust the frequency range. But there is another way to adjust the frequency range without using a slug - in this case a trimmer capacitor (3..25 pF or 8..30 pF) can be used instead of the capacitor C3.
The super-regenerative stage based on the transistor VT2, it is similar to the previously described circuit (see Fig. 1). Use the potentiometer R4 to set the operating point. The quench frequency depends on the value of the capacitor C5. The low-pass filter R6C6R7C7 prevents the quench frequency from going through the audio amplifier, to avoid its overload and distortion of the signal.
The audio signal at the output of the super-regenerative stage has very low amplitude, so it requires two stage audio amplifier. But in this circuit, transistors of the audio amplifier work in micro-current mode (note the high value resistors R9, R10, R11), because of this the audio amplifier has lower gain, so it takes three stage audio amplifier to get required amplification. The audio amplifier is based on transistors VT3...VT5 connected directly to one another. Resistors R12, R13 provide a negative feedback, that stabilizes the operation point. The capacitor C9 reduces the negative feedback, the potentiometer R14 allows to adjust the amplification of the amplifier.
The output stage of the audio amplifier uses a pair of germanium transistors VT6, VT7 of opposite type conductivity in the push-pull circuit. This transistors works without any bias voltage, but there is no crossover distortion problem, because germanium transistors have very low base-emitter voltage 0.15 volts (compare it with 0.5 volts for silicon transistors) and because the low-pass filter does not suppress the quench frequency completely, so the quench frequency (with smaller amplitude) works similar to the generator for magnetizing in a tape recorder - it blurs the crossover distortion.
This receiver takes headphones with impedance 1000 Ω and higher, to keep low battery consumption. If it's not a goal, then any one suitable audio amplifier and low-impedance headphones can be used with this receiver.
The start of the adjustments begins with the audio amplifier. Disconnect it from the circuit. Match the resistor R13 to get voltage at the bases of transistors VT6, VT7 equal to half of supply voltage (1.5 V). Be sure there is no self-excited oscillation in the audio amplifier circuit. Feed a sine wave audio signal with the amplitude of some mV to the input of the audio amplifier to see if there is no crossover distortion and the output signal is symmetric.
Connect the super-regenerative stage to the circuit, rotate the potentiometer R4 to hear a noise in the headphones (the output noise voltage is about 0.3 volts). It's worth to note that any HF silicon p-n-p transistor can work in the RF amplifier and in the super-regenerative stages.
Connect the antenna to the resonant tank using a capacitor of 1 pF or less (or use a coupling coil), then try to tune to a radio station. After that, connect the RF stage to the circuit, adjust the frequency band by changing the the inductance of the coil L2 or match the capacitor C3.
V. Polyakov, Moscow