Regenerative Shortwave receiver - The Last of the Mahicans

Radio, 1997, 04, p. 20, 21

It seems that the time of regenerative receivers has gone, very long ago - somewhere in late 1960s. But in the middle of 1990s the MFJ-8100 World Band Regenerative Receiver was launched on the US market - the model "MFJ-8100". That was very unexpected - probably it was "the Last of the Mahicans", revived the interest to this type of circuits.

In the time after World war II, for lots of radio amateurs regenerative receivers were the first design they ever made. Despite the disadvantages (especially very unstable work), regenerative receivers have very simple design with minimal components and a great sensitivity, that allows to receive DX stations. In the end of 1960s, direct-conversion receivers (DCR) became popular, they were able to receive SSB and CW signals. It was the end of the regenerative receiver's time. Direct-conversion receivers became so popular because they are simple (not more complicated than a regenerative receiver), they are easy to build and working very stable and they are used in software-defined radio (SDR) designs.

To be honest, there is more disadvantages of regenerative receivers - the reception is very bad near strong radio stations because of the direct detection of AM signals or TV signals. Because the audio amplifier of a regenerative receiver is very sensitive, there is a problem with a humming sound (a low-frequency 50/60Hz mains interference). But it would be unfair to ask for more for this simple designs.

One more disadvantage of direct-conversion receivers is that they are not suitable for steady reception of AM broadcast stations (because of the frequency beating). That's why they are interesting only for hams (in nowadays they almost never use AM). Probably, the interest to regenerative receivers is rising because of the same reason.

But whatever it was, the US company MFJ produced the regenerative shortwave receiver and the kit to build it. The receiver is based on modern electronic components (FET transistors and integrated circuit), so this is very simple and stable device.

This regenerative receiver can receive AM, SSB, CW stations in frequency range from 3.5 MHz to 22 MHz. The frequency range is divided into 5 sub-ranges: 3,5...4,3. 5.9...7,4, 9.5...12, 13.2...16,4 and 17,5...22 MHz. This sub-ranges includes broadcast bands and Ham bands.

Circuit diagram of regenerative Shortwave receiver MFJ-8100

Figure 1. Circuit diagram of MFJ-8100 World Band Receiver

VT1..VT3 - J310;
IC1 - LM386;
VD1 - a LED;
R1, R3, R5, R8, R9 - 10k; R2 - a potentiometer of 10k; R4, R11 - 1k; R6 - 10 Ω; R7 - 1MEG;
R10 - a potentiometer of 100k; R12 - 15 Ω; R13 - a potentiometer of 250 Ω; R14 - 2.2k; R15 - 22 Ω;
C1, C11, C13, C15, C20, C21 - 0.1 μF; C2 - 30 pF; C3 - 47 pF; C4 - 3..258 pF; C5 - 75 pF; C6 - 1 μF; C7 - 33 pF;
C8, C9, C10, C11 - 10 nF; C12, C14 - 3.3 nF; C16 - 100 μF; C18 - 22 μF; C19 - 470 μF;
L1 - 10 μH; L2 - 3.3 μH; L3 - 1 μH; L4 - 0.47 μH;
L5 - 8 turns of 0.7 mm wire, wound on a toroid T-52-2 or on a former of 12 mm in diameter;
Antenna - a wire of 3..6 meters long.

The circuit diagram of the regenerative receiver MFJ-8100 is shown in Figure 1. It has 3 stages: RF amplifier, the regenerative detector and the audio amplifier. The circuit is designed with high input impedance FET transistors, the use of this type transistors provides in first two stages very simple schematic. Because there are no tapped coils or second windings, the simple single deck rotary switch SA1 is used.

The RF amplifier is based on transistor VT1 in the common gate configuration. The potentiometer R2, connected between the the antenna terminal and capacitor C5, provides adjustment of the signal level at the antenna. This potentiometer is mounted on the rear of the receiver. This adjustment needs only when you connect a new antenna.

The rotary switch SA1 is used to select a band, it switches coils L1..L5 in the source-drain circuit of the transistor VT1. The resonant tank, formed by this coils and by capacitors C2..C4 is connected in the output circuit of the RF amplifier, and in the same time it is connected in the input circuit of the regenerative detector (it is based on transistors VT2, VT3). The coil L1 has a high Q (quality factor) is shunted by a resistor R1 to stabilize operation of the RF amplifier and for smoother regeneration.

The common-drain stage (VT2) and the common-gate stage (VT3) provide necessary phase shift (0°) in the regenerative detector. Of course, the regenerative detector can be built using only one transistor, but in this case it takes an additional deck of the rotary switch SA1 for switching the feedback circuit. So, the additional transistor allows to avoid this problem.

The potentiometer R8 is needed to adjust the operation point of the regenerative detector (the oscillation threshold), the trimmer potentiometer R10 is used to adjust the regeneration range in a position where the oscillation amplitudes grow smoothly (the smooth regeneration control).

The audio signal is developed across the resistor R9 and passes through the bandpass filter C12R11C14 to the input of the audio amplifier.

The audio amplifier is the LM386 integrated circuit, connected in the configuration with a gain of 200.

There is 5 induction coils are used in this receiver: L1 - 10 μH; L2 - 3.3 μH; L3 - 1 μH; L4 - 0.47 μH; L5 - 0.33 μH (8 turns of 0.7 mm wire, wound on a toroid T-52-2 or on a former of 12 mm in diameter). Number of turns for coils L1..L4 can be calculated using online calculator - see How to calculate induction coils. You can use an antenna made of 3..6 meters of hookup wire.

The appearance on the market of the MFJ-8100 receiver pushed radio amateurs forward in designing of regenerative receivers. Some simple regenerative receivers were published in radio amateur magazines. It seems that the most popular design is the Charles Kitchin regenerative receiver (its circuit diagram is shown in Figure 2) that was published in EDN magazine of August 18, 1994. Actually, the detector circuit in this circuit is the common envelope detector (for reception of AM radio signals; for reception of SSB and CW signals the detector turns in the mixer). The first regenerative stage, based on the transistor VT1, this is the Q-multiplier circuit that was quite popular in 1960s. The detector circuit is based on the diode VD1. It is very important that this is the germanium diode, because of the lower forward voltage (it is about 0.2 V), and because germanium diodes have lower reverse resistance values. Diodes VD2..VD4 is used as a low voltage Zener diode to stabilize voltage of the regenerative stage.

Circuit diagram of the simple regenerative receiver

Figure 2. Circuit diagram of the simple regenerative receiver ($10 receiver has microvolt sensitivity by Charles Kitchin)

VT1..VT3 - 2N2222; VD1 - 1N34A; VD2..VD4 - 1N4148;
C1 - 2..5 pF; C2 - 5..250 pF; C3, C7 - 1 nF; C4, C6, C10 - 10 nF; C5, C9 - 10 μF; C8 - 33 μF;
R1 - 1k; R2 - a potentiometer of 100k; R3 - 470k; R4, R7 - 100k; R5 - 150k; R6 - 2k; R8 - 200 Ω;
Headphones with high impedance.

The design of the audio amplifier is very simple - it comprises only two transistors VT2, VT3. The load of the audio amplifier is a high impedance headphones.

In the first stage (VT1) can be used any modern HF transistor, and in the second stage can be used any modern small-signal transistors. The induction coil L1 is wound on a plastic form, 25 mm in diameter. The coil has 12 turns (8+4) of AWG 20 (0.8 mm) insulated hookup wire.

The interest of radio amateurs to regenerative receivers also includes and super-regenerative receivers for VHF band. The circuit diagram of this type receiver is shown in Figure 3. As all super-regenerative receivers, it can receive FM and AM signals.

Super-regenerative receiver for 100...150 MHz band

Figure 3. Circuit diagram of the super-regenerative receiver for VHF band (100...150 MHz)

VT1-VT2 - BF245; VT3 - BC107;
C1, C2, C5 - 1 nF; C3 - 100 μF; C4 - a trimmer of 3..30 pF; C6 - 25 μF;
C7 - 2..10 pF; C8 - a trimmer of 2..10 pF; C9 - 4.7 nF; C10, C11 - 10 nF; C12 - 2 μF;
R1 - 330 Ω; R2, R3 - 1k; R4 - a potentiometer of 22k; R5, R7 - 10k; R6 - 15k; R8 - 2.2 MEG;
L1 - 2 turns, L2 - 4 turns with length of 18 mm, a copper wire of 1 mm in diameter, the diameter of both coils is 12 mm;
L3 - 35 turns of 0.8 mm wire on a plastic former of 8 mm in diameter (≥ 2.2 μH);
Headphones with high impedance;
A whip-type antenna.

In this circuit, just like in the circuit diagram of MFJ-8100 receiver, the first stage is based on the FET transistor VT1, connected in the common gate configuration. The RF amplifier stage prevents RF radiation from the antenna in both circuits.

The super-regenerative detector is based on the transistor VT2, connected in the common gate configuration. Use the trimmer C8 to adjust the feedback gain to a point, where the potentiometer R4 provides smooth regeneration control. The third stage is the single transistor audio amplifier based on VT3 (BC107). It takes high-impedance headphones.

The frequency range of this receiver is 100 MHz to 150 MHz. Its sensitivity is less than 1 μV. Coils L1 and L2 are formerless (they are wound on a removable mandrel of 12 mm in the diameter), L1 has 2 turns, L2 has 4 turns of a copper wire of 1 mm in diameter. The length of the coil L2 is 18 mm. The coil L3 has 35 turns of 0.8 mm wire, it wound on a plastic former of 8 mm in diameter.

Of course, regenerators and super-regenerators are not the future of radio amateurs. But they still have a place under the Sun.

The used literature: "CQ ham radio", "Technium" and "Electron"