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Regenerative Receiver Design Blocks

Updated on August 3, 2013

Regenerative Receiver Design Blocks

Here is a collection of design blocks for creating a regenerative receiver to your own specification. From maximum frequency stability and selectivity to minimum component count and best sensitivity, the choice is yours. Design notes are also provided allowing you to tailor the component values further.

The Core Requirement

A stable Q Multiplier circuit is the core requirement for any regenerative receiver. It is particularly important that the amplifier in a Q Multiplier provides small signal compressive gain. The gain the amplifier provides must actually decrease as the signal strength increases. This is necessary to provide smooth control of the regeneration effect as you transition through the threshold of oscillation. To do this directly without resorting to tricks such as changes in bias level with signal strength is quite difficult using BJT transistor technology. However the circuit in figure 1 is a direct solution.

Figure 1

 Q1 and Q2 form an AC linked differential pair.  The amount of regeneration is controlled by altering the voltage at the base of Q2. L2 and C9 form the LC resonator.  The Q of the LC resonator is boosted by positive feedback from the tickler coil.
Q1 and Q2 form an AC linked differential pair. The amount of regeneration is controlled by altering the voltage at the base of Q2. L2 and C9 form the LC resonator. The Q of the LC resonator is boosted by positive feedback from the tickler coil.

The input impedance at the base of Q1 is about 5k at a collector current of 1mA. The 10:1 ratio of the capacitor tap C3,C4 increases the input impedance by approximately the square of that. Giving an apparent damping impedance of 500k across the LC resonator and a minimal impact on Q. The emitter resistors can vary between 220 ohms and 10k with other component being altered accordingly. At high collector currents the transition to oscillation is very sharp and you need to decrease C4 to 33pF. At low collector currents the transition into oscillation is very smooth but noise levels are higher. An asymmetry in the emitter resistors can be beneficial in providing smooth regeneration.

A Q Multiplier which shows bad backlash is shown in figure 2. The problem with this circuit is that the small signal gain increases with signal strength. This results in the circuit suddenly transitioning from a non-oscillating state to a very strongly oscillating state as the regeneration control is advanced. All is not lost however when using it to create regenerative receiver. It needs to be coupled to an AM detector that gives increasing damping of the LC resonant circuit with signal strength.

Figure 2

A BJT Armstrong topology Q Multiplier.  Some advantages of this circuit are low noise, simplicity and low phase shift.  The disadvantages are the abrupt transition from the non-oscillating to oscillating state and possible overload conditions.
A BJT Armstrong topology Q Multiplier. Some advantages of this circuit are low noise, simplicity and low phase shift. The disadvantages are the abrupt transition from the non-oscillating to oscillating state and possible overload conditions.

AM Detectors

The requirements for AM detection are also somewhat strict. The input impedance of the AM detector must be stable over time to prevent low frequency oscillations from occurring. Ideally it should also have a slight dynamic damping effect. The input impedance of the AM detector should decrease with signal strength helping to make control of the regeneration effect even smoother. For non-ideal Q Multipliers such as the one in figure 2 a strong dynamic damping effect is required to bring the system under control. The best place to do AM detection in a regenerative receiver is directly at the LC resonator. Using a buffer stage is likely to cause excess noise or instability.

The best AM detector to use is voltage source biased drain bend FET detector, biased just at the pinch off voltage. This gives least damping of the LC resonator and maximum frequency stability. See figure 3.

Figure 3

Pinch off drain bend FET AM detector.  The FET is biased just to its pinch off voltage by adjusting the potentiometer R6.  It is important that the source is biased by a relatively low impedance circuit or the input impedance will be unstable.
Pinch off drain bend FET AM detector. The FET is biased just to its pinch off voltage by adjusting the potentiometer R6. It is important that the source is biased by a relatively low impedance circuit or the input impedance will be unstable.

A somewhat more sensitive circuit can be made by operating the FET at higher drain current. See figure 4.

Where you need a strong dynamic damping effect such as with the Q Multiplier in figure 2 the BJT square law detector in figure 5 is ideal.

Figure 4

Drain bend FET AM detector.  This circuit has better sensitivity than the pinched off circuit. However is also has a higher dynamic damping effect on the LC resonator that reduces selectivity somewhat.
Drain bend FET AM detector. This circuit has better sensitivity than the pinched off circuit. However is also has a higher dynamic damping effect on the LC resonator that reduces selectivity somewhat.

Figure 5

A BJT square law AM detector that give strongly increasing damping of the LC circuit with signal strength.  Using a high Ft transistor for Q1 will give less damping and greater frequency stability.  Even a 2SC3356 (Ft=7Ghz) is Ok in this role.
A BJT square law AM detector that give strongly increasing damping of the LC circuit with signal strength. Using a high Ft transistor for Q1 will give less damping and greater frequency stability. Even a 2SC3356 (Ft=7Ghz) is Ok in this role.

Regenerative Receiver Circuits

Two possible regenerative receiver circuits built from the circuit blocks are given in figures 6 and 7.

Figure 8 is a useful audio filter circuit for regenerative receivers. It allows you to reduce 5KHz and 10KHz heterodyne tones from strong adjacent carriers and gives a treble boost that is useful with high selectivity circuits.

Figure 6

The Sens-o-dyne regenerative receiver.  This circuit has very good selectivity and frequency stability.
The Sens-o-dyne regenerative receiver. This circuit has very good selectivity and frequency stability.

Figure 7

Dynamically damped regenerative receiver.  The Q Multiplier relies on the BJT square law AM Detector to provide sufficient dynamic damping to give smooth control of regeneration.  In practice this is not a problem.
Dynamically damped regenerative receiver. The Q Multiplier relies on the BJT square law AM Detector to provide sufficient dynamic damping to give smooth control of regeneration. In practice this is not a problem.

Figure 8

A useful audio filter circuit.  The audio input  should have a relatively low impedance to give correct filter performance.  The output from an emitter follower is fine.
A useful audio filter circuit. The audio input should have a relatively low impedance to give correct filter performance. The output from an emitter follower is fine.

Note about the voltage supply

All the circuits shown above must be powered from a stable voltage source. The gain of the Q Multipliers is directly affected by the supply voltage. The AM detector and filter circuits have no supply line noise rejection ability. The use of a voltage regulator chip is advised. If you are using batteries and don’t wish to use a voltage regulator then use separate battery packs for the regenerative receiver and audio power amplifier.

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      macgenio 3 years ago

      Hi! I'm following your articles about radio technology and design.

      I found them very interesting and useful!

      I ope to find soon other new.

      Thanks a lot from an Italian radiolover!

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