Regenerative and Reflex Receivers
The super-regenerative receiver is designed to allow maximum regeneration while automatically preventing free oscillation. There are several variations of the principle of super-regeneration, one type being shown in Fig. 1.
Neglecting for the present the coils A and B and the condensers M and N at the lower part of the diagram it will be seen that the receiver is of the ordinary regenerative type. Signal energy is collected by the loop, the loop tuning being accomplished by the variable condenser. The grid coil is tuned with the variable condenser C. Feedback of energy from plate circuit to grid circuit is secured by coupling the tickler coil to the grid coil.
With the parts of the circuit so far considered it is possible for the tickler coil to couple with the grid coil closely enough to produce regeneration which will almost instantly build up into oscillation. Maximum amplification will be secured just before regeneration changes into oscillation. In actual operation the receiver allows regeneration to start and to build up to a point that sends great energy into the grid circuit of the tube. While regeneration is continuing to build up, but before it changes into oscillation, the additional circuit in the lower part of the diagram absorbs so much power from the grid circuit that regeneration is completely stopped. The absorption of power is then stopped and regeneration once more starts building up.
Coils A and B together with their condensers M and N allow the tube to act as an oscillator. Coil B is in the plate circuit and coil A is in the grid circuit. The two are coupled together so that continuous oscillations are generated. The frequency of these oscillations is determined by the inductances of the coils and by the capacities of the condensers M and N. The frequency of the oscillations is made of some value above audibility, fifteen thousand to twenty thousand cycles being suitable values.
The oscillation voltages are impressed on the grid of the tube so that the grid voltage is alternately positive and negative. While the grid voltage is negative the regenerative action in the grid and tickler coils builds up rapidly and applies the signal to the grid of the tube with great power. As soon as the oscillator voltage swings to the positive half of a cycle the grid becomes positive and absorbs power. This absorption of power stops the regeneration just before it changes to oscillation. This action keeps on as long as the receiver is in operation.
The super-regenerative receiver is difficult to control, very critical in its adjustments, and because of the peculiar action in the grid circuit it lacks selectivity. It's great advantage is in the extreme amplification possible from single tube. When audio frequency amplification is added it becomes necessary to place a filter circuit between the super-regenerative tube and the first audio frequency tube to prevent the oscillations from coming through and being amplified with great volume.