Regenerative and Reflex Receivers
Action And Principle Of Regeneration
Regeneration is the action by which a part of the energy from the plate circuit of a tube is fed back into the grid circuit of the same tube. The plate circuit energy is added to the energy already in the grid circuit.
Fig. 1 shows a tube having one inductance coil in the grid circuit and another inductance coil in the plate circuit. The energy in the plate circuit is several times greater than the energy in the grid circuit. The grid circuit is called the input circuit of the tube and the plate circuit is called the output circuit of the tube. The signal coming to the tube is introduced into the grid circuit and the voltage changes in the signal cause corresponding voltage changes on the grid of the tube. These voltage changes on the grid control the flow of current in the plate circuit.
The strength of the output from the tube is proportional to the strength of the signal input. If the signal voltage impressed on the grid is made stronger by any means, it will be followed by a greater output in the plate circuit. Signal strength may be increased through many causes outside of the receiver. For example, a stronger signal will be received from a nearby or powerful broadcasting station than from a distant or weak broadcasting station.
By means of regeneration the tube itself is made to increase the input voltage. In Fig. 2 the two coils of Fig. 1 have been rearranged so that they are brought close together. The one magnetic field now includes both coils. They are coupled and energy from the plate coil is fed back into the grid coil.
If the grid circuit of the tube is tuned to resonance with the frequency of the incoming signal, as is the case in radio frequency amplifiers and in detectors, the inductive reactance and the capacitive reactance in the grid circuit neutralize each other and leave only the resistance of the conductors in the circuit to oppose flow of current. Were it possible to reduce this resistance to zero nothing would remain to oppose the current flow and when oscillating voltages were once introduced into the grid circuit they would continue to flow indefinitely.
It is evident that the same results may be secured by adding just enough energy to that already in the grid circuit so that this additional energy overcomes the loss due to resistance. As an example, supposing the resistance of the grid circuit caused a power loss of five watts and suppose that just enough of the plate circuit energy were fed into the grid circuit to make up for this five-watt loss. Then the signal voltage originally brought into the grid circuit would set up oscillations which would continue on and on without diminishing.
It is possible to feed energy from the plate circuit back to the grid circuit and reinforce the voltages in the grid circuit because the frequency in the plate circuit is exactly the same as the frequency in the grid circuit.
After enough plate circuit energy has been fed back to just overcome the grid circuit resistance still more may be fed back to increase the grid circuit voltages to almost any desired extent. The power fed back from the plate circuit may be made sufficient to maintain oscillations in the grid circuit without the help of any outside voltage, such as an incoming signal voltage. Under such conditions the tube will maintain oscillations in its circuits as long as the filament batteries and plate batteries hold out. The tube is then oscillating.
As long as the grid circuit absorbs power from the incoming signal we have regeneration with a feedback in use. But just as soon as the feedback energy is great enough to sustain oscillation without outside help we have gone beyond regeneration and have oscillation in the tube. The feedback energy is then able to keep the tube's circuits in continuous oscillation.
It is apparent that regeneration allows an exceedingly weak signal to be built up until it is as effective as a powerful signal. Thus regeneration increases the sensitivity of a receiver many times. Regeneration also increases the selectivity of the receiver as may be seen from Fig. 3. The curve at the left side indicates the response of a receiver to various frequencies when the receiver is tuned to a frequency of 750 kilocycles. When tuned to this frequency the circuits have the least possible reactance at 750 kilocycles. At points below and above this frequency the response of the receiver will not be so powerful because the reactance has not been eliminated by the process of tuning to resonance.
The effect of regeneration is shown at the right in Fig. 3. The frequency of 750 kilocycles is being fed back from plate circuit to grid circuit and the signal at this one frequency is built up to great volume. Since the feedback is occurring only at the tuned frequency other frequencies below and above the resonant points are not increased in strength. Therefore the relative strength of the 750 kilocycle signal with regeneration is several times as great as without regeneration. Any signals attempting to enter the receiver at other frequencies are relatively weaker under the conditions shown at the right in Fig. 3.
The feedback of energy from the plate circuit to the grid circuit may be made through inductive coupling, through capacitive coupling or through resistance coupling. Inductive coupling and capacitive coupling are the types generally used because resistance coupling is not effective at radio frequencies. With the more commonly used methods of obtaining regeneration an inductive coupling between two coils or two parts of one coil is employed. Capacitive coupling through the capacity existing between the plate and the grid inside of the tube is used in a few instances.
There is always a feedback of energy from plate circuit to grid circuit through the capacity between the tube's plate and grid. This capacity feedback is independent of any external means for additional feedback. Since the reactance of any capacity is less at high frequencies than at low frequencies, the capacity feedback at high frequencies will be much greater than at low frequencies because of this change of effective reactance in the tube's internal capacity.
Regeneration and oscillation occur more easily at high frequencies than at low frequencies. Therefore less feedback will always be required to produce regeneration at the high frequencies or low wavelengths. Any control for regeneration provides for increasing the feedback as the frequency is lowered. The lower the frequency or the higher the wavelength the more regeneration will always be needed to produce a given strength of signal in the tube's output.