Regenerative and Reflex Receivers
It is possible to use a single vacuum tube for the amplification of two different frequencies at one time, this being called reflexing. The principle is shown by the diagram in Fig. 1. The operation of a reflex amplifier is shown in Fig. 2. The two frequencies travel as follows: High frequency or radio frequency is introduced from the winding 1 which is coupled with the winding 2 to form a radio frequency transformer. Voltage changes in winding 2 are impressed on the grid of the tube. The grid circuit is completed to the filament through the bypass condenser A which carries the high frequency around the high impedance of the iron core transformer.
The high frequency output from the plate of the tube passes through the winding 3 which is coupled with winding 4 to make a transformer. The high frequency circuit is completed through the bypass condenser B from winding 3 to the filament circuit of the tube. Plate voltage from the B-battery is applied through the winding of the right hand air-core radio frequency transformer. The amplified high frequency appears in the winding 4.
Still referring to Fig. 2 low frequency or audio frequency is introduced through the left hand audio frequency transformer. The audio frequency voltages pass to the grid of the tube through winding 2, the grid circuit being complete through the winding of the audio frequency iron-core transformer and winding 2 of the air-core radio frequency transformer. Bypass condenser A is of small capacity which offers a very high reactance to the low frequency, therefore does not bypass it but forces it through the winding of the audio transformer. Winding 2 of the air-core radio frequency transformer is of comparatively few turns, has no iron core, and is therefore of low reactance to the audio frequency voltages and offers practically no opposition.
The audio frequency output from the plate of the tube passes through winding 3 of the right hand radio frequency transformer. The reactance of this winding is very low to the audio frequency and it passes through with practically no opposition until the right hand bypass condenser B is reached. This condenser, being of small capacity, offers such great reactance that the audio frequency is forced through the winding of the right hand iron-core audio frequency transformer. The audio frequency output then appears in the secondary of this transformer.
Reflex receivers provide two paths for the grid voltages and two paths for the plate currents of all reflexed tubes. One path carries the radio frequency current. This path is of low reactance to the radio frequency and of high reactance to audio frequency. The other part carries audio frequency current and is of high reactance to the radio frequency. The two paths meet in the tube and in the batteries. The radio frequency circuit is always carried around the windings of iron-core transformers, speakers, etc. by bypass condensers.
Reflex Receivers.- A complete single-tube reflex receiver with crystal detector is shown in Fig. 3. Windings 1, 2, 3 and 4 of Fig. 3 correspond to similarly numbered windings of Fig. 2. The radio frequency output of transformer 3-4 passes through the crystal detector. The rectified output from the detector passes through the primary of the audio transformer. The audio frequency output from the secondary of the audio transformer reaches the grid of the tube through winding 2 and is amplified by the tube. The audio frequency output from the plate of the tube passes through winding 3 and to the jack to which is connected the speaker or headphones. Tuning is accomplished with the variable condenser across winding 2. Winding 1 is in the antenna circuit. The bypass condensers C in these receivers are usually of .001 microfarad capacity. The best value for the bypasses may be found by experimenting with condensers of from .00025 to .002 microfarad capacity.
Fig. 4 shows the circuits for a two-tube reflex receiver. This receiver employs a tube for its detector but is otherwise the same as the arrangement of Fig. 3. In Fig. 3 the radio frequency output from winding 4 is carried through the crystal. In Fig. 4 the output of winding 4 is carried to the grid of the detector tube. The output from the plate of the detector tube is carried to the primary of the audio frequency transformer, the B-battery or plate voltage supply for the detector being connected to the other end of this primary winding.
The output from the secondary of the audio frequency transformer is carried to the grid of the left hand amplifier tube and is amplified at audio frequency. From the plate circuit of this left hand tube the amplified audio frequency passes through winding 3 to the jack just as in Fig. 3. Tuning is accomplished by two variable condensers, one across winding 2 and the other across winding 4.
A three-tube reflex receiver is shown in Fig. 5. Tube number 1 is the first radio frequency tube and the second audio frequency tube. Tube number 2 is the second radio frequency tube and the first audio frequency tube. Tube number 1 thus carries the lightest radio frequency load and the heaviest audio frequency load. In tube number 2 this condition is reversed by carrying the heaviest radio frequency load and the lightest audio frequency load. This division of load is the inverse duplex principle devised by David Grimes. Tube number 3 is the detector and is not reflexed.
In this three-tube receiver tuning is done with the three variable condensers. Bypass condenser A may be from .002 to .005 microfarad capacity, bypass B is of one microfarad or even greater capacity, bypasses C are of .001 microfarad capacity, and bypass D is of .002 microfarad capacity.
Regeneration may be applied to any reflex receiver either in the detector circuit or in the radio frequency tube circuits. Connections of tickler coils for regeneration are shown by broken lines in Fig. 4. While only the tickler coil method is shown, any kind of regeneration control may be applied to these receivers.
Reflex receivers have the advantage of saving in the number of tubes required for a given amount of amplification. For example, the single-tube receiver of Fig. 3 consists of one radio frequency stage, one audio frequency stage and a crystal detector. The receiver of Fig. 4 consists of one audio frequency and one radio frequency stage with a tube detector. The receiver of Fig. 5, while using only three tubes, provides two radio frequency stages and two audio frequency stages. A crystal detector might be substituted for the tube detector in Figs. 4 and 5. Reflex receivers are generally rather unstable and are more inclined to oscillate than receivers using separate tubes for radio frequency and audio frequency amplification.