Regenerative and Reflex Receivers
Methods Of Obtaining Regeneration
It is plain that the amount of feedback must be under the control of the operator. For strong incoming signals little or no feedback may be required while for very weak signals the maximum allowable feedback and the maximum regeneration must be used. There is always a capacitive feedback through the plate to grid capacity of the tube and the amount of regeneration through this tube capacity varies according to the construction of the tube. The added means for feedback must be controlled so that the feedback energy combined with the energy passing through the tube capacity will equal the desired or needed value.
Regeneration is usually applied only to the detector tube and in the following diagrams showing the various methods of obtaining regeneration the plate of the tube is shown connected to the primary winding of an audio frequency transformer as would be the case with the detector plate. If choke coil coupling or resistance coupling is used in the audio amplifier following the detector, a choke or a resistance would be substituted for the audio frequency transformer. The part of the detector circuit in which regeneration is obtained would not be altered by this substitution.
Tickler Coil Control.- Fig. 1 shows regeneration obtained by a tickler coil connected in the plate circuit and coupled to the tuned coil of the grid circuit. The construction of the tickler coil unit is shown in Fig. 2. The tuned winding, which is the secondary of a radio frequency transformer, and the primary winding of this transformer are wound on a stationary form in the usual way. The tickler coil is wound on a form which rotates within the stationary form. A shaft is attached to the tickler coil form and extends through to a control knob. If the tickler coil is small, consisting of ten turns or less, it must be placed close to the secondary coil. If the tickler is large, containing fifteen to thirty turns, it may be placed farther away from the stationary coil.
As the tickler is turned to increase its coupling to the stationary coil the effective inductance of the tuned stationary coil is increased. Therefore, the tuning point at which the circuit becomes resonant to a certain frequency will change with changes of tickler adjustment. This is a rather serious disadvantage of this method for obtaining regeneration since a receiver cannot be logged unless a note is made of the tickler coil setting.
The tickler coil adjustment should be such that oscillation may be caused at the lowest frequency or highest wavelength to be received. If oscillation cannot be obtained when the tickler coil is turned to the position of maximum coupling, it will be necessary either to increase the number of turns on the tickler or to move it closer to the stationary coil.
The position of the tickler coil in relation to the fixed coil must be such that increase of coupling between the two will increase the feedback, will increase regeneration and finally cause oscillation. If turning the tickler coil into line with the fixed coil reduces the signal strength by reducing regeneration, the connections to the tickler coil should be reversed or it should be rotated in the opposite direction to increase regeneration.
When the axis of the tickler coil is in line with the axis of the grid coil, there is maximum coupling between the two. If the voltages in the tickler coil and in the grid coil are in phase, the tickler will reinforce the grid coil and there will be maximum regeneration. But if the voltages in the two coils are in opposite phase, the tickler coil will oppose the grid coil and the signal strength will be reduced.
Some tickler coils are arranged so that they may be given one-half of a complete revolution, starting with the axes of the two coils in line and ending with them again in line. Other ticklers are arranged for only one-quarter of a revolution, starting with the axes at right angles and ending with them in line.
The greatest range of control will be obtained when the tickler coil is allowed a half revolution. With the tickler coil axis and the grid coil axis in line at one extreme of rotation the voltages will reinforce each other and there will be maximum regeneration. With the tickler turned half way around, so that the two coils are again in line, the voltages will oppose each other, there will be a reversed feedback and minimum signal strength.
If the tickler is allowed only a quarter revolution, it is necessary that the voltages be in phase when the coil axes are in line. Minimum coupling and minimum regeneration will be obtained with the coils at right angles but it will be impossible to make the voltages oppose for a reversed feedback effect.
The feedback from plate circuit to grid circuit is at radio frequency. This radio frequency will not pass through the high impedance of the primary winding in the audio transformer or choke. Therefore, a bypass condenser is connected from the line between tickler and transformer to one of the filament terminals on the tube. This bypass should have at least .001 microfarad capacity.
Fig. 3 shows the method known as reversed feedback. The construction is exactly like that shown in Fig. 2. But now the tickler coil is placed in such a relation to the stationary coil that its energy opposes the energy in the stationary or tuned coil. The constants of the tuned circuit are such that it normally tends to oscillate at the lowest frequency or highest wavelength to he received. This may he accomplished by using a large primary winding on the radio frequency transformer and making the coupling between the primary and secondary of this transformer very close.
When the reversed tickler is in the position for maximum coupling, its feedback effect will be a minimum because it is opposing the voltages in the tuned coil. When the reversed tickler is at right angles to the fixed coil, regeneration will be maximum because then all of the opposing effect of the reversed tickler will have been removed.
If a tickler coil used in the manner of Fig. 1 is rotated to the right to increase regeneration, rotating it to the left will cause it to act as a reversed tickler and the system will then correspond to Fig. 3.
Resistance Control.- Fig. 4 shows control of regeneration by a variable resistance unit placed in the tickler circuit. This unit should have a resistance which is variable up to 50,000 ohms. Units providing still higher resistance will be equally satisfactory. The plate of the tube is connected directly to the primary winding of the audio frequency transformer. The resistance unit is in series with the tickler coil and this tickler circuit connects to one of the filament terminals through a bypass condenser having a capacity not less than .001 microfarad.
In the case of Fig. 4 the coupling of the tickler to the tuned coil is not variable. The tickler coil is wound on one end of the form that carries the secondary, this being shown in Fig. 5. The less space between the tickler winding and the tuned winding the fewer tickler turns will be required to obtain satisfactory regeneration. The number of turns and the distance of the tickler winding from the tuned winding should make it possible to obtain oscillation at the lowest frequency or highest wavelength when the control unit is adjusted for lowest resistance. If it is impossible to obtain Oscillation when using the least possible resistance, it will be necessary to increase the number of turns on the tickler winding or to move this tickler winding closer to the tuned winding.
With resistance units giving up to 50,000 ohms the tickler coil may usually be placed so that the nearest turns of tickler and tuned winding are separated by three-sixteenths to one-quarter an inch. From ten to thirty turns will be required on the tickler coil.
Fig. 6 shows the use of a resistance control shunted across the tickler winding. The construction of the tickler and the tuned coil is the same as shown in Fig. 5 and the adjustment of tickler turns and position is the same as for the method of Fig. 4. The resistance of Fig. 6 forms a bypass for the radio frequency energy from the plate circuit. The smaller the amount of resistance used in Fig. 6, the less will be the regeneration obtained. In Fig. 4 the greater the resistance, the less the regeneration. The two methods operate equally well as controls for regeneration.
Condenser Control.- In Fig. 7 the regeneration is controlled by a variable condenser used as a bypass for the radio frequency energy in the plate circuit. The tickler coil should be mounted so that its coupling with the tuned coil may be varied. The method of Fig. 2 makes a satisfactory mounting, but any other adjustable coil mounting may be used. The variable condenser should have a capacity of .001 microfarad, the old style forty-three plate units being just right. If a smaller variable condenser is used, it will be necessary to increase the number of turns on the tickler coil.
The connections are made exactly as shown in Fig.7. The plate of the tube is connected to the tickler and the other side of the tickler is connected to the stator plates of the control condenser and to the primary of the audio transformer. The rotor of the condenser is connected to either filament terminal of the tube.
When making the preliminary adjustment for the system of Fig. 7, the condenser should be turned to maximum capacity with its plates fully in mesh. Connections to the tickler should then be reversed and tried both ways. The connections are left in the way that produces maximum regeneration or oscillation. With the condenser still at maximum capacity the tickler is coupled closer and closer to the fixed coil until oscillation takes place. Oscillation may then be prevented and regeneration controlled by varying the condenser. The less the condenser capacity, the less will be the regeneration and the greater the condenser capacity, the more regeneration will be obtained. If it is impossible to obtain sufficient regeneration at the lower frequencies or higher wavelengths, it will be necessary to increase the coupling or the number of turns on the tickler coil
The regeneration control of Fig. 8 is very similar to that of Fig. 7 and all of the constructional details given for Fig. 7 apply equally well to Fig. 8. The only difference between the two methods is in the connections between plate, tickler and condenser.
Fig. 9 shows still another method of controlling regeneration with a variable condenser. Here the tickler winding forms part of the tuned coil winding. The tickler winding should have a number of turns equal to about one-fourth the number of turns in the tuned portion of the coil. For broadcast reception this method of Fig. 9 is not as satisfactory as the methods of Figs. 7 or 8.
Link Circuit Control.- Fig. 10 shows regeneration obtained through a link circuit coupled at one end to the plate circuit and at the other end of the grid circuit. It is necessary to insert an additional air-core coil between the plate of the tube and the audio frequency transformer. This coil has two windings, both of the same number of turns, and closely coupled by winding them end to end or one over the other. Twenty turns on each winding will usually be about right. If the coupling between these two windings is to be varied to control regeneration, this unit may be made of a split variometer.
The tickler coil proper, which is coupled to the tuned coil of the grid circuit, is fixed in position as shown in Fig. 5. It should consist of ten or more turns. The number of turns on the tickler and its closeness of coupling to the tuned coil are such as to allow oscillation at the lowest frequencies or highest wavelengths to be received.
Three different methods of control are shown in Fig. 10, although only one of them would be employed at any one time. As already mentioned it is possible to control regeneration by varying the coupling between the coil in the plate circuit and the coil in the link circuit. With variable coupling neither the variable resistance nor the variable condenser would be used.
If the variable condenser is placed in the link circuit of Fig. 10, neither the resistance nor the variable coupling would be used. The resistance would likewise be used without either the variable condenser or the variable coupling.
Control of Plate Circuit.- Fig. 11 shows regeneration control by limiting the energy passing into the grid circuit to a value low enough so that the total energy in the grid and plate circuits of the tube, even with the feedback through the tube capacity, is not sufficient to allow oscillation. A variable resistance, which may be adjusted from about 10,000 to 100,000 ohms, is connected between the B battery or plate voltage supply unit and the primary of the radio frequency transformer. Increasing the resistance lessens the regeneration while lessening the resistance increases the regeneration. Since this method acts to change the direct current voltage applied to the plate circuit of the preceding tube it must not be allowed to interfere with passage of radio frequency currents through its circuit. Therefore, the resistance is bypassed with a one microfarad condenser through which the radio frequency currents pass unhindered.
Fig. 12 shows another method of regeneration control applied to the plate circuit of one or more radio frequency tubes. The primary of the radio frequency transformer is divided into two parts, one part being stationary and the other being rotated. Rotation of the movable part of the primary winding allows it either to assist the stationary part, to oppose the stationary part, or to have any intermediate effect. With the movable part of the primary opposing the stationary part regeneration is cut to a minimum. With the two parts acting together regeneration is maximum.
The split primary winding of Fig. 12 has been used for automatic control of regeneration by attaching the movable part of the winding to the shaft of the tuning condenser. More regeneration is always required for low frequencies than for high frequencies, consequently the connection is made so that the two parts of the primary act together for maximum regeneration at low frequencies or high wavelengths.
Inefficient Methods.- The control methods shown in Figs. 1 to 12 allow efficient operation of the receiver since they introduce the least possible added resistance and loss into the grid circuits. The methods to be shown immediately following are classed as inefficient since they add considerable resistance directly or indirectly to the grid circuit. This causes a loss of signal strength and broadens the tuning of the receiver.
Fig. 13 shows the use of a variable resistance unit in the oscillatory portion of the tube's grid circuit. This resistance may be a rheostat or a potentiometer used as a rheostat. The amount of resistance needed to control regeneration and prevent oscillation depends on the size and construction of the coil and condenser, also on the wiring in the grid circuit. Resistances as low as ten to twenty ohms may be sufficient or it may be necessary to use two or three hundred ohms.
Fig. 13 also shows the use of a variable resistance between the ground connection and the antenna coil, this method being applied to the first tube of the receiver. This resistance should have a maximum value of 200 to 400 ohms. A potentiometer or any variable resistance reaching this value will be satisfactory. Increasing the amount of the resistance will reduce regeneration while reducing the resistance will increase regeneration and produce oscillation.
Fig. 14 shows the use of a variable grid leak for controlling regeneration. This grid leak should be constructed so that its resistance may he reduced below 100,000 ohms or one-tenth of a megohm. Reducing the resistance of the grid leak lessens regeneration while increasing this resistance will increase regeneration and produce oscillation
In Fig. 15 a potentiometer is used in the grid return circuit. Turning the potentiometer arm to the side connected to the negative filament terminal places a negative grid bias on the tube, increases regeneration and increases the tendency to oscillate Turning the potentiometer arm toward the positive side provides a positive grid bias and allows the grid circuit to consume power. This reduces regeneration. This use of a potentiometer broadens the tuning and distorts the signal. It also weakens the incoming signal
Fig. 16 shows the use of an absorption circuit for controlling regeneration. The absorption circuit consists of a coil and a variable condenser. The coil is loosely coupled to the tuned coil in the grid circuit. The absorption coil may be mounted on the grid coil form as in Fig. 5. The coupling of the grid coil to the absorption coil should be close enough so that oscillation may be prevented at the highest frequencies to be received The absorption coil's inductance and the capacity of its tuning condenser must be of such values that they tune to the highest frequency or lowest wavelength to be received.
As the regeneration control condenser is tuned more and more closely to the frequency being received, the power absorbed from the grid circuit will increase and regeneration will be reduced.
Variometer Controls.- Fig. 17 shows one of the first methods used for regeneration control in broadcast receivers. This is known as the tuned plate method. A variometer is inserted in the plate circuit between the plate terminal of the tube and the audio frequency transformer. As the inductance of the variometer is increased, the voltages across it are increased proportionately. The feedback is obtained through the capacity between the plate and the grid in the tube. This capacity is indicated in broken lines.
As the variometer's inductance is increased, the feedback through the tube capacity increases so that additional energy is sent back into the grid circuit. Reducing the variometer's inductance reduces the regeneration.
Fig. 18 shows the use of a plate variometer connected and operated in the same way as the variometer in Fig. 17. The grid circuit also contains a variometer whose inductance is used for tuning the grid circuit to the frequency being received.
Automatic Control of Regeneration.- Inasmuch as it is desirable to increase the amount of regeneration with decrease of the frequency being received, the regeneration control may be attached to the tuning control so that both move together. Tuning is usually done with a variable condenser whose capacity is increased for the reception of higher wavelengths or lower frequencies. If regeneration is controlled with a condenser, this control condenser may be connected to the tuning condenser so that the feedback is increased as tube capacity of the tuning condenser is increased. The types of control shown in Figs. 7, 8, 9, 10, 12 and 16 are well adapted to automatic regeneration.
Automatic regeneration is always attended with considerable difficulty because of changes introduced by altering the antenna, by using different tubes, by movement or any coils, or by changes of any nature whatsoever in the receiver.
In Figs. 7, 8, 9, and 10, increasing the capacity of the control condensers increases the regeneration. Were these control condensers to be connected to the tuning condenser the two condensers should increase their capacities together so that regeneration would automatically increase at the higher wavelengths or lower frequencies. The size of the tickler coil and its coupling to the grid coil are matters for experiment. The proper values will differ for each circuit to which automatic regeneration control is being adapted.
With the control condenser fully in mesh, at lowest frequency, the tickler coil should be given just enough turns or its coupling should be made just loose enough to bring the circuit to maximum regeneration while preventing oscillation. The tuning condenser and control condenser are then turned to their lowest capacities at which reception is expected. If oscillation occurs at this point, it will be necessary to reduce the tickler coupling, to reduce the number of turns on the tickler coil, or to use a control condenser of lower minimum capacity.
Regeneration with a Loop.- Feedback regeneration may be obtained in any loop receiver by the method shown in Fig. 19. The number of turns on the loop is increased above the number ordinarily used by adding from one-fourth to three-fourths the original number of turns. The connection from the loop and the tuning condenser to the grid of the first tube is not disturbed. A tap is provided at the junction between the old and new parts of the loop winding. From this tap a connection is made to the filament circuit of the first tube and the loop tuning condenser. From the outer end of the added turns a connection is made through a variable condenser to the plate terminal of the first tube. This condenser may have a capacity between .00025 and .0005 microfarad.
Increasing the capacity of the added regeneration condenser will increase the feedback and the regeneration. Reducing the capacity of this condenser will lessen regeneration. It should be mentioned that this system will cause the loop to radiate sufficiently to bother nearby receivers. This system of regeneration may be added to tuned radio frequency receivers or to superheterodyne receivers. When added to a superheterodyne the connection from the added portion of the loop through the control condenser is made to the plate of the first detector tube.
Producing Regeneration in Balanced Circuits.- Various kinds of receivers are provided with small condensers which balance the feedback through the plate to grid capacity of the tube with an external feedback of equal voltage but of opposite phase. These receivers include those using the Neutrodyne, Roberts, Rice, Sampson and similar circuits. The Neutrodyne, the Roberts, and the Rice are shown respectively in Figs. 20, 21 and 22. In each case the balancing condenser has been replaced with a variable condenser marked "Control."
With this control condenser adjusted to the capacity which exactly balances the internal capacity of the tube the receiver will be balanced and regeneration will be prevented. As soon as the control condenser is adjusted to provide either more or less capacity than the amount required for balancing, regeneration will take place. Increasing the capacity of the control condenser will allow the external feedback to be greater than the internal feedback. Reducing the capacity of the control condenser will allow the external feedback to be less than the internal feedback. Regeneration will take place in either case. To cause regeneration at the lower frequencies or higher wavelengths it is usually necessary to increase the control condenser capacity to provide a comparatively large external feedback.
It will be unnecessary to provide regeneration in more than one of the radio frequency stages. The best results will be obtained by unbalancing the circuit which immediately precedes the detector, this being the second radio frequency stage.
Multiple Regeneration.- While regeneration is applied only to the detector grid circuit as a general rule, there is no reason why it cannot also be applied to the grid circuits of any radio frequency tube including the one immediately following the antenna.
Systems have been designed in which variable regeneration control is applied to the detector grid circuit and fixed or semi-fixed regeneration is applied to one or more of the radio frequency stages preceding the detector. One method substitutes for a single radio frequency tube two tubes having their grid circuits in parallel. The plate circuit of one of these tubes is connected through a transformer to the following stage as usual. The plate circuit of the other tube is connected to a tickler coil in the tube's grid circuit and is not connected to the following stage. To be effective in increasing signal strength and selectivity, regeneration must be increased as the received frequency is decreased, consequently no method of fixed regeneration is of much value except at some one frequency among all those to he handled.
One of the simplest and easiest ways of controlling regeneration and preventing oscillation is by the use of a variable rheostat for the tube in which regeneration is desired. Any radio frequency stage in which the tube is fitted with a variable rheostat may be made to regenerate and if this system is used on two or more radio frequency stages we will have multiple regeneration.