# An oscillator frequency control circuit

How the Discriminator-Detector Circuit Works.^The discriminator-detector network as the name implies, discriminates between applied intermediate frequencies which are too low and thpse which are too high, and produces a corresponding direct current or voltage whose polarity depends upon the direction of frequency departure from a prescribed intermediate frequency. This d.c. voltage is applied to a control element which in turn causes a shift in frequency of the local oscillator such as to bring the i.f. signal to very nearly the correct inter- WG. 1—Conventional block diagram of an automatic radio frequency control circuit.

mediate frequency. Since production of the d.c. voltage is due to departure from the resonant or center frequency of the i.f. system, obviously the correction cannot be strictly complete; but in the system described a correction ratio of more than 100 to 1 is feasibre.

In other words, when the dial of the receiver is mis-tuned 100 k.c. for the received signal, the automatic correction may be made to bring the actual i.f. signal frequency to only 100 cycles off resonance in the i.f. system. Of course that is easily sufficient.

The Frequency Discriminator.—A method for obtaining differential d.c. potentials (or currents) whose magnitude and polarity are determined by the amount and the sign, respectively, of the difference between an applied frequency and the true intermediate frequency is described herewith. Side circuits tuned above and below the center frequency are not used.

The action depends upon the fact that a 90° phase difference exists between the primary and secondary potentials of a double-tuned, loosely-coupled transformer when the resonant frequency is applied and that this phase angle varies as the applied frequency varies. Thus if the primary and secondary voltages are added vectorially, the absolute magnitude of the resultant vector will be greater on one side of resonance then on the other.

The vector sum of the primary and secondary voltages may be physically realized by connecting the two parallel tuned, coupled circuits in tandem, applying the input potentials to one circuit and taking the output across both circuits in series. In this manner, an action similar to that of a side circuit is produced even though the primary and secondary are both tuned to the center frequency.

The potentials at either end of a secondary winding with respect to a center tap on that winding are 180° out of phase. Therefore, if the center tap, rather than one end, of the secondary is connected to the primary, two potentials may be realized, one maximizing above and one maximizing below the center frequency. See fig. 2.

If a transformer is connected in this manner and the resonant frequency is applied to the primary the two resulting output potentials will be equal in magnitude. If these are then applied to two separate, like detectors and the resulting d.c. voltages are added in opposition, the sum will be equal to zero. If, however, the applied frequency departs from responance, the sum of their outputs will be some real value whose polarity will depend upon the sign of the frequency departure.  FIG. 2—Diagram and plotted curves illustrating how the potentials at either end of secondary are 180° out of phase.

Referring to fig. 3, the action is as follows: If the resonant or center frequency is applied to the grid of the amplifier tube, equal amplified voltages will exist between the point A and ground and between the point B and ground. These are rectified by the diodes and direct currents will flow in the resistors Rj and R2 in opposite directions with respect to ground. Thus, the net d.c. potential produced by the two IR drops between E, and ground is equal to zero. If, however, the applied frequency departs from resonance the potentials across the diodes will be unequal in magnitude, unequal IR drops will be produced in the two resistors and a d.c. potential will exist between E and ground, the polarity of which will depend upon the sign of the frequency departure. If a carrier at the resonant frequency with normal intensity modulation, but without frequency modulation, is applied to the system, the a.f. as well as the d.c. voltages across i?i and i?2 will be equal and opposed. Therefore at resonance there will be no a.f. potentials between E and ground, and as far as audio components are concerned, the system acts exactly as though point E, were grounded with the outputs of the two diodes acting in parallel. Actually if is sufficiently large to have negligible reactance at the lowest modulating frequency, this is the case. Then the point F, becomes a potent source of audio voltages to supply the a.f. amplifier-system and no other audio detector is necessary. It can be seen that the d.c. potential between ground and the point F, will have the proper polarity to be used for avc action, and that this potential will bear the same ratio to the developed audio voltages as is found in the conventional diode detector avc system. The fact that it maximizes at one side of resonance is of no significance if automatic frequency control is used. When the afc is cut out of circuit (manually) point E, is grounded. This causes the d.c. potential at F, to maximize on resonance.

The Control Circuit.-—A circuit, which'will convert d.c. discriminator voltages into changes in oscillator frequency is shown in fig. 4. In this figure 7\ is the oscillator tube and 7\ the control tube. The combination of Rx and Ci connected across the oscillator tank circuit produces a voltage on the grid of the control tube 90° out of phase with that existing across the tank circuit. Variations in grid bias of the control tube (obtained from the discriminator) vary the plate current of that tube. This plate current is 90° out of phase with the tank circuit voltage and therefore the control tube acts like a reactance in shunt to the tank circuit. The magnitude of the,reactance and therefore the oscillatory frequency are varied by the con-' trol tube grid bias.

With the circuit shown in fig. 4 the control tube is equivalent to an inductance in parallel with the tuned circuit. An increase in mutual conductance of the control tube produces a decrease in the magnitude of this equivalent inductance and consequently an increase in the oscillator frequency.

Control Tube.—The amount of control is proportional to Gm, but is also affected by the control grid voltage for this Gm, since a high value of bias permits Ri or Ci to be smaller for a given oscillatory voltage. Consequently maximum control is proportional to the product of Gm and Ec. Sensitivity of control is however, another important requirement, since it is desired that the frequency change be as large as possible for a given change in bias. This means the control tube should be of the short cut-off type. Further requirements are high rv, linear change of Gm with bias, and for economy, low plate and screen currents.

All of these requirements are best met by the short cut-off, r.f. pentodes such as 57, 77, 6C6 and 6/7. By proper choice of Ri and Ci the maximum amount of frequency correction can be adjusted to suit required conditions.

The frequency control readily obtainable by this circuit is of the order of 9.5% of the oscillator frequency in the broadcast band and 1.5% in the region of 10 megacycles.

In a receiver it has been found that a discriminator sensitivity of 100 volts per k.c. and a control sensitivity of 7 k.c. per volt can be easily obtained, so that an overall control ratio of 700 to 1 results. A tuning misadjustment of 7 k.c. will therefore result in only a 10 cycle shift of the intermediate frequency.

The use of afc on the short-wave bands has the very much needed advantage of making the tuning operation easier. The tuning control has to be moved only until the frequency is close enough to resonance that the discriminator will develop sufficient voltage to bias the control tube the amount required for the departure from resonance. Short-wave stations are thus spread out on the dial, making them easier to locate and easier to hold.

In the broadcast band this characteristic would have the disadvantage that the receiver would appear to laymen to be broad in tuning in comparison with receivers without afc. This apparent disadvantage can be eliminated by combining the aft switch with the tuning mechanism so that the afc automtically becomes inoperative during the tuning operation.

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