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Fig. 252. Schematic diagram of a beat frequency oscillator which is frequency modulated by the motor driven variable condenser in the lower left corner of the diagram. This type of oscillator, when frequency modulated, provides a band width that is constant for any output frequency.

In contrast to the single oscillator type of frequency modulated source of test signal, there is in use the dual oscillator system, operated with a motor driven frequency modulator condenser, whereby a constant band width is secured. This is the beat frequency unit mentioned earlier in the text. Operation of the system can best be comprehended by referring to the elementary circuit shown in figure 252 and the following explanation.

The circuit consists of two oscillators, a mixer tube and the motoi driven frequency modulator. One of these oscillators, say Osc. 1, ii fixed tuned at some frequency, say 700 kc. Connected across the tuning condenser of this unit is the motor driven frequency modulator condenser. The capacity of the rotating condenser is so designed that when in operation, the mean frequency of the oscillator is 700 kc. and the frequency limits of the output are 690 kc. and 710 kc., a band width of 20 kc. or 10 kc. each side of the mean frequency. In effect this oscillator is a frequency modulated r-f. oscillator, fixed tuned to 700 kc. and frequency modulated over a 20 kc. band.

As is evident, provision is made to feed the frequency modulated output voltage to one section of a 6A7 tube. It is to be remembered that the circuit shown does not contain all of the frills and filters actually found in the commercial units. The circuit in figure 252 is the basic operating system. The diagrams of the commercial frequency modulated r-f. oscillators will be shown later in this chapter.

Oscillator 2 is the continuously variable frequency unit. This is the one which employs the wave changing mechanism, so that it may be tuned over the entire i-f., broadcast and high-fsequency or short-wave spectrum. The tuning condenser in this unit is operated by hand, just as if it were a conventional oscillator without even the remotest association with any frequency modulating condenser. As is shown in the diagram, the output of the oscillator is fed to the mixer tube through a suitable attenuating system. The 6A7 in this case is used as a mixer tube only. One of the oscillators feeds into one of the control grids, grid number 1, and the other oscillator feeds into the other control grid, grid number 4.

Let us say that oscillator 2, the continuously variable -.nit, is adjusted to 960 kc. When this is done, two signals are fed into the mixer tube. One of them is of 960 kc. and the other is the 700 kc signal frequency modulated over a 20 kc. band. These two voltages are mixed and as a result of rectification, there is present in the plate circuit of the 6A7, a voltage which varies in frequency from 250 kc. to 270 kc., or a 260 kc. signal, the beat frequency, which is frequency modulated over the 20 kc. band, or 10 kc. each side of the mean frequency. This frequency modulated signal is produced because one of the signals fed into the mixer is modulated plus and minus 10 kc. We might mention that the selection of a 20 kc. band width for the frequency modulation of the fixed oscillator, is purely illustrative. It can just as readily be 10 kc. or 30 kc.

As a result of the fixed relation between the frequency modulating condenser capacity and the circuit capacity of the fixed frequency oscillator, the band width of the frequency modulated signal is constant, at whatever the design provides. Since the beat signal retains the modulation characteristics of the modulated signal, which is mixed with the sine-wave signal, the output beat voltage, irrespective of frequency, will retain the same modulation characteristics, which in this case means modulation at plus and minus 10 kc. This is how the band width of the frequency modulated output of this dual oscillator arrangement is constant at whatever the design provides.

Electrically Operated Frequency Modulated Oscillators

In contrast to the motor driven condenser type of frequency modulator, several units are in use, wherein frequency modulation is secured without the use of a motor driven condenser. One such system is used in the Egert unit. Two oscillators are used to produce the constant band width frequency modulated signal, just as in the motor driven affair, illustrated in figure 252, except that the motor driven frequency modulating condenser is replaced by an electrical circuit.

Fig. 253. Schematic diagram of a frequency modulated oscillator, wherein the spreading of the frequency over a band is accomplished by using the variation in permeability of the iron core in the No. 2 oscillator oscillation transformer.

The process of beating a variable frequency oscillator against a fixed frequency oscillator is the same. An idea of how this electrically operated type of frequency modulator develops the modulated wave can be gathered from figure 253 and the following text. The circuit shown is basic and is offered solely for illustrative purposes. The complete circuit of the Egert unit is given later in this chapter.

Fig. 253. Schematic diagram of a frequency modulated oscillator, wherein the spreading of the frequency over a band is accomplished by using the variation in permeability of the iron core in the No. 2 oscillator oscillation transformer.

Oscillator number 1 is the variable frequency unit, which covers the various bands. Oscillator number 2 is the fixed frequency oscillator, which is frequency modulated over a constant band width, say 20 kilocycles. Oscillator number 1 uses the oscillator portion of a 6A7 tube. The mixer or pentode portion of this same tube receives the frequency modulated output of oscillator number 2, so that the two signals are mixed and, as a result of the rectifying action within the tube, the plate circuit contains the difference frequency modulated plus and minus 10 kc.

If you examine the wiring diagram, you will also note two significant facts: First the presence of an iron core for the oscillator number 2 coils. Second, the use of a rectifier and that a portion of the a-c. voltage present across the input condenser is fed into the oscillation transformer used in oscillator number 2.

This iron core and the rectified a-c. voltage serve to frequency modulate the output of the fixed frequency oscillator, by employing the variation in permeability of the iron core to change the frequency of the oscillations generated by oscillator number 2. The winding connected to the power supply furnishes a magnetizing current, which varies at the frequency of the a-c. voltage across condenser C. Since the rectifier is a half-wave system, this magnetizing current varies 60 times per second or at whatever the frequency of the power supply may be. By suitable design of the iron cored oscillation transformer, operation upon the straight portion of the permeability curve is secured. The normal permeability of the iron core establishes a fixed inductance for the oscillation windings and the oscillator circuit is tuned in the regular manner, except that, since a fixed tuning condenser is used, the resonant frequency is fixed and is of a single value. Now, when an a-c. voltage is established across condenser C, current will flow through coil L and the magnetizing current will change the permeability of the iron core and influence the inductance of'the oscillator number 2 winding, thereby changing the frequency of the output voltage from oscillator number 2. Since the magnetizing current secured from the power supply is a varying current, the frequency of oscillator number 2 varies over a range and by designing the magnetizing current supply circuit and the iron core so that the current and the permeability varies between pre-determined limits, the frequency of oscillator number 2 will also vary between pre-determined limits. In the actual unit, the band width provided is 22 kilocycles, or 11 kilocycles each side of the mean frequency.

Since this signal is of constant band width and is fed into the mixer tube, where it beats against the variable frequency oscillator, the resultant beat frequency signal is a frequency modulated signal of the stated band width, without the use of a motor driven condenser.

Another type of r-f. frequency modulation, which is electrically operated and which does not use a motor driven frequency modulating condenser, employs the dynamic input capacity of a tube to vary the frequency of the oscillator output. According to information we can gather, a voltage secured from the 60-cycle power supply varies the operating potentials upon the auxiliary tube. The grid-to-plate capacity of a tube, being reflected across the input circuit of the same tube and being amplified to an extent determined by the operating potentials, is caused to vary over a certain range, as a result of the change in operating potential. The net result is a variation in the dynamic input capacity and since this capacity is a part of the tuning capacity, which is fixed, the frequency of the voltage generated is varied over a range determined by the change effected as a consequence of the varying operating potential. In a sense this circuit is very much like the automatic noise suppression and tone control system used in some radio receivers. Full details of the exact system are not available and we were not in possession of one of the units during the preparation of this volume.

So much for the general outline of how the frequency modulation of r-f. and i-f. test signal voltage is produced. Let us now consider the frequency modulator, which, in the case of motor driven condensers, is the unit comprised of the motor, the condenser and the synchronous pulse generator. In the case of the electrically operated frequency modulated oscillator, such as that which employs variation of the permeability of an iron core, the frequency modulator is the a-c. supply unit, the magnetizing winding and the iron core.

Motor Driven Frequency Modulators

Speaking in generalities, the motor driven frequency modulator unit, employed in connection with cathode-ray oscillographs used for alignment purposes, consists of the drive motor, the variable condenser and the synchronous pulse generator. It might be well to state that all such units do not employ synchronous pulse generators. Those which employ such a generator utilize the linear sweep circuit in the cathode-ray oscillograph to sweep the spot across the screen. Those which do not employ a synchronous pulse generator, do not use the linear sweep in the cathode-ray oscillograph to sweep the spot across the viewing screen. Instead, a supplementary sweep circuit, associated with the motor driven condenser system, generates the voltage which sweeps the spot across the screen.

Perhaps we should mention that all reference to a frequency modulator means the unit which is responsible for the change in circuit constants so as to change the frequency. The motor driven condenser is an example of the frequency modulator. The combination of die frequency modulator and the oscillator forms what is designated as a frequency modulated r-f., i-f. or a-f. oscillator.

The circuit of the RCA TMV 128-A motor driven frequency modulator is shown in figure 254. The two condensers provide a range of

Courtesy RCA Mfg. Co., Inc. Fig. 254. Schematic diagram of the RCA Model TMV-128-A motor driven frequency modulator.

capacity. The "Hi-Lo" switch connects one or both condensers into the circuit. When set to "Lo," one of the condensers, C-l, is in the circuit; when set to "Hi", C-2 is shunted across C-l and the band width, for any one setting, is increased. Inclusive of the connecting cable, the capacity range for the two positions is

"Hi" 65 to 110 mmfd.

An armature, which rotates between the two pole pieces of an induction generator, is joined to the shaft of the condenser rotor and naturally to the shaft of the motor. This induction generator furnishes the impulse, which is fed to the linear sweep circuit within the cathode-ray oscillograph, for synchronization between the frequency modulated voltage, which is the vertical deflection, and the linear sweep voltage, which is the horizontal deflection. In this way the spot is swept across the screen at a rate in conformity with the variation in frequency, as the frequency modulating condenser rotor progresses through its arc of travel. Figure 255 illustrates two cycles of the waveform of the voltage developed by the impulse generator. This illustration may serve as reference data in the event of some trouble with the unit.

This frequency modulator is intended for use with the RCA model TMV-97-C all-wave oscillator, which is already furnished with an input jack to take the plug connection between the frequency modulator and the oscillator. However, the unit also is suitable for use with

Fig. 255. Two cycles of the waveform produced by the impulse generator, which is that part of the RCA frequency modulator that brings about synchronization between the rate of frequency modulation and the linear sweep voltage.

any other oscillator, providing proper connections are made. As a matter of fact, we used the unit with a number of different oscillators.

The circuit of the Clough-Brengle frequency modulator system, inclusive of the sweep used when checking alignment, is shown in figure 256. There is no provision for a change in the width of the band. The maximum amount of capacity present in the circuit is definitely fixed. Attached to the rotor is a short circuiting switch, consisting of a rotating and a stationary contact. The rotating contact makes electri-

Fig. 256. The circuit of the Clough-Brengle frequency modulator. Due to the short-circuiting switch on the shaft, the sweep circuit is shorted during one half of the rotating condenser's travel.

cal connection with the stationary contact during 180 degrees of the 360-degree arc made by the rotating condenser rotor. A high d-c. voltage is fed to condenser C through resistor R, charging the condenser.

The rate at which the condenser C develops the sweep voltage is not of importance providing that full charge is not reached before the contacts close. The rotating short circuiting contacts are closed during half of the arc of travel of the rotating condenser rotor, so that vertical spot displacement takes place during one half of the complete frequency modulating cycle. During the other half of the cycle there is no displacement of the beam in the horizontal direction, because the sweep voltage circuit is shorted.

This circuit arrangement is related to the proper development of the resonance curve, that is synchronization between the rate of frequency change in the oscillator circuit and the movement of the spot in the horizontal direction. Figure 257 illustrates the character of the

sweep voltage used in the Clough-Brengle model OM frequency modulated r-f. oscillator. This photograph was made by taking the voltage out of the aforementioned sweep system and observing it on another cathode-ray oscillograph.

Referring to figure 257, the jagged horizontal line represents the 180 degrees of time, during which the shorting contacts are closed.

Fig. 257. An oscillogram of the sweep voltage employed in the Clough-Brengle frequency modulator. The horizontal line represents that period of the cycle when the shorting contact is closed and the rising portion of the curve is the time when the condenser is charging.

The rising portion of the pattern is the 180 degrees of time during which the condenser charges. This image was made with the linear sweep in another cathode-ray oscillograph, adjusted to twice the frequency of the frequency modulator motor, in order to show the relative periods during a complete cycle, when the charging action and the shorting action existed. Since the horizontal axis of figure 257 represents time, you can readily see that the duration of the charging action is as long as the discharging or shorting action. Just how the synchronizing pulse previously mentioned and the sweep, now being discussed, actually function when developing resonance curves, will be shown later.

Synchronization between the sweep of the spot across the cathode-ray tube screen and the frequency change in the Egert VRO unit is accomplished by securing the sweep voltage from the source which supplies the voltage that causes the flow of the magnetizing current through the magnetizing coil. The nature of this sweep voltage can be gleaned by examination of figure 41. Synchronization is automatically accomplished, because, as a result of the circuit connections, the frequency of the modulated oscillator changes over its prescribed band during the time that the spot is sweeping across the screen. This is so during the charging as well as the discharging portion of the sweep cycle, because during the charging portion of the cycle, current is flowing through the magnetizing winding and causing the same change in frequency of the fixed oscillator output, as during the charging portion of the sweep cycle. So much for that. Let us now consider the development of the resonance curve trace upon the cathode-ray tube screen

and see how each part of the complete system contributes its share and how the patterns, which appear upon the screen, differ from one another.

Relation Between Rotating Condenser and Frequency Sweep

In order to present this subject properly for most complete comprehension, let us first establish certain facts, with which you are by this time familiar. First, that the frequency modulator, operated in conjunction with the oscillator, constitutes a source of a continuously variable frequency signal of supposedly constant amplitude or level. This is so, irrespective of the exact design of the system, that is, if it employs a single oscillator tube, or if it employs two oscillator tubes and a rectifier, so that the resultant beat signal is frequency modulated over a pre-determined and constant band width.

If we measured the voltage from a system which is supposed to provide a frequency modulated signal covering the 450 to 470 kc. band, and plotted this voltage against frequency, the resultant graph would appear like that shown in figure 258. It is possible that a slight

Fig. 258. The horizontal JJ line of this graph indi- ^ cates that the voltage output from a frequency J modulated oscillator is constant over the fre-quency band covered.

Fig. 258. The horizontal JJ line of this graph indi- ^ cates that the voltage output from a frequency J modulated oscillator is constant over the fre-quency band covered.

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