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Fig. 275. Daring one half of each revolution of the variable condenser in the frequency modulator of Fig. 256, the sweep circuit is shorted and only cycle N spreads the resonance curve, giving a single instead of a double pattern.

Fig. 275. Daring one half of each revolution of the variable condenser in the frequency modulator of Fig. 256, the sweep circuit is shorted and only cycle N spreads the resonance curve, giving a single instead of a double pattern.

circuit, which is shorted during the inactive portion of the cycle, is the sweep and NOT THE FREQUENCY MODULATING CONDENSER. This is shown in figure 275. Note that the timing oscillator is active only during one half of the complete cycle of the rotating frequency modulating condenser rotor. Frequency modulation of the oscillator is taking place during the complete cycle of the rotating condenser rotor, but each spot position is related to one frequency only and no matter how asymmetrical the pattern, only a single line trace appears. However, this trace is also accompanied by a vertical line, which represents the vertical displacement during that portion of the frequency modulating cycle, when there is no horizontal displacement.

Let us assume for the sake of illustration that the sweep circuit is synchronized in such manner that it shows the response curve during frequency modulation in the upward direction, from 450 kc. to 470 kc., ats shown in figure 275. Horizontal displacement exists during the time that the oscillator is being frequency modulated in this di-

Fig. 276, left, Fig. 277 right. The resonance curve in Fig. 276 is non-symmetrical and that in Fig. 277 is symmetrical. These oscillograms were made using the frequency modulator and sweep described in Fig. 275. The vertical line on the right of each oscillogram indicates the vertical displacement due to change in frequency, but as the Sweep was shorted, it is not spread across the screen.

rection. When the frequency modulating condenser rotor has reached the position where a 470-kc. signal is being generated, the shorting contacts close and there is no horizontal displacement during the time that frequency modulation takes place in the downward direction. However, voltage is established across the circuit being tested during both halves of the frequency modulation cycle. The pattern or image appearing upon the screen, when checking a non-symmetrical circuit under these conditions, is shown in figure 276.

In accordance with the specifications stated, concerning the direction of frequency modulation, the resonance curve shown in figure 276 is for this half cycle, whereas the straight line is the vertical displacement without horizontal displacement, during the O sweep cycle of figure 275. The pattern for a symmetrical circuit is shown in figure 277. You now have a picture of the manner in which the resonance curve is developed with the Clough-Brengle OM frequency modulated r-f. oscillator.

Commercial Frequency Modulated R-F. Oscillators

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