Beat-Frequency Oscillators

Author: J.B. Hoag

Fig. 29 I. A beat-frequency oscillator

In Fig. 29 I, the voltage output from two oscillators is fed into a common detector. The result is a " beat-note " whose frequency is equal to the difference in frequency between those of the two oscillators. This can be understood from Fig. 29 J. By simple addition, point by point, of the voltages of the two oscillators, the detector's input voltage can be plotted, as in the third curve from the top. It will rise and fall at the difference-frequency previously mentioned. Because the detector passes only the positive half-cycles, and because condenser C (Fig. 29 I) bypasses the higher frequency pulses, the voltage fluctuations across B follow only the average, rectified curve (dotted line Fig. 29 J) of the input voltage (as indicated in the fourth curve down from the top of Fig. 29 J). The fluctuating voltages across R are passed through a condenser to the amplifier and on to the output circuits.

Fig. 29 J. Principle of beats

Suppose the frequency of the fixed oscillator of Fig. 29 I were established at 200,000 cycles per second, while that of the variable oscillator was changed (by varying its tank condenser) from 200,000 to 210,000 cycles per second. Then the beat-frequency heard in a loudspeaker connected to the output terminals would change from zero to 10,000. Thus, with a single dial control on the condenser of the variable oscillator, the entire audio band can be covered.

When the two oscillators have nearly the same frequency, they tend to " pull-in," the stronger one forcing the weaker one to assume its frequency. This tendency can be overcome by use of a buffer-amplifier (which is just like any other amplifier covering the proper frequency range) and by injecting the two oscillator voltages into the detector on two separate grids, with a screen grid between, as indicated in Fig 29 I

The production of beats is referred to as a heterodyne process. Hetero means to mix and dyne is a unit of force; hence — a mixture of the outputs of the two oscillators.

Suppose the fixed frequency was 1,000,000, and the other frequency was 1,000,401 cycles per second. The beat-note would have a frequency of 401 cycles per second. Let this be beat a second time against a 400-cycle oscillator. The second beat-note would be 1 cycle per second. Now change the variable oscillator by only one part in a million, to 1,000,402. The second-beat would change from 1 to 2 cycles per second, a very noticeable difference. It does not require a very large change in the capacitance of an oscillator to shift its frequency by this small amount. If the plates are moved closer together by one one-millionth of an inch, the change can be detected. Any other shift in the variable oscillator, in its capacitance, resistance, inductance, or voltages can be observed with equal delicacy.