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Having followed the signal thus far
and having seen it well on its way in
the primary of the i.f. transformer, we
might figuratively stand aside for a
little while and with our inward eye
observe other signals as they pass
after they are tuned in. "We're all
intermediate now!" would be a good
slogan for radio signals in this part
of the circuit.
Now the chief agent in "creating all signals equal" is the local oscillator. It generates an unmodulated wave of just the right frequency to combine with the incoming signal so that the difference is always that to which the i.f. channel is tuned. Furthermore, when the weak incoming signal joins with the strong oscillator voltage, the union produces an i.f. signal that is the difference of the two. So, the offspring has a new identity and a far greater strength but it never loses the character, the modulation, of the signal. The oscillator itself is a highly important component, not only in the receiver but in the broadcasting station also. Before the invention of the vacuum tube, the generation of high-frequency waves presented a serious problem. At one time, huge rotating alternators were used. Now the 50 kw. output of a powerful broadcasting station is initiated and controlled by a small glass tube that can be tucked into a man's pocket. Fundamentally the oscillator is an amplifier excited by its own output. It consists of an ordinary vacuum tube containing plate, grid, and cathode. The associated circuit of coils and condensers comprise a resonant circuit capable of vibrating or oscillating electrically. The energy is supplied by the local battery or "B"-supply. With the proper testing equipment it is easy to determine whether the circuit is oscillating and at what frequency. Since the output is unmodulated it can not be detected with an audio tracer. However, sudden or sharp changes in output (or in plate current) can be heard. With the probe at G, a click should be heard when the oscillator condenser plates are shorted with a small screwdriver and again when the short is removed. In the 7A8 tube of Fig. 4, the cathode, grid No. 1, and grid No. 2 are the oscillator cathode, grid, and plate, respectively. The plate voltage may be measured directly. Control grid voltage will show negative if the tube is oscillating. |
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