Letters of a Radio-Engineer to His Son

by John Mills

Consider for example the portion of circuit shown in Fig. 127. The wires a and b act like small plates of condensers. What we really have, is a lot of 243 tiny condensers which I have shown in the figure by the light dotted-lines. If the wires are transmitting high-frequency currents these condensers offer tiny waiting-rooms where the electrons can run in and out without having to go on to the grid of the next tube. There are other difficulties in high-frequency amplifiers. This one of capacity effects between parallel wires is enough for the present. It is perhaps the most interesting because it is always more or less troublesome whenever a pair of wires is used to transmit an alternating current.

In the case of a multi-stage amplifier of audio-frequency current there is always the possibility of the amplification of any small variations in current which may naturally occur in the action of the batteries. There are always small variations in the currents from batteries, due to impurities in the materials of the plates, air bubbles, and other causes. Ordinarily we don’t observe these changes because they are too small to make an audible sound in the telephone receivers. Suppose, however, that they take place in the battery of the first tube of a series of amplifiers. Any tiny change of current is amplified many times and results in a troublesome noise in 244the telephone receiver which is connected to the last tube.

In both types of amplifiers there is, of course, always the chance that the output circuit of one tube may be coupled to and induce some effect in the input circuit of one of the earlier tubes of the series. This will be amplified and result in a greater induction. In other words, in a circuit where there is large amplification, there is always the difficulty of avoiding a feed-back of energy from one tube to another so that the entire group acts like an oscillating circuit, that is “regeneratively.” Much of this difficulty can be avoided after experience.

If a multi-stage amplifier is to be built for a current which does not have too high a frequency the “capacity effects” and the other difficulties due to high-frequency need not be seriously troublesome. If the frequency is not too high, but is still well above the audible limit, the noises due to variations in battery currents need not bother for they are of quite low frequency. Currents from 20,000 to 60,000 cycles a second are, therefore, the most satisfactory to amplify.

Suppose, however, one wishes to amplify the signals from a radio-broadcasting station. The wave-length is 360 meters and the frequency is about 834,000 cycles a second. The system of intermediate-frequency amplification solves the difficulty and we shall see how it does so.

245At the receiving station a local oscillator is used. This generates a frequency which is about 30,000 cycles less than that of the incoming signal. Both currents are impressed on the grid of a detector. The result is, in the output of the detector, a current which has a frequency of 30,000 cycles a second. The intensity of this detected current depends upon the intensity of the incoming signal. The “beat note” current of 30,000 cycles varies, therefore, in accordance with the voice which is modulating at the distant sending station. The speech significance is now hidden in a current of a frequency intermediate between radio and audio. This current may 246be amplified many times and then supplied to the grid of a detector which obtains from it a current of audio-frequency which has a speech significance. In Fig. 128 I have indicated the several operations.

We can now see why this method permits sharper tuning. The whole idea of tuning, of course, is to arrange that the incoming signal shall cause the largest possible current and at the same time to provide that any signals at other wave-lengths shall cause only negligible currents. What we want a receiving set to do is to distinguish between two signals which differ slightly in wave-length and to respond to only one of them.

Suppose we set up a tuned circuit formed by a coil and a condenser and try it out for various frequencies of signals. You know how it will respond from our discussion in connection with the tuning curve of Fig. 51 of Letter 13. We might find from a number of such tests that the best we can expect any tuned circuit to do is to discriminate between signals which differ about ten percent in frequency, that is, to receive well the desired signal and to fail practically entirely to receive a signal of a frequency either ten percent higher or the same amount lower.