In this latter figure you will notice a small air-core coil, LR, between the oscillator and the modulator tube. This coil has a small inductance but it is enough to offer a large impedance to radio-frequency currents. The result is, it does not let the alternating currents of the oscillating tube flow into the modulator. These currents are confined to their own circuit, where they are useful in establishing similar currents in the antenna. On the other hand, the coil LR doesn’t seriously impede low-frequency currents and therefore it does not prevent variations 241 in the current which are at audio-frequency. It does not interfere with the changes in current which accompany the variations in the resistance of the plate circuit of the modulator. That is, it has too little impedance to act like LA and so it permits the modulator to vary the output of the oscillator.
The oscillating circuit of Fig. 125 includes part of the antenna. It differs also from the others I have shown in the manner in which grid and plate circuits are coupled. I’ll explain by Fig. 126.
The transmitting set which I have just described involves many of the principles of the most modern sets. If you understand its operation you can probably reason out for yourself any of the other sets of which you will hear from time to time.
242LETTER 23
AMPLIFICATION AT INTERMEDIATE FREQUENCIES
Dear Son:
In the matter of receiving I have already covered all the important principles. There is one more system, however, which you will need to know. This is spoken of either as the “super-heterodyne” or as the “intermediate-frequency amplification” method of reception.
The system has two important advantages. First, it permits sharper tuning and so reduces interference from other radio signals. Second, it permits more amplification of the incoming signal than is usually practicable.
First as to amplification: We have seen that amplification can be accomplished either by amplifying the radio-frequency current before detection or by amplifying the audio-frequency current which results from detection. There are practical limitations to the amount of amplification which can be obtained in either case. An efficient multi-stage amplifier for radio-frequencies is difficult to build because of what we call “capacity effects.”