Transmitters

Author: J.B. Hoag

In preceding chapters, the component parts of transmitters have been presented in some detail. Here, the method of linking oscillators, amplifiers, modulators, antennas, and microphones into a single unit, called a transmitter, becomes the main problem; together with observation of any new phenomena which result from the mutual influence of these parts upon each other.

In a broad way, we realize that the transmitter must contain an oscillator in order to generate currents whose frequency is sufficiently elevated that appreciable electromagnetic radiation can take place from a suitably designed antenna, and that some means must be devised to add human intelligence to the emitted energy. Our transmitter (of the amplitude modulated type) must be so designed and adjusted that its frequency will remain constant; also the carrier wave, when unmodulated, must not change in amplitude. In other words, we desire stable output. If we attempt to devise a single oscillator of great power, we find a unit which is beautifully flexible, but not particularly stable. Hence it has become recognized that a more satisfactory plan is to use a very low-power oscillator, with inherent self-stability, followed by either straight amplifiers and/or frequency multiplying stages. Between the oscillator and the frequency multiplier, it is found necessary to introduce a buffer amplifier to prevent interaction of the load upon the oscillator.

In planning a transmitter, one of the first steps is to choose the frequency at which the oscillator is to be operated. The circuit, the tube or tubes, and the numerical values of the component parts can then be determined. The second step in planning the transmitter is the choice of the output power of the entire unit. This, of course, fixes the requirements of the final amplifier stage. For this stage, a suitable circuit is chosen, and a suitable tube capable of handling the desired power. From the tube manufacturer's data one can determine the required grid driving-power. This sets the requirements for the preceding stage. The design is then followed, stage by stage, back to the low-power " master " oscillator. If the intelligible message is to be transmitted by means of code signals, it is necessary to decide at what point, and how, to break the carrier wave up into the dots and dashes. This is generally accomplished at a low-power stage in the transmitter. On the other hand, if the communication is to be carried on by means of a phone, one must plan the point at which the modulator tube ties into the radio frequency assembly. If the speech amplifier has but small output, then low-level modulation must be used. It is often common practice, however, to modulate the r.f. at or near the final amplifier stage (high-level modulation); in which case the speech amplifier must of-itself have considerable output power.