Receivers

Author: J.B. Hoag

In the preceding chapters, the component parts of receivers have been presented in some detail. In this chapter we propose to study the most suitable combinations of radio, intermediate, and audio-frequency amplifiers, and also oscillators, for the amplification and detection of the weak voltages picked up by the receiving antenna. The simple crystal detector circuits described in the chapter on Principles of Detection are hardly suited for modern reception.

When a radio wave cuts across the wires of the receiving antenna, the voltages generated will only drive an exceedingly small current through the receiver's input end. It is necessary to resonate or augment these by means of inductance-capacitance tuned circuits. In addition, the use of a "pre-selector" circuit at the beginning of a receiver makes it possible to tune in only one of the many carrier waves which may be on the air at the same time.

The next step is to amplify the voltages developed across the capacity in this tuned circuit, and to extract or unscramble the useful intelligence which was superimposed upon the carrier wave at the transmitter.

Fig. 32 A. A simple receiver circuit

Figure 32 A shows one of the simplest receiver circuits which has proven satisfactory after much experimentation with different kinds of circuits. This is seen to be a regenerative circuit, together with a one-stage audio amplifier. For the very high frequencies, the simplest usable circuit has been found to be the super-regenerative type.

In order to increase the amplification and the ability of the receiver to select but one of several carrier waves, cascade amplifying systems have been built. A simple receiver will consist of a three-stage unit, as in Fig. 32 B.

Fig. 32 B. A simple receiver consisting of a one-stage r.f. amplifier, a detector, and a one-stage a.f. amplifier

The first stage is the radio frequency amplifier, with its resonant circuits; the second stage is a simple detector circuit, serving to rectify the signals so that its output consists only of the audio or modulated signal, and the last stage is an audio frequency amplifier of the "power" type. The function of the latter is to deliver enough power to operate a loudspeaker.

If a continuous wave (dots and dashes = c.w.) is received with the circuit of Fig. 32 B, it will be found that sound will not come out of the loudspeaker.

Fig. 32 C. An unmodulated carrier wave will not operate the receiver of Fig. 32 B

The reason for this will be clear from a study of Fig. 32 C. If, however, a local oscillator is used in connection with the detector, as suggested in Fig. 32 D, and its frequency is adjusted to a value differing from that of the incoming signal by some convenient audio frequency (say, 500 to 1,200 cycles), then a beat-note will be produced. This audio-frequency beat-note is amplified and delivered to the loudspeaker.

Fig. 32 D. For the reception of c.w.

Therefore, when continuous waves, such as dots and dashes of a code message, have appeared at the input of the receiver, beat-notes will be produced throughout the comparatively long time intervals of each dot and each dash, thus rendering the signal audible. The local oscillator must not be used, of course, when voice modulated waves are received, for then the beat-note will be superimposed upon the speech, and will cause a jumble of sound. It is possible to have the local oscillator in operation if its frequency is adjusted to be exaxctly that of the carrier, for then the beat-frequency will equal zero. In this case, the signal strength will be augmented. Due, however, to the difficulty of maintaining both frequencies continuously at the same value, this method is not commonly used.

Before we approach our study of the more common modern type of receiver (the superheterodyne), we must specify certain desirable properties of a receiver more accurately than we have yet done.