Amplitude-Modulation Radio Receivers

The Tuned-Radio-Frequency Receiver. This is shown in the block diagram of Fig. 12. The signal voltage from the antenna is impressed on several stages of tuned-radio-frequency voltage amplifiers that increase the magnitude of the carrier and the two sidebands constituting the received message or program signal. This signal is then impressed on the demodulator, or detector, where it is distorted, and sum and difference frequencies are produced. If the carrier and the sidebands are as listed on page 489, the sum frequencies are in the vicinity of 2 megacycles, and the difference frequencies will constitute two bands from 100 to 5000 cycles. These two bands are in phase and combine to produce the desired audible output signal voltage. This is passed by the audio-frequency amplifier, which incorporates both voltage and power amplification, and the output drives the loudspeaker (or headphones) reproducing the message or program transmitted.

Figure 12. Block diagram of a tuned-radio-frequency radio-receiving set.

The demodulator, or detector, may be one of several types. It is possible to use vacuum-tube plate-circuit demodulation similar to that used in early carrier telephone systems (page 421). However, in the tuned-radio-frequency receivers that once were used, the distortion required for demodulation was produced in the grid circuit of the tube. The detector tube was operated with negligible grid bias, and, when the amplified received signal composed of the carrier and the sidebands was impressed, distorted grid current flowed to cathode. A parallel circuit composed of a resistor and capacitor was placed in series with the grid. The circuit constants were such that, when the distorted grid current flowed through it, the difference frequencies (that is, the desired audio signal) caused an audio-frequency voltage drop across the resistor-capacitor combination, but radio-frequency components caused negligible drop. This audio-voltage drop, being between the grid and cathode, was amplified in the plate circuit of the detector tube and, after additional amplification, drove the loudspeaker. This circuit was called the grid-leak detector.

A tuned-radio-frequency receiving set is simple and has advantages for some purposes such as fixed-frequency communication. For broadcast reception in the standard amplitude-modulation range, it must tune from 550 to 1600 kilocycles, and this tuning offers design problems. The set is not very selective in its usual form, and the early type of detection caused serious distortion on signals with high percentage modulation.