Superregenerative receivers radiate a strong, broad, and rough signal. For this reason, it is necessary in most applications to employ a radio frequency amplifier stage ahead of the detector, with thorough shielding throughout the receiver.
The Fremodyne The H a z e 11 i n e - Fremodyne Detector superregenerative circuit is expressly designed for reception of FM signals. This versatile circuit combines the action of the superregenerative receiver with the superhetrodyne, converting FM signals directly into audio signals in one double triode tube (figure 4). One section of the triode serves as a superregenerative mixer, producing an i-f of 22 Mc., an i-f amplifier, and a FM detector. The detector action is accomplished by slope detection tuning on the side of the i-f selectivity curve.
This circuit greatly reduces the radiated signal, characteristic of the superregenerative detector, yet provides many of the desirable features of the superregenerator. The pass-band of the Fremodyne detector is about 400 kc.
Because of its superiority and nearly universal use in all fields of radio reception, the theory of operation of the superheterodyne should be familiar to every radio student and experimenter. The following discussion concerns superheterodynes for amplitude-modulation reception. It is, however, applicable in part to receivers for frequency modulation.
Principle of In the superheterodyne, the in-Operation coming signal is applied to a mixer consisting of a non-linear impedance such as a vacuum tube or a diode. The signal is mixed with a steady signal generated locally in an oscillator stage, with the result that a signal bearing all the modulation applied to the original signal but of a frequency equal to the difference between the local oscillator and incoming signal frequencies appears in the mixer output circuit. The output from the mixer stage is fed into a fixed-tuned intermediate-frequency amplifier, where it is amplified and detected in the usual manner, and passed on to the audio amplifier. Figure 5 shows a block diagram of the fundamental superheterodyne arrangement. The basic components are shown in heavy lines, the simplest superheterodyne consisting simply of these three units. However, a good communications receiver will comprise all of the elements shown, both heavy and dotted blocks.
Superheterodyne The advantages of super-Advantages heterodyne reception are directly attributable to the use of the fixed-tuned intermediate-frequency (i-f) amplifier. Since all signals are converted to the intermediate frequency, this section of the receiver may be designed for optimum selectivity and high amplification. High amplification is easily obtained in the intermediate-frequency amplifier, since it operates at a
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