The superheterodyne

Chapter The superheterodyne

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • The superheterodyneThe superheterodyne uses one or more mixers to convert an incoming signal, re-gardless of its frequency, to an identically modulated signal at some other, constant fre-quency. The signal frequency can be heterodyned once, twice, or even three times.Thus, you might hear of a single-conversion, double-conversion, or triple-conver-sion superheterodyne receiver.A single-conversion superhetA block diagram of a single-conversion receiver is shown in Fig. 27-13.The superheterodyne51527-13A single-conversion superheterodyne receiver.The incoming signal first passes through a sensitive, low-noise, tunable front-endamplifier. The tuning range of this amplifier must be sufficient to cover all the desiredreception frequencies fIN.The second stage is a mixer/LO combination. The LO has a variable frequency thattunes over the received-signal range plus 9.000 MHz. The LO frequency control is themain tuning control for the entire receiver. The LO tuning might track along with thetuning of the front end, or the front end might tune independently by means of a sepa-rate preselector control. The mixer output is always at 9.000 MHz, no matter what theincoming signal frequency.The intermediate frequencyThe 9.000-MHz mixer output signal is called the intermediate frequency (IF) of thesuperhet. This signal has the same modulation waveform, and the same bandwidth, asthe incoming signal. The only difference is that it might be “upside down”; LSB wouldbe changed to USB, or the sense of FSK would be reversed. But this is an inconsequen-tial difference insofar as it has no effect on the quality of the received signal.