THE SUPERHETERODYNE BASIC INFORMATION

The superheterodyne uses one or more mixers to convert an incoming signal, regardless of its frequency, to an identically modulated signal at some other, constant frequency. The signal frequency can be heterodyned once, twice, or even three times. Thus, you might hear of a single-conversion, double-conversion, or triple-conversion superheterodyne receiver.

A single-conversion superhet

The incoming signal first passes through a sensitive, low-noise, tunable front-end amplifier. The tuning range of this amplifier must be sufficient to cover all the desired reception frequencies fIN.

The second stage is a mixer/LO combination. The LO has a variable frequency that tunes over the received-signal range plus 9.000 MHz. The LO frequency control is the main tuning control for the entire receiver.

The LO tuning might track along with the tuning of the front end, or the front end might tune independently by means of a separate preselector control. The mixer output is always at 9.000 MHz, no matter what the incoming signal frequency.

The intermediate frequency

The 9.000-MHz mixer output signal is called the intermediate frequency (IF) of the superhet. This signal has the same modulation waveform, and the same bandwidth, as the incoming signal.

The only difference is that it might be “upside down”; LSB would be changed to USB, or the sense of FSK would be reversed. But this is an inconsequential difference insofar as it has no effect on the quality of the received signal.

The IF is easy to process because its frequency never changes. Several IF amplifier stages, along with filtering, provide the best possible sensitivity and selectivity. This part of the receiver is the IF amplifier chain or IF chain.