Dual conversion receiver

There are many interesting receiver concepts. The double-conversion is one of several options.
For example, shortwave receivers to cover 0.1 to 30 MHz are better with first stage operating as an upconverter, then another conversion is done down from say ~50 MHz. Filtering unwanted signals is one of most important requirements.
A typical double superhet converts the RF input say at 50 or 500 MHz down to the first IF at 10 or 30 MHz, then again, to 455 kHz or some other second IF where the signal is demodulated. Using good filters, unwanted responses are suppressed.
Many spectrum analyzers are wideband receivers that utilize up to four down-conversion stages. Such instruments need to measure signal power over a very narrow band (selectable from MHz to 0.1 Hz) while the filters must reject any unwanted response.

A high first IF makes it easier to get good image rejection but is more difficult to get narrow bandwidth. A dual conversion receiver starts with a high first IF then converts it down to a lower frequency where it's bandwidth can be more easily tailored.

Brian.