1. The following block diagram shows, in simplified form, the major building blocks of a typical Tunable IF communication receiver used in amateur radio stations from the 1960s onwards and capable of multi-mode and multi-band operation:

3. The various functional blocks shown above may be designed and assembled individually from own designs or kits that are available from commercial suppliers. Consideration must be given to the design of modules that may require to be bi-directional or interconnected in order to be part of a transceiver.

The main receiver is shown above to the right and the crystal controlled converter to the left. The receiver will cover a single tuning range which may be an amateur band or not and the converter will cover as many ranges as is required. In the late 1970s this receiver technique on the HF bands was replaced by designs using synthesised local oscillators and a single frequency conversion to the IF containing the main selectivity block in order to improve the dynamic range. However, local oscillator noise in the synthesiser restricted the overall dynamic range that could be obtained.

Modern mixer and filter design from the 90s onwards means that the tunable IF is still an option for good receiver performance without the use of synthesiser techniques and their attendant noise problems. The H-Mode mixer and its associated RF filters by G3SBI and PA3AKE has demonstrated perfectly adequate HF receiver sensitivity without the need for an RF stage.

2. The following block diagram shows a dual conversion communication receiver with synthesised local oscillators. The second IF is optional but does provide a mechanism for providing significant gain at a lower frequency to the mode filters with a minimal risk of instability.

For continuous coverage receivers the first IF needs to be outside the receiver tuning range and is frequently in the 40 - 70Mhz region. As crystal filters are complex and expensive in that frequency range the mode filters would be moved to the second IF with only a roofing filter present at the first IF.

However, for amateur band only applications a first IF of 9Mhz is popular as crystal filters are available commercially and may also be made by home constructors.