Down conversion is often used to allow reception of ultra-high-frequency (UHF) and microwave signals (above 300 MHz). The UHF or microwave input is mixed with an LO to provide an output that falls within the tuning range of a shortwave VHF receiver.
A block diagram of a down converter for UHF/microwave reception is shown in Fig. 27-11B.
At B, a down converter that allows UHF/microwave reception on a shortwave receiver.
This converter has an output that covers a huge band of frequencies. In fact, a single frequency allocation at UHF or microwave might be larger than the entire frequency range of a shortwave receiver.
An example is a UHF converter designed to cover 1.000 GHz to 1.100 GHz. This is a span of 100 MHz, more than three times the whole range of a shortwave radio.
To receive 1.000 to 1.100 GHz using a down converter and a shortwave receiver, the LO frequency must be switchable. Suppose you have a communications receiver that tunes in 1-MHz bands.
You might choose one of these bands, say 7.000 to 8.000 MHz and use a keypad to choose LO frequencies from 0.993 GHz to 1.092 GHz. This will produce a difference-frequency output at 7.000 to 8.000 MHz for 100 segments, each 1 MHz wide, in the desired band of reception.
If you want to hear the segment 1.023 to 1.024 GHz, you set the LO at 1.016 MHz. This produces an output range from 1023 − 1016 = 7 MHz to 1024 − 1016 = 8 MHz.
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