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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