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My first shortwave radio was a Nanaola 10NT504, a receiver that uses superheterodyne technology. In that mode, a radio is tuned by mixing the incoming radio signal with a locally generated variable frequency carrier signal (from a variable frequency oscillator or VFO) in a “mixer” stage, producing two output signals equal to the sum and the difference of the two originals. Those are passed on to a filter tuned to one fixed frequency (often 455 kHz) called the intermediate frequency, which is then amplified, demodulated into audio, amplified again, and sent to a speaker. The VFO signal can be on the high side or the low side of the incoming signal, but in most radios it is on the high side, or 455 kHz + the desired frequency for receiving.
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