Which description best defines the intermediate frequency (IF) in a receiver?

• A. The fixed frequency to which the received signal is down-converted for processing.
• B. The frequency of the local oscillator
• C. The variable frequency used in the mixer
• D. The original RF frequency

Answer: (A)

The key idea is that the intermediate frequency is the fixed, lower frequency that the received signal is converted to inside the receiver for easier processing. In a superheterodyne receiver, the incoming radio frequency is mixed with a locally generated oscillator signal. The mixer outputs two new frequencies—the sum and the difference of the RF and LO frequencies—and the design selects the difference that matches a predetermined, steady IF. Keeping this IF constant allows the receiver to use highly selective, stable filters and amplifiers optimized for that frequency, which improves selectivity and sensitivity across the tuning range. The local oscillator frequency is what’s varied to bring different RF signals down to that same IF, while the original RF frequency is simply the signal being received before conversion.