How it works
The received RF signal is amplified by a tuned RF stage, then mixed with the local oscillator (LO) signal in a non-linear mixer to produce sum and difference frequencies. The difference frequency fIF = fLO − fRF is always 455 kHz for AM broadcast receivers (10.7 MHz for FM). This fixed IF passes through a crystal or ceramic IF filter with a 3 dB bandwidth of about 10 kHz for AM, providing sharp selectivity. The IF amplifier provides most of the receiver gain — typically 60–80 dB. A demodulator (envelope detector for AM) then recovers the audio, which is amplified in an audio stage. AGC voltage fed back to the IF and RF amplifiers keeps output level constant across signal strengths.
Key points to remember
Image frequency = fRF + 2·fIF for a high-side LO (LO above RF); for a 720 kHz AM station, the image is at 720 + 910 = 1630 kHz. Image rejection is provided by the pre-mixer RF tuned stage and is typically 40–60 dB in a well-designed receiver. Sensitivity refers to the minimum detectable signal — typically expressed as μV at the antenna terminal for a given SNR, often 10 dB SINAD. Selectivity is the ability to reject adjacent channels and is determined by the IF filter bandwidth. Double conversion superheterodyne receivers use two mixer stages with a high first IF (e.g., 10.7 MHz) to improve image rejection, then a low second IF (455 kHz) for selectivity.
Exam tip
Every Anna University communication systems exam asks you to draw the complete superheterodyne receiver block diagram and calculate image frequency for a given station — draw all blocks from antenna to speaker, label fIF = 455 kHz, and show the image frequency calculation clearly.