Radio Receivers (AM/FM)
What this domain tests
Radio Receivers (AM/FM) focuses on superheterodyne architecture: RF amplifier, mixer, local oscillator, IF amplifier, detector, AGC, AFC, and alignment; AM/FM broadcast bands and receiver performance.
Core rules
- A superheterodyne receiver converts incoming RF to a fixed IF: AM commonly 455 kHz, FM 10.7 MHz.
- The local oscillator usually runs above the signal for high-side injection: fLO=fRF+fIF.
- Image frequency = fRF+2fIF for high-side injection; RF preselection and IF filtering reject it.
- AGC holds audio output steady as signal strength varies; AFC corrects local oscillator drift in FM.
- Sensitivity is the weakest usable signal; selectivity is rejection of adjacent channels; tracking keeps RF and LO tuned together.
- FM detector: ratio detector or discriminator; a limiter removes amplitude noise.
Worked example
An AM receiver is tuned to 1000 kHz with an IF of 455 kHz. High-side LO=1000+455=1455 kHz. Image=1455+455=1910 kHz, or fRF+2fIF=1000+910=1910 kHz. A station at 1910 kHz can pass the RF stage and mix to 455 kHz, so a tuned RF filter is needed.
Traps
Calculating the image as fRF+fIF is wrong. Do not assume the LO is always below the signal. IF is not audio; it is the fixed intermediate frequency before detection. Aligning IF stages with the signal generator on the wrong frequency spoils selectivity. FM capture ratio helps reject co-channel interference but does not replace proper IF alignment.