Fig: TRF receiver: antenna cascaded tuned RF amplifiers envelope detector AF amplifier speaker; all resonators track togetherFig: Superheterodyne receiver: antenna RF amp mixer (fed by the local oscillator) fixed-IF amp detector AF amp speaker, with AGC feedback to the RF/IF stagesFig: Mixer frequency conversion: f_RF and f_LO in; the IF filter selects the difference f_IF=f_RF-f_LO and discards the sumFig: Superheterodyne FM receiver: RF amp mixer/LO IF amp limiter discriminator de-emphasis AF amp speaker; a slow discriminator error feeds AFC back to the LO