Superheterodyne AM receiver; why standard AM in broadcasting

A superheterodyne receiver converts the incoming RF signal of any station to a fixed intermediate frequency (IF), 455 kHz for AM broadcast, where most of the gain and selectivity are obtained.

Block diagram

 Antenna
   |
 [RF amplifier]--->( Mixer )--->[ IF amplifier ]
   ^  fs              ^             455 kHz
   |                  | fLO             |
   +--- ganged ---[Local osc]      [ Detector ]
        tuning                          |
                    AGC <---------------+
                                        |
                  Speaker <--[ AF amplifier ]

Working principle

1. Antenna and RF amplifier: the tuned RF stage selects the desired station frequency $f_s$ (535–1605 kHz), amplifies it, improves SNR and rejects the image frequency.
2. Local oscillator and mixer (frequency changer): the LO is ganged with the RF tuning so that it always runs IF above the signal:

$$f_{LO}=f_s+f_{IF}$$

The mixer produces $f_{LO}\pm f_s$; the difference $f_{LO}-f_s=455$ kHz is selected. The modulation (sidebands) is kept unchanged, only shifted to IF. 3. IF amplifier: several fixed-tuned stages at 455 kHz with bandwidth about 10 kHz. Since frequency is fixed, they give high, uniform gain and sharp selectivity (adjacent-channel rejection) for all stations. 4. Detector: a diode envelope detector recovers the audio from the IF signal. 5. AGC (automatic gain control): a DC voltage proportional to the carrier level from the detector controls the gain of RF and IF stages, keeping output nearly constant for strong and weak stations and preventing overload. 6. AF amplifier and loudspeaker: amplify the audio to the required power and convert it to sound.

Example: to receive 1000 kHz, $f_{LO}=1455$ kHz. The image is $f_s+2f_{IF}=1910$ kHz, which must be rejected by the RF stage.

Advantages: constant selectivity and sensitivity over the band, high gain without instability, good adjacent-channel rejection. Disadvantages: image-frequency problem, mixer noise, tracking alignment needed.

Why standard AM is used in AM broadcasting

1. Simple, cheap receivers: standard AM (DSB-FC) can be demodulated by a simple envelope detector; no carrier recovery or synchronization is needed.
2. One transmitter, millions of receivers: it is economical to put extra power (the carrier) in one costly transmitter so that every listener can use a cheap receiver.
3. The transmitted carrier also helps AGC and tuning.
4. Historical compatibility with the huge base of existing receivers.

The cost is poor power efficiency (at most 33.3%) and twice the bandwidth of SSB, which are acceptable for broadcasting.