Requirements of a good radio receiver; superheterodyne

Requirements of a good radio receiver

1. Sensitivity: ability to pick up weak signals and give a usable output (measured in µV for a given output and SNR).
2. Selectivity: ability to select the wanted station and reject adjacent channels.
3. Fidelity: ability to reproduce all message frequencies equally, without distortion.
4. Image frequency rejection: high rejection of the image $f_{si} = f_s + 2f_{IF}$.
5. Good SNR / low noise figure in the front end.
6. Stability and simple, single-knob tuning over the whole band.
7. Automatic gain control so output stays steady as signal strength changes.

Superheterodyne receiver

A superheterodyne receiver converts every incoming station to one fixed intermediate frequency (IF) (455 kHz for AM broadcast), and does most of the amplification and filtering at that fixed frequency.

Antenna
  |
[RF amp]-->[Mixer]-->[IF amp]-->[Det]-->[AF amp]-->Spk
  fs          ^      455 kHz   |
  |           |        ^       |
  |       [Local osc]  +--AGC--+
  |       fLO=fs+fIF
  +--ganged tuning--+

Blocks

1. RF stage: a tuned amplifier at the signal frequency $f_s$. It improves sensitivity and noise figure and rejects the image frequency before mixing.
2. Local oscillator and mixer: the oscillator is tuned together with the RF stage (ganged) so that $f_{LO} = f_s + f_{IF}$. The mixer is a non-linear stage giving sum and difference frequencies; the difference $f_{LO} - f_s = f_{IF}$ is selected. The message is kept, only the carrier is moved.
3. IF amplifier: several fixed-tuned stages at 455 kHz with bandwidth about 10 kHz. Since the frequency is fixed, they give high gain and sharp, constant selectivity for all stations. Most of the receiver gain comes from here.
4. Detector: an envelope detector recovers the audio. It also gives a DC voltage proportional to carrier strength.
5. AGC: this DC level controls the gain of RF and IF stages, keeping the output nearly constant for strong and weak stations.
6. AF amplifier and speaker: raise the audio to the power needed to drive the loudspeaker.

Example: for a station at 1000 kHz, $f_{LO} = 1455$ kHz and $f_{IF} = 455$ kHz. The image is $1000 + 2(455) = 1910$ kHz, which the RF stage must reject.

Advantages: uniform selectivity and sensitivity across the band, high gain without instability, simple tuning. Problems: image frequency and local oscillator tracking.