Superheterodyne Receiver

Signal Chain Walkthrough

The superheterodyne receiver, invented by Edwin Armstrong in 1918, remains the most widely used receiver architecture in RF engineering. It converts the incoming RF signal to a fixed intermediate frequency (IF) where filtering, amplification, and demodulation are performed more efficiently.

Preselector (RF BPF)

A bandpass filter at the antenna input selects the desired frequency band and rejects out-of-band interference. It provides initial protection against strong out-of-band signals that could overload the LNA.

Image Reject Filter

The critical filter between the LNA and mixer. It must reject the image frequency, which is separated from the desired signal by 2×f_IF. If the IF is 70 MHz and the desired signal is at 1 GHz, the image is at 1.14 GHz (or 0.86 GHz). This filter must provide 40-60 dB of image rejection.

IF Bandpass Filter

Defines the channel bandwidth and provides the receiver's selectivity. At the fixed IF frequency, high-Q crystal, ceramic, or SAW filters can achieve very narrow bandwidths that would be impractical at RF. This is the key advantage of the superheterodyne architecture.

AGC (Automatic Gain Control)

The IF amplifier typically includes AGC to maintain constant output level over a wide range of input signal strengths. AGC feedback adjusts the gain of the IF amplifier (and sometimes the LNA) based on the detected signal level.

Demodulator

Extracts the information from the IF-modulated carrier. The demodulator type depends on the modulation scheme: envelope detector for AM, discriminator or PLL for FM, or I/Q demodulator for digital modulations.