Direct-conversion receivers sound elegant — mix your RF signal with a local oscillator tuned to the carrier, low-pass filter, and you have baseband. But building high-Q, tunable filters at hundreds of MHz is nearly impossible, and DC offsets from LO leakage swamp weak signals. Edwin Armstrong's 1918 solution: mix the incoming RF down to a fixed intermediate frequency (IF), then do all the heavy filtering and amplification there. Nearly every radio built since — AM broadcast, FM, cellular, GPS front-ends — uses this trick.

The signal chain:

• RF preselector — a broad bandpass filter rejecting out-of-band garbage, especially the image frequency.
• LNA — sets the receiver's noise figure. Low noise, modest gain (10–20 dB).
• Mixer — multiplies RF by LO, producing sum and difference frequencies.
• Local oscillator (LO) — the only tunable element. Usually a PLL synthesizer.
• IF filter — high-Q crystal, ceramic, or SAW filter defining channel selectivity. This is the receiver's brain.
• IF amplifier — most of the receiver's gain (60–100 dB) lives here, at one fixed frequency.
• Demodulator — envelope detector, product detector, FM discriminator, or I/Q sampler.

The image problem: mixing RF with LO produces both f_RF − f_LO and f_LO − f_RF. But a signal at f_image = f_LO + f_IF (if RF is below LO) also maps to the same IF. Once folded in, it's indistinguishable from the desired signal. The RF preselector must reject the image before the mixer.

Design example — classic AM broadcast receiver: IF = 455 kHz, receiving 1000 kHz. LO tunes to 1000 + 455 = 1455 kHz. Image is at 1455 + 455 = 1910 kHz. The preselector must attenuate 1910 kHz while passing 1000 kHz — easy at these frequencies with a simple LC tuned circuit. To tune to 1500 kHz, LO moves to 1955 kHz; the IF filter never moves.

Rule of thumb: pick IF such that image rejection is easy at the front end and selective filters are cheap at the IF. Higher IF = easier image rejection but harder channel filtering. Lower IF = tighter channel filters possible but images get closer to the wanted signal. Classic compromises: 455 kHz (AM), 10.7 MHz (FM), 70 MHz (microwave links), 140 MHz (satellite downlink first IF in dual-conversion designs).

Dual conversion uses a high first IF (great image rejection) followed by a low second IF (great selectivity) — the best of both, at the cost of another mixer and LO.