The superheterodyne ("superhet") receiver converts every signal you tune to onto one fixed intermediate frequency (IF). Amplifiers and filters built for that single frequency can be excellent, and they never need retuning. Nearly every analog and many digital receivers, from broadcast radios to HF rigs, work this way.
Tuning by moving the LO
Worked example with a 9 MHz IF, tuned to a CW signal at 14.200 MHz. Everything right of the mixer stays on 9 MHz however you tune.
The mixer multiplies the incoming signal by the local oscillator and produces the sum and difference of their frequencies (see Mixers and frequency conversion). The IF filter keeps only the difference. To tune a different station you change the LO so that the new signal also lands on 9 MHz:
Dial frequency
LO (IF + RF)
Difference
7.000 MHz
16.000 MHz
9.000 MHz
14.200 MHz
23.200 MHz
9.000 MHz
21.200 MHz
30.200 MHz
9.000 MHz
Tune the local oscillator to pull a different signal onto the fixed IF. The image is the unwanted second one.
Wanted signal14.20 MHz
Image32.20 MHz
Moving the LO slides the whole band past a fixed window. This is why the selecting filter can be a fixed, narrow, high-quality one. The LO itself is usually a synthesizer, so tuning is accurate and stable.
The image problem
The mixer cannot tell which side of the LO a signal is on. A station at 14.200 MHz gives 23.200 − 14.200 = 9 MHz. But a station at 32.200 MHz gives 32.200 − 23.200 = 9 MHz as well. That unwanted second frequency, 2 × IF above the signal, is the image. Once it is through the mixer it cannot be separated from the wanted signal, so it must be stopped before the mixer, by the RF band-pass filter.
The image lies 2 × IF from the wanted signal. A higher IF moves it far enough away for the front-end filter to reject.
That is where the IF choice matters:
A low IF (say 455 kHz) puts the image only 910 kHz away (at 15.110 MHz for a 14.200 MHz signal), too close for a front-end filter to reject.
A high IF puts the image far away, so simple front-end filters stop it. But narrow, steep filters are hard to make at high frequencies.
Double conversion
The usual answer is two conversions. A first, high IF pushes the image far out of the way and lets a roofing filter remove strong neighbours. A second mixer then brings the signal down to a low IF (or to audio in DSP), where narrow filters are easy and cheap. Many modern HF radios go further and use a very high first IF, well above the top of HF, so all the images fall outside the bands they cover.
Pitfalls
Image and other spurious responses show up as signals that appear on the wrong dial frequency.
Birdies: harmonics and mixing products of the radio's own oscillators land on certain frequencies as steady whistles or carriers with no antenna attached.
Each conversion adds noise and distortion, so a good design keeps the number of stages small.