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AGC, noise blankers and noise reduction
Taming level changes and noise.
Taming level changes and noise.
Signals vary enormously in strength, and noise rides along with them. Three receiver features deal with this: AGC (automatic gain control) keeps the audio level steady, a noise blanker removes short sharp pulses, and noise reduction (usually DSP) thins out continuous hiss. Each suits a particular problem and does nothing for the others.
Signals at the antenna can differ by 100 dB or more, but your ears want a fairly steady volume. The AGC circuit measures the signal level late in the IF chain, turns it into a control voltage, and uses that to reduce the gain of earlier stages when the signal is strong. The S-meter in many receivers reads the same control voltage (see Signal reports: RS, RST and the S-meter).
Two times set its behaviour: attack (how fast it turns the gain down, a few milliseconds) and decay or release (how fast the gain comes back after the signal drops).
| Setting | Decay (typical) | Good for |
|---|---|---|
| Fast | a fraction of a second | CW and digital modes: weak signals return quickly between strong ones |
| Slow | a second or two | SSB and AM: steady volume through pauses in speech, without noise rushing up between words |
| Off | none | rare: working a very weak signal near a strong one, with RF gain set by hand |
Too fast an AGC on SSB makes background noise surge up in every pause. A strong noise burst or nearby signal also turns the gain down, so a weak signal can seem to "pump".
Impulse noise (ignition sparks, electric fences, arcing power lines, some switching supplies) arrives as very short, strong pulses. The blanker detects each pulse and turns the signal path off for its duration, a matter of microseconds or milliseconds. Because the pulse is so short, you hear almost nothing missing.
It has to sit ahead of the narrow filters: a pulse is only short and sharp while the signal is still wide. It does nothing against continuous noise, and a very strong nearby signal can look like a pulse and get chopped, causing distortion. Turn it off if the audio sounds ragged.
Continuous noise (hiss, buzz, the "grass" on a band) has no pulses to blank. DSP noise reduction works on the sampled audio: voice and CW have structure that repeats from moment to moment, noise does not. The processor emphasises the repeating parts and suppresses the random ones.
It cannot create signal that is not there, and strong settings make speech sound "watery" or digitally garbled. A modest setting can make weak signals easier to copy.
A notch filter is the tool for a single steady carrier or whistle: it cuts a very narrow slice out of the passband while leaving the rest.
| Problem | Try |
|---|---|
| Fast clicks and pops | noise blanker |
| Steady hiss | noise reduction, narrower filter |
| A single tone or carrier | notch filter |
| Loud adjacent signal | narrower filter, passband shift |
| Strong signal fading everything | AGC, or less RF gain |
| Noise from your own house | find and fix the source (see RFI and EMI overview) |