AGC Explained: Fast, Slow, Attack and Hang Time on HF

Almost every HF receiver you will ever use has a control that most operators leave in the default position for years: the AGC. In this AGC explained guide we look at what automatic gain control actually does, why it can make a good receiver sound pumping and unpleasant, and how to choose between fast and slow settings for CW, SSB, digital modes and DX pileups. The short version is that the AGC decides how loud strong and weak signals are relative to each other, and it makes that decision continuously and in real time. Understanding its three timing settings, attack, decay and hang, is one of the cheapest ways to improve how a station copies, because it needs no new hardware and costs no money.

What Automatic Gain Control Does

Radio signals on the HF bands vary in strength by an enormous range. A weak DX station may be a few microvolts at the antenna, and a local station on the same band may be a hundred thousand times stronger. If a receiver amplified both by the same fixed amount, one would be inaudible and the other would blast your ears and overload the stages that follow. The automatic gain control circuit is a feedback loop that measures the signal level, uses it to turn the amplifier gain down when the signal is strong and lets the gain rise again when the signal is weak. The effect is that the output level stays reasonably constant over a very wide input range.

In a typical receiver the control voltage comes from the detector or from a dedicated AGC detector, and it is applied to the intermediate frequency amplifiers and sometimes to the RF stage. Modern radios do the same thing in software or in the digital signal processing chain, but the idea has not changed. The general principle is that weak signals get maximum gain, and the gain is reduced progressively as the signal strength rises. Many designs leave the front-end gain alone until stronger signals appear, because reducing gain very early can hurt the signal-to-noise ratio on weak signals.

The AGC also drives your S-meter in most radios. The meter reads the amount of gain reduction, and that is the reason an S-meter is only a rough guide to signal strength. Our article on RST signal reports explains why you should treat the S-meter as a relative indicator, not as a calibrated measurement.

The Three Timing Settings

A loop that reacts instantly would follow the audio itself and distort it, and a loop that never reacts would not help at all. The compromise is set by time constants, and amateur transceivers expose them in a small number of ways. Learn these three terms and the menus in any radio become understandable.

Attack

Attack is how quickly the AGC reduces gain when a stronger signal arrives. A fast attack clamps down on a sudden strong signal before it can overload the receiver or hurt your ears. A slow attack lets the start of a strong signal through at full gain, which you hear as a click or thump. Most modern radios use a very fast attack, in the order of milliseconds, and do not offer it as a user setting. The attack time is the reason an AGC can protect your hearing, although it is not a substitute for proper front-end linearity.

Decay

Decay, sometimes called release or recovery, is how quickly the gain climbs back after a strong signal stops. If the decay is fast, the receiver recovers almost instantly, and the weak signals that were suppressed by a strong neighbour reappear quickly. If the decay is slow, the gain stays turned down for a while, and the background noise and the weak stations are quieter until the receiver recovers. This is the setting you notice most when you tune across a band.

Hang Time

Hang time is a delay before the decay begins. During the hang the gain is held where it is, which prevents the receiver from rushing back to full gain in a pause between words or between dits. Hang is what lets SSB speech sound smooth, because the noise does not rise in every gap between syllables. Radios that expose an AGC menu usually combine the hang and decay into the choices of fast, medium and slow, and a few let you set the numbers yourself.

Fast, Medium and Slow in Practice

The names are shorthand for a combination of decay and hang. The values differ from one radio to another, so use the descriptions below as a guide to behaviour and try each setting on the air.

• Fast: Gain returns quickly. Noise and weak signals reappear between strong signals, and the receiver feels lively and responsive. The price is that the noise floor seems to pump up and down, and fast CW at high speeds can sound choppy if the AGC reacts to each element.
• Medium: A balance between the two. Good for general SSB listening, ragchews and many digital sessions.
• Slow: Gain recovers gently. The audio is calm and smooth, which is very comfortable for long listening and for stable speech, though a strong signal can leave the receiver deaf to weak stations for a moment.
• Off: The gain is fixed and you control it with the RF gain knob. This is the traditional setting of serious weak-signal operators on CW, but it demands attention, because strong signals will not be held down.

The Pumping Problem

When a strong signal keys up, the AGC reduces gain quickly. When it stops, the gain increases again, and the background noise swells in the gap. This rise and fall of the noise floor is called pumping, and a very fast recovery makes it worse on a crowded band. It is tiring to listen to and also hides the weak signals you want just after the strong one stops. A slightly slower decay or a longer hang time often cures the symptom, and it explains why many experienced operators sit on the medium or slow setting during casual listening and switch to fast only when a pileup demands it.

Choosing a Setting by Mode

The right setting depends on what you are trying to hear. The table below summarises the starting points that many operators use. Treat them as places to begin, and adjust with your own ears and your own band conditions.

For digital modes the principle is a little different. A decoder wants a steady audio level, and an AGC that changes the level during an exchange can reduce the decoding sensitivity. This is one of several reasons that the audio chain for FT8 deserves care, as described in our FT8 operating guide, and why many operators choose a slow AGC or switch it off for those sessions and watch the audio level in the software.

AGC and Pileups

A DX pileup is where the AGC earns its keep. The rare station is usually weak, and the callers around you can be very strong. If the receiver turns down the gain for each strong caller and then recovers slowly, the weak DX disappears behind the pumping. A faster decay, with a short or medium hang, helps you hear the DX station in the gaps between the callers, and that is also why the CW and SSB filters in the radio matter as much as the AGC. Narrowing the receive bandwidth removes nearby signals before they can drive the AGC at all, which is the best protection against pumping. The tactics in our guide to working a pileup pair naturally with these settings.

The AGC threshold is another control found in some radios, and it determines the signal level at which the gain reduction starts. Setting it higher lets weak and medium signals pass at full gain, and only the strong ones are controlled. Setting it lower makes everything more uniform in loudness, which is pleasant for casual listening and poor for judging relative signal strength.

Relationship to Dynamic Range and Overload

The AGC is not an overload cure. If a very strong signal drives the first mixer or the analogue-to-digital converter into compression, the damage happens before the stages the AGC controls can respond. The AGC can reduce the audible level, but it cannot undo distortion that is already in the signal. For that reason the front-end design and the settings of the preamplifier and attenuator matter. Our article on receiver dynamic range covers blocking, intermodulation and the way a good receiver is built to resist them.

In some radios an AGC loop can be driven by signals outside the passband, which makes the audio level drop for no reason you can hear. A roofing filter, which is described in the roofing filter entry, reduces this problem by removing strong off-channel signals early in the receiver. In practice, if the gain seems to dip when no signal is audible on your frequency, a strong signal nearby is probably the cause, and you can check by narrowing the bandwidth or switching on the attenuator.

RF Gain and the Attenuator

Two companion controls deserve a mention. The RF gain control reduces the gain of the receiver manually, and with the AGC off it is how you set the level. The attenuator reduces the signal before the first amplifier. On a noisy low band, a few decibels of attenuation can actually improve readability, because the atmospheric noise is far above the receiver noise floor and strong signals no longer push the input stage toward overload. If the band is quiet and the signal is weak, leave the attenuator off.

How to Adjust Your Radio Step by Step

1. Start with the AGC on medium and the receive filter set for the mode, such as 2.4 kHz for SSB or a few hundred hertz for CW.
2. Tune to a quiet part of the band and listen to the noise. Does the background rise sharply after strong signals? If so, try a slower decay or a longer hang.
3. Tune across a busy area and listen to how quickly weak stations return after a strong one. If they come back too slowly, move toward fast.
4. In a pileup or a contest, switch to fast or medium and narrow the bandwidth before changing anything else.
5. For digital modes, check the audio level in the software. If it fluctuates with the signal, choose a slow AGC or switch it off and set the level with the RF gain control.
6. Write down the settings that worked for each mode, and store them as memories if the radio allows it.

Take care with one thing. If you turn the AGC off, remember that a very strong signal will not be held down, and a sudden loud signal in headphones can hurt. Lower the AF gain first, and raise it slowly.

Common Mistakes

• Leaving the AGC on fast all the time. It sounds sharp and lively, but the noise pumping can mask weak signals and causes fatigue.
• Using the S-meter as a measurement. It reflects the AGC action and the radio calibration, and is not a precise measure of signal strength.
• Blaming the AGC for overload. A good setting helps, but strong off-channel signals need filtering and attenuation, and no AGC time constant fixes a saturated front end.
• Switching it off without lowering the audio. A strong signal then comes through at full gain.
• Ignoring the filter. A bandwidth that is too wide lets noise and neighbours drive the AGC, so narrow the filter before touching the AGC settings.

Another source of confusion is the noise reduction function, because it also changes how the noise sounds. Noise reduction and noise blanking act after or alongside the AGC and can interact with it, as shown in our comparison of noise blanker and noise reduction. Change one control at a time so that you know which one is responsible for the sound you hear.

The Bottom Line

The AGC is a feedback loop that keeps the audio level steady while the signals on the band rise and fall. Its attack protects you from sudden loud signals, its decay controls how fast weak stations return, and its hang time keeps the audio smooth between syllables. Medium is a good everyday choice, fast helps in pileups and contests, and slow or off suits weak-signal CW and digital modes. Keep the filter narrow, use the attenuator when the band is loud, and remember that the AGC does not cure overload. A few minutes spent comparing the settings on a live band will teach you more than any table, and the result is a receiver that sounds calmer and copes better when you need it most.

Frequently Asked Questions

What does AGC stand for in a ham radio receiver?

It stands for automatic gain control. It is a feedback circuit that lowers the receiver gain when a signal is strong and raises it again when the signal is weak, so the audio level stays reasonably steady.

Should I use fast or slow AGC?

Use fast or medium for pileups, contests and fast CW, and medium or slow for relaxed SSB listening and digital modes. Try both on a busy band and keep the one that lets weak stations through without noisy pumping.

What is AGC hang time?

Hang time is the pause before the gain starts to recover after a signal drops. It prevents the noise from rising in every gap between words or elements, which makes speech and CW sound smoother.

Why does my receiver go quiet when a strong signal appears?

The AGC has reduced the gain to protect the receiver, and it recovers according to its decay and hang settings. A narrower filter, a faster decay or an attenuator can reduce the effect.

Should I turn AGC off for CW?

Some weak-signal operators do, using the RF gain control instead. It gives a stable noise floor, but strong signals are not held down, so lower the audio level before you switch it off and watch the volume.

Does AGC affect FT8 decoding?

It can. A decoder works best with a steady audio level, so a slow AGC or a manual gain setting with a checked audio level is usually preferred. The level should stay in the range that the software recommends.