ALC
| ALC | |
|---|---|
| Stands for | Automatic level control |
| What it does | Turns down a transmitter's gain as the output nears its set level, so later stages are not overdriven |
| Where found | Nearly every SSB transceiver; also a control line from an external amplifier to the transceiver |
| Shown on | The transceiver's ALC meter |
| Typical settings | |
| SSB voice | Some movement on voice peaks, within the zone the manual shows |
| Digital modes | Zero, or as close to zero as the radio allows |
| Amplifier ALC | A negative DC voltage sent from the amplifier to the transceiver's ALC input |
| Sign of too much | A harsh, distorted, wide signal and splatter |
ALC, or automatic level control, is a feedback circuit in a transmitter that holds the output at or below a set level. It samples the signal, and when the power approaches the limit it develops a control voltage that turns down the gain of an earlier stage. On single sideband voice, ALC keeps speech peaks from overdriving the final amplifier. The same name is used for a control line between an external amplifier and the transceiver that drives it.
How it works
An amplifier stage can produce only so much power. Pushed beyond that, it stops reproducing its input faithfully and the tops of the waveform are flattened, an effect called flat-topping. A flat-topped SSB signal is distorted, and the distortion creates intermodulation products, new signals that spread beyond the normal bandwidth onto neighboring frequencies. That spreading is called splatter.
ALC prevents most of it. A detector measures the output and produces a DC voltage that rises as the output nears the limit. That voltage reduces the gain of a stage further back in the transmitter, so the final stage is never asked for more than it can deliver. The circuit must respond quickly enough to catch the start of a speech peak, but not so quickly that it distorts the waveform itself.
ALC is a limiter and a protection circuit, not a volume control. A little action on voice peaks is normal. When the microphone gain is turned up so far that ALC is cutting back hard all the time, the circuit becomes a source of distortion in its own right, and the signal sounds harsh and occupies more spectrum than it should.
The ALC meter
Most transceivers can show ALC action on their meter. On many models the scale has a marked zone, and the owner's manual explains what reading is normal for voice. Radios differ in how the meter is scaled, so a reading that is fine on one model may be too much on another, and the manual is the best guide.
For SSB, a common procedure is to set the power control first, then speak normally into the microphone and adjust the microphone gain so the ALC meter moves on voice peaks but stays within the indicated zone. Levels set while transmitting into a dummy load are just as valid as those set on the air, and they cause no interference. Speech processing interacts with ALC: a processor raises the average level of the audio, so the microphone gain usually needs rechecking after the processor is turned on.
Digital modes
For FT8, RTTY, PSK31 and other modes generated by a computer as audio, the usual advice is to keep the ALC meter at zero, or as close to zero as the radio allows. These signals are meant to leave the transmitter exactly as the software produced them. ALC action, and the overdriven audio stages that usually come with it, distorts them and creates unwanted signals on nearby frequencies. A badly overdriven FT8 signal can appear in other operators' waterfall displays as a cluster of weaker copies alongside the real one.
The procedure is described under Digital mode interface: set the radio's output power, raise the audio from the computer until the output reaches that power, then back the audio off until the ALC reading drops to zero. Speech processing should be off for data. If the radio cannot reach the intended power without ALC action, an audio level somewhere in the chain is set wrongly and should be found, rather than leaving ALC to do the limiting.
Amplifier ALC
Many HF amplifiers have an ALC output that connects by cable to the ALC input on the back of the transceiver. If the transceiver drives the amplifier too hard, the amplifier sends back a negative control voltage that reduces the transceiver's output. The amplifier's manual explains how to set the point at which this begins, typically at or just above the amplifier's rated output.
External ALC is a safety net, not the normal way to set power. The cleanest arrangement is to set the transceiver's power control so the amplifier reaches its rated output with no ALC action at all. Relying on the external loop to hold the power down can let a burst of excess power through at the start of each transmission, before the loop has time to respond, and a poorly matched loop can make the output pump up and down. Some operators leave amplifier ALC disconnected and simply set the drive with care; others connect it as protection against a mistake, such as forgetting to turn the power down after operating barefoot, meaning without the amplifier.
Amplifier ALC outputs and transceiver ALC inputs do not all use the same voltage range, so check both manuals before connecting a particular pair.