Explaining FT 991A ALC and AGC
The ALC and AGC concepts were something that took me some time to understand properly - and how to control them in order to have the best transmission and reception. The lack of information in manuals and sparse (and sometimes even wrong) Internet information does not helps a lot too. I wrote this article aiming to help fellow amateur radio colleagues to properly understand these two very important functionalities of amateur radio transceivers.
ALC means Automatic Level Control. The ALC actuates during transmission only.
Think of ALC as a regulator. It is a circuit that automatically ducks a strong signal, by reducing the input signal gain/amplification (ALC attack) - and keeping the signal output within a safe maximum threshold . And when this strong signal ceases, the ALC slowly increases the input amplification (ALC release) to allow the output of this weaker signal . Usually, ALC is quick to attack, and slow to release. You can notice this behavior in your equipment meter - if you yell "PA!" at the mic while transmitting, you will see the ALC needle/meter jump quickly and fall slowly. Check this Maxim's application note for more details.
Its actuation and thresholds are defined on the factory alignment.
ALC is very important - especially in SSB (USB, LSB) modes; voice or data. It is intended to keep the input signal gain within a safe envelope, avoiding overdrive, distortion, signal clipping or even damage to your equipment's transmit stage - as well keeping the transmitter's final amplification stage nonlinearities that distorts your signal, wastes power and generates splatter under reins.
Other than protecting your radio and ensuring a clear signal, the ALC meter will provide you a visual cue on how you are driving your equipment so you can adjust your gain settings to an optimal range, given the mode that you are operating.
Some equipment takes advantage of the ALC control over the final gain stage and mixes the SWR signal with the ALC signal, basically injecting the SWR information into the ALC circuit, and causing the radio to fold-back the transmission power in order to avoid damage to your equipment. This is sometimes referred as "Reverse ALC" (RALC).
The ALC indication is only measured during transmission. Select your radio meter control/button/menu/knob to ALC.
The ALC scale is usually a sub-scale up to almost half the meter. In FT-991A it is a blue bar under the S-Meter scale.
FT-991A ALC Meter. The ALC blue bar range is highlighted in the orange square
The ALC is not something that you directly control via knobs - but influence instead. In order to set a proper ALC level you use the radio gain controls - In FT-991A you use the MIC GAIN for voice modes. For digital modes, use preferentially the volume control in your digital software. If your digital modes software does not provide a TX Volume control, then use the DT GAIN control.
And - surprise - there are different ALC requirements for Voice and Data modes. Keep reading.
Due to the nature of the human voice and how we perceive it, it is interesting (and optimized in terms of power) to keep the meter within the ALC range, to the point that your peak modulation reaches the end of the ALC scale - but not exceeding it. If your transmission is barely tickling the ALC meter, that means that you are driving way too low signal to your radio; crank up the microphone gain.
Configure your microphone gain to the point that your meter swings happily through the ALC range as you speak, not too shy, not exceeding it - that's the optimal ALC setting for your voice communication.
Digital modes like FT8, PSK, etc - have entirely different requirements in terms of ALC.
As explained above, the ALC circuit acts by ducking the 'offending' signal above a threshold - and this is not desirable at all in digital modes. Excessive signal drive causes signal clipping, distortion, ringing and can yield an undecipherable signal from your station.
Excessive signal drive, way past ALC range, yielding a distorted signal, hampering the decode of your transmission
The right ALC level for digital modes is slim (up to a third of ALC scale) to no ALC indication in your meter. If you are getting past half the scale, you are way off optimal settings.
Optimal ALC settings, meter indicating a slight ALC presence
Simply, because other than the problems presented above - it is of no use to have more ALC. It will not yield a more powerful or higher range signal.
I would like to suggest a quick test, so you can see by your own eyes:
- Transmit your digital tone with half or past half ALC indication
- Check the power meter
- Now, reduce the gain until you have little to no ALC indication
- Check the power meter again
The power meter indication in both steps 2 and 4 will show the same output power - with the catch that the signal quality in 4 is better than 2.
Yes - and that's the reason you don't have ALC meter in your FM-only or HT transceiver.
AM, FM and CW outputs a constant power - no matter if you are muted or yelling at the microphone. The AM, FM and CW ALC levels are defined in factory alignment, and can't be controlled or influenced by your modulation. If your radio has an ALC meter and is capable of AM / FM / CW (and is properly aligned and with an proper antena), when transmitting full power the radio will display a ALC reading at the end of the "good" ALC range.
Conversely, SSB modes (LSB and USB) does not output any power if you key down and say nothing. So, in SSB modes silence equals no power being transmitted. That's why you see your power meter swing happily as you talk on the radio - and this doesn't happens in AM FM or CW, where you will see a constant transmitted power output - in these modes it's all (transmitting) or nothing (receiving).
Due to the SSB modulation characteristic, difference of microphones, operating phone or digital modes etc, you are expected to set your proper ALC levels so the transceiver can yield its best performance for each application.
At the receiving stage, we have ALC's twin brother: The AGC, or Automatic Gain Control.
Pretty much like ALC, AGC ducks a strong receiving signal, keeping the reception in a comfortable level. In a more detailed fashion, the AGC acts (attack) a strong signal at the reception stage, reducing the input amplifier gain, so this strong signal can be comfortably listened. In the absence of a strong signal, the AGC increases the receive gain, in order to listen to weak signals.
The flip side of this behaviour is that a nearby strong signal can attack the AGC, ducking the gain - and you were trying to copy a weaker signal. Think of you listening to a weak signal in 7,105 - and then suddenly a strong signal pops up in 7,107.50. That 7,107.5 signal will probably attack your AGC reducing the receiver gain, and depending on the strength of the interfering signal, wiping out your 7,105 signal. That's why high-end equipments have narrow-band filters, so it can shrink the radio receiving bandwidth and defend from these neighboring strong signals (hoping it doesn't spew out-of-band artifacts...).
Most equipments provide AGC controls ranging from Fast to Slow. When to use them?
If you are having that ragchew and receiving the other side with medium to strong signals, you will find that your reception will be more comfortable with a Slow AGC. As we talked about, the silence in SSB means no transmitted power. During small pauses between spoken words, your slow AGC setting will increase the gain slowly, hardly reaching the noise floor - making these small pauses more 'silent' instead of a quick AGC recovery, where you would likely listen to the 'idle band' noise.
So, Voice mode? Comfortable RX signal? Use Slow AGC.
Fast AGC is a must during digital modes and challenging reception.
If you are operating Digital Modes, receiving with static crashes, nearby strong signal or you are trying to copy a station with marginal signal - then fast AGC is a must. The fast mode will cause the reception amplifier to raise its gain quickly, returning to noise floor or the interest signal quicker.
Reinforcing: Digital modes? Static crash hampering the reception? Marginal signal? Fast AGC.
If you want to make a SSB phone contact even more comfortable, with medium to marginal conditions, counteract with RF gain, to the point that the S-Meter stops swinging during the other party transmission.
Hope that helps; 73 de PY2RAF.
