Content provided by: FlexRadio Systems Engineering
The intent of an external ALC (automatic level control) interface is to reduce an exciter’s RF drive level to ensure that it does not exceed levels that would damage the input stages of the amplifier or cause “splatter” from overdriving the amplifier. RF power amplifier manufacturers implement ALC in their products in an attempt to protect them from operators that apply to much RF drive from the exciter driving the amplifier beyond linearity and from exciters that produced high RF transient pulses which appear on the leading edge of the RF envelope due to the exciter’s internal slow response ALC circuitry.
Even though the FLEX-6000s have a hardware ALC for exciter control, it is not recommended that it be used except when operating an amplifier that does not have internal ALC or other protection mechanisms.
Reason #1: The first reason is external ALC is not universally recommended by RF amplifier engineers as an effective mechanism for protecting the RF input stage of an amplifier, therefore, it is not an operational requirement to have one. In many cases, it is not possible to get the exciter to respond quickly enough to ALC voltages because it can take too much time for the amplifier to generate the ALC control voltage. Even with an analog transceiver that utilizes ALC, you will find that there is a time delay before the exciter responds to the ALC control voltage and subsequently turns down the gain of the exciter's PA stage, which may exceed the time threshold that would prevent damage to the amplifier.
A case in point, Acom states in their Acom 2000 manual "The ALC output of the amplifier normally remains unused. In case of need of ALC for your transceiver, WHICH WE DO NOT RECOMMEND, you may contact your dealer or see the Technical Supplement".
Most of the high-end amplifiers manufactured today, especially solid-state amplifiers, contain their own ALC and protection circuitry to prevent high RF exciter drive levels from damaging them, not relying on an external process. Many of the older tube-type amplifiers easily tolerate 100 watts of RF input, therefore, ALC is not usually necessary unless the exciter is capable of exceeding the amplifier's maximum RF input for a period of time that would cause damage the amplifier. The only exceptions to this are amplifiers that have a very low control-grid dissipation rating and no input protection what so ever.
FlexRadio Systems’ products have been and are used with just about all of the new solid-state amplifiers manufactured today without external ALC and do not have any operational issues. The FlexRadio Systems engineering team has tested our radios with the Icom PW1, Yaesu Quadra, and Tokyo High Power 1.5K-FX successfully. These amplifiers worked as they should and behave properly when the AF input signal path (microphone configuration) in SmartSDR (and PowerSDR) is properly setup. The setup is a bit more tedious on solid-state amplifiers because the internal ALC on the amp and DRIVE control in PowerSDR need to be properly matched.
Reason #2: The second reason that FlexRadio Systems’ products do not recommend using an external ALC feature is due to how it is implemented for a software defined radio as opposed to how it is done today with radios that have analog controlled PA stages. An external ALC process utilizes a voltage control mechanism to “alert” the exciter to reduce RF drive levels. For traditional legacy type transceivers, external ALC is an analog process, which is interfaced to the exciter’s analog controlled PA stage. In the case of a software defined radio, the analog voltage control signal would have to be read and converted to a digital (binary) value and that value passed to the controlling SDR software telling it to reduce the drive level of the PA. Each one of these steps in the process, including the analog to digital conversion of the ALC voltage control signal, has small inherent delays associated with them that are somewhat greater in total than an analog implementation of external ALC. The sum of these delays in a digitally controlled system, in some cases, would exceed the time threshold needed to reduce the exciter drive level sufficient enough to prevent damage to the amplifier. This incremental cumulative delay makes it challanging to effectively implement an external ALC drive control solution in a software defined radio and be confident to a high degree of certainty that the external ALC control mechanism would reliably protect the amplifier. Therefore, another set of mechanisms are needed for software defined radios to ensure that the amplifier is not being over-driven with too much RF input and that there are no high RF transient pulses being generated by the exciter.
Reason #3: The last reason is FlexRadio Systems’ products have essentially absolute RF power management and a reliable fast-attack internal software ALC mechanism that has much greater control of the RF envelope that what is possible with analog exciters, which essentially negates the need for external ALC control of the exciter.
If you set the DRIVE level in the SDR control software to a value of “50”, it will generate 50% of the rated RF power, so in the case of the FLEX-6000, that would be 50 watts RF output. Once the DRIVE level is set, the power output will typically not exceed 50 watts even if the AF (microphone) input is overdriven as the internal software ALC, which controls the RF power output of the radio, is very aggressive and works as a fast-attack RF limiter.
The one thing that a software defined radio can not do is control the initial input drive level that the operator chooses to use. It is good amateur radio practice to make sure that the drive level is set correctly for the amplifier to prevent QRM from "splatter". Some operators set the RF drive level at or slightly above the maximum level relying on the ALC to cut back the exciter’s drive to the safe level in an attempt to squeeze out the last few watts out of the amplifier. This is a bad amateur radio practice since those last few watts really make no difference in your talk power and you are relying on an inherently slow protection mechanism to ensure you do not damage your expensive RF amplifier. There is no real substitute for adhering to good amateur radio practices in this situation.