Content provided by: FlexRadio Engineering
The intent of an external hardware ALC (automatic level control) interface is to reduce an exciter’s RF drive level to ensure that it does not exceed levels that could possibly 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 too much RF drive from the exciter driving the amplifier beyond linearity and from exciters that produce high RF transient pulses, which appear on the leading edge of the RF envelope due to the exciter’s internal slow-response ALC circuitry.
Reason #1: The first reason is that 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, especially for DSP/Digital radios. 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 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 analog 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 some time, which would cause damage to the amplifier. The only exceptions to this are amplifiers that have a very low control-grid dissipation rating and no input protection whatsoever.
FlexRadio products have been and are used with just about all of the new solid-state and most tube-based amplifiers manufactured without using external ALC, and do not have any operational issues. The FlexRadio 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 behaved properly when the AF input signal path (microphone configuration) in SmartSDR was properly set up. The setup is a bit more tedious on solid-state amplifiers because the internal ALC on the amp and the DRIVE control in SmartSDR 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 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 interfaced with the exciter’s analog-controlled PA stage. In the case of a software defined radio, the analog voltage control signal would have to be sampled (read) and converted to a digital (binary) value and 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 it that are somewhat greater 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 sufficiently to prevent damage to the amplifier. This incremental cumulative delay makes it challenging 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 is needed for software defined radios to ensure that the amplifier is not being over-driven with too much RF input and no high RF transient pulses are being generated by the exciter.
Reason #3. An amplifier's Automatic Level Control (ALC) can begin to oscillate when Radio Frequency Interference (RFI) enters the ALC control loop. This creates a negative feedback instability, causing the output power to fluctuate and distort the signal. RFI, often from the amplifier's own RF output, can couple back into the ALC control loop. This can happen through poor grounding, inadequate shielding, or improper component placement. The amplifier's gain control sees the fluctuating control voltage and responds by changing the output power up and down. This, in turn, changes the amount of RFI picked up by the ALC circuit, completing a loop that sustains the oscillation.
Reason #4: The last reason is that FlexRadio 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 than 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-8000, 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.
In addition, the operator can set specific maximum power levels per band using a TX (transmit) profile and use the global Max Power setting as an upper-end threshold for a two-stage protection configuration. This allows for greater grainularity for maximum power level management.
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 ensure the drive level is set correctly 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 from the amplifier. This is a bad amateur radio practice since those last few watts make no difference in your talk power, and it relies 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.