Many people use HF transceivers in situations where they back the power down significantly from full output. Examples are driving an amplifier that may require 20 or 30 watts for full output, or using a transverter that might want just a few milliwatts.
The method many modern HF transmitters use to reduce power is to adjust the ALC (automatic level control) circuit. This circuit is a feedback loop that adjusts the transmitter to maintain a desired maximum output power. The detected power at the output drives the automatic gain setting. This circuit doesn’t respond instantly; there’s a time constant of a few milliseconds. As a result, some rigs overshoot when their output power is reduced — there is a full-power spike at the beginning of a transmission before the ALC kicks in to back down the power.
This spike can damage amplifiers and transverters. The question was whether the Icom IC-7300 exhibited this behavior. I did a set of simple tests to find out.
I connected the output of the IC-7300 to a power attenuator with 40 dB of loss. The attenuator output went to the vertical input of a digital oscilloscope. I set the scope to “peak detect” mode, set the trigger level and trigger time appropriately, pushed the “SINGLE” sweep button, and then keyed the transmitter either via the built-in CW keyer, or with a whistle to the microphone. This let me capture the very beginning of the RF output.
I didn’t see any sign of overshoot at any of the power settings I tried. The ‘7300 seems to use something other than ALC for its power control, which given its SDR architecture makes sense; it’s really easy to control levels in the digital domain.
Here’s a screen capture of a CW “dit” at 30 WPM with a power setting of 100%:
Here’s the power set to 10%. The only change in the waveform is that the leading edge rises a bit more slowly:
I zoomed in to the leading edge of the dit at 10% power to see if there was any really, really short overshoot. Nothing seen (note that the V/div is different here):
After doing all this work, I wondered if the CW path might be different than the one used for voice, so here are two shots showing a whistle into the microphone. The first is 100% power, and the second is 10%; the same scale settings were used for both:
So, based on this I don’t think the IC-7300 poses any concerns for use at low power settings to drive amplifiers or transverters.
There’s no sign of overshoot with this rig, either.