[NLRS] Class C amp and digital modes

[James Anderson]

Hi Jon..

There seems to be some disagreement about this.

In order for class-C amplification to work for FT8, transitions between tones must be constant amplitude. This is actually the case. So, we’re in good shape.

However, if the PA is actually linear with an accompanying ALC, there can be problems. The difficulty is ALC operation is based upon a feedback control loop that, depending upon settings, can exhibit significant overshoot and undershoot. As I’m sure you’re well aware, overshoot and undershoot are themselves amplitude variations that might cause a nonlinear PA to generate artifacts.

The key to all this is recognition a linear PA driven to a point where ALC is engaged is no longer linear.

The conclusion is perhaps a bit anti-intuitive; If the PA is pure class-C, (i.e., w/out ALC), it will likely work well. However, if it’s a linear-with-ALC, and the output power isn’t backed-off to an extent ALC corrections remain minimal, there will likely be a substantial distortion.

Thanks for reminding me of this topic!

Cheers/73 de JB/k8os

From: nlrs-bounces at mailman.qth.net <nlrs-bounces at mailman.qth.net> On Behalf Of Jon Platt via NLRS
Sent: Sunday, August 10, 2025 5:58 PM
To: NRLS Reflector <nlrs at mailman.qth.net>
Subject: [NLRS] Class C amp and digital modes

A technical question for the brain trust.  Because slow digital modes like FT8, Q65, and WSPR use a single tone one at a time, its my understanding that it would be OK to use a Class C amplifier with these modes.  Fast modes like MSK144 use multiple tones at one time so no Class C there.  Right?

73, Jon
W0ZQ
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