Transmitter pulse generator for SSB RF PA adjustment

This article describes a pulse generator for adjustment of SSB RF power amplifiers.

The need

Valve RF power amplifiers usually use high voltage power supplies with poor regulation, and typically the voltage may sag by 10% or more on full power CW output, whilst on SSB telephony the voltage may sag a quarter of that.

The effect is that finding PA loading conditions for maximum power output on a key down CW signal optimises the loading for conditions that are significantly different to SSB telephony and not only is the maximum power output likely to be lower for key down CW, but it will be lower when used for SSB telephony than if it were adjusted using a drive that created full output power without sagging the power supply more than speech would.

Additionally, RF PAs intended for the amateur market cannot sustain key down CW for very long before overheating and sustaining damage forcing very short adjustment sessions. Adjustment at continuous maximum power puts great demands on a dummy load if one is being used.

So, to solve these problems, there are three objective:

  • create a drive / load scenario that is similar to SSB telephony conditions;
  • operate at reduced duty cycle to reduce internal heating of valves and power supply;
  • reduce the average dissipation requirements of a dummy load.

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