Automating the matching network

It is often desirable to control the matching network from a remote location and for the matching network to be able to tune to zero reflected power without human intervention. Sometimes it is desirable that tuning should take place in a very short time, faster than an operator could manage. Sometimes the aim is to allow operation by an unskilled operator. An automatic matching network and controller will allow this. In our automatic matching network the manual control knobs of the manual design are replaced by precision servomotors. 

Sensing the error

A device called a 'Phase and Magnitude detector' (PMD) measures whether the system is in tune or not. This is a device similar to the directional coupler used to sense reflected power in the RF Generator but it senses the polarity of the reflected power. A typical design measures the RF current and voltage separately and compares them to produce a phase error signal and a magnitude error signal. A more sophisticated detector design might consist of a phase detector that processes the forward and reflected signals from a separate directional coupler and converts them to phase and magnitude signals.

The controller

In it's simplest form the Automatic Matching Network (AMN) controller amplifies the error signal from the 'Phase and Magnitude detector' and feeds it to the motors. In addition a full system will:

• Limit the travel of the motors to within the mechanical limits of the capacitors.
• Display the current position of the capacitors.
• Allow the unit to be switched to manual operation for testing
• Park the capacitors in a predetermined 'base' position when RF is inactive.
• Allow the base position to be adjusted.
• Allow the base positions to be set by external equipment
• Allow the current positions to be read by external equipment

Driving the capacitors

The most common drive method is a geared down DC motor. The motor gear ratio will be chosen according to the application. Air-vane type capacitors require little torque and can be moved by small motors. The vacuum variable capacitors used in some designs require high torque, and also multiple turns. This demands a more powerful motor and can even require an upgrade of the standard controller in order to supply the increased power. In other applications a match time of below 500ms may be called for, in which case low gear ratios will be chosen for maximum speed. In high speed applications the biggest strain on the output amplifiers is not driving the motors but slowing them down. 

It is undesirable allow the motor to wind beyond set limits. In an air vane system the capacitors can be turned full circle but only half the capacitor's travel is actually useful. Vacuum capacitors can be damaged by excess travel. Limits are required, sometimes mechanical switches, and sometimes relays operated when the position signal exceeds its limits.

On a high speed system a simple end point relay or micro-switch is insufficient to stop the motor at a consistent position and instead the motor speed must be tapered down by an adaptive limit as it approaches the end of its travel.

Remote control