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provide a DC voltage proportional to RF power. These two voltages are read by the ADCs in the
microprocessor, and are used to compute SWR in real time.
The relays are powered by the 12VDC input provided by CAT interface cable. The relays
are a latching type, and so they consume no current when not actively switching.
Although the microprocessor’s oscillator runs at 20 MHz, which allows the main tuning
routine to execute in only a few milliseconds, the relays require several milliseconds of settling
time for every combination of inductors and capacitors. Thus, it may take several seconds before
all relay combinations are exhausted, in the case of a difficult tune.
The tuning routine uses an algorithm to minimize the number of tuner adjustments. The
routine first de-energizes the high/low impedance relay if necessary, and then individually steps
through the inductors to find a coarse match. With the best inductor selected, the tuner then steps
through the individual capacitors to find the best coarse match. If no match is found, the routine
repeats the coarse tuning with the high/low impedance relay energized. The routine then fine
tunes the inductors and capacitors. The program checks LC combinations to see if a 1.5:1 or
lower SWR can be obtained and stops when it finds a good match.
The microprocessor runs a fine tune routine just after the tuner finds a match of 1.5:1 or less.
This fine tune routine now tries to the SWR as low as possible (not just to 1.5); it takes about
half a second to run.
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