Dear Andy, LF Group,
Aha... wondered why you hadn't mentioned it before -
unfortunately, the guard circuit is not shown in the simplified diagram in the
LF handbook.
What it is - the tank coil has a link winding on it.
This feeds the 'AC' side of a bridge rectifier made up of 4 fast diodes, via a
series leakage inductance tuning capacitor. the 'DC' side is connected to some
big plastic film reservoir capacitors, and then across the PA supply rail. In
fig 3.20 in the handbook, it is assembled on the 3 heatsinks largely concealed
by the tank capacitor boards. I think it works like this:
When the tank circuit current (determined by the
load
impedance) exceeds a certain ratio with the supply voltage, the
voltage across the link winding forward biases the guard circuit
diodes, and returns power to the DC supply. This effectively
clamps the voltage across the tank circuit, and due to the
impedance transforming properties of the LC circuit, appears to the
PA as a non-linear resistance in series with the load which increases
rapidly when the output current exceeds a certain point. If you monitor the
'guard', PA input, and supply currents as the load is decreased, the guard and
PA currents increase, but the difference between them (ie. the supply current)
decreases. With a dead short on the output, the PA current is increased by
about 50%, eg, after the unit was re-tuned:
With 50ohm load, 60V supply -
Total PA supply current for 3 PA's = 21.8A = PSU supply current
Total guard current = 0.1A
With short circuit load -
Total PA current = 34.9A
Total guard current = 28.3A
Supply current from PSU = 6.5A
With the full supply voltage and a short circuit load, the PA modules run
quite a bit warmer than normal, but not dangerously so, so this is an
impressively rugged design!
I can send you a photocopy of the full diagrams if you like - also the
details of how I tuned it up for a different frequency and load impedance, if
anyone's interested.
Cheers, Jim Moritz
73 de M0BMU
