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
