"Fred McKenzie" wrote in message
...
...
I agree with Allison that you should look at the power bus. You often
find
designs with two or three parallel filter capacitors with values spread
over
the range of 100 pf to 10 uf, all with shortest possible leads including
PCB
tracks.

This can be an issue as well. When a cap is above its parallel resonant
freq., it looks inductive and can resonate with one of the other caps. A
parallel resonance has a higher impedance than the cap. While this
resonance may be of poor quality, the extremely low impedance of the
transistor collector makes an ohm or two rather significant. Our standard
practice was to place a small bead between the caps. They were about 1/8
inch in dia and had about 50 ohms real Z above a few Mhz. This helped to
destroy any higher freq resonances while preserving the low freq impedance
of the parallel caps.

In earlier days, on VHF, we would sometimes put a "banana" from base to
collector. This was our slang for a series orange drop (I think mylar) cap
and two (one on each end) low valued resistors. Don't remember all the
values, but the idea was to kill the low freq gain. The cap blocked the
collector voltage and had enough inductance and in combo with the two (I
think 33 ohm) resistors was high relative to the base and collector
impedances. I think the cap may have been .01, but memory faded... The low
freq collector load in this situation is the supply bypass method...or the
power supply leads, in lew thereof....

Heck. I remember running a Development Engineernig prototype 100W Micor on
30 Mhz for many months, wondering why I seemed to have battery problems,
when I found large (size -wattage) and quite low valued resistors directly
from base to collector of the finals (which were always tied to the
battery). Guess someone was trying a flight fix and didn't think any DC
blocking was necessary! Transmitter worked fine. I just had a high static
drain.

--
Steve N, K,9;d, c. i My email has no u's.