Hi all the technical chat about PA operation has reminded me of an
unfinished project idea that someone might like to pick-up. It was
stimulated by the need to tune a Class-E stage for maximum efficiency (usual
technique to tune for PA FET drain waveform on a 'scope), but I guess it
could be a useful tool in any Class of PA.
Simply, the efficiency is approximately the PA (DC) current divided by the
aerial of feeder current, so there is some "computation" to do. I considered
a simple analogue multiplier, but then remembered the circuits of analogue
computers from my youth. I am thinking that these latter circuits might be
more rugged and easier to understand for most. ( no software or clever
programmers required !!) A KISS solution ??
The idea goes like this....... use a high-side current measurement chip
....there is one made by Zetex but I have not doubt there are others. Some
extra circuitry will be necessary to get it to sit 60 volts of so "up in the
air" (see the Zetex app note) The voltage produced by this part is turned
into a PWM stream (maybe a 555) referenced to ground. The feeder current is
monitored with a current transformer (like Jim's tuning aid) but the output
to a moving coil meter is interupted (chopped) by the PWM stream (100Hz or
so). With the correct "polarity" of the pulse stream the level integrated by
the moving coil meter will be a measure of the efficiency. Hence as the DC
current increases the chopper gate is closed for less time giving a lower
reading. I know it will not be an accurate measure of efficiency, but I was
seeking a "peaking" meter to easily tune a Class-E stage correctly without
the use of a 'scope.
I have not patented this idea and offer it for the good of mankind and
hopefully a help to LF operators.
Yup I know you could do it with a PIC or some other device with a
mega-bucketfull of transistors inside.....but I though this conceptually
simple idea would appeal to the "hot solder-iron" fraternity (like me), and
would be relatively easy to breadboard up. There may of course be a
"theoretical hole" in my bucket....but I have no doubt someone will soon
tell me !!
Cheers de Alan G3NYK