Dick, I am not totally sure of the design, but I believe the transformer
does NOT resonate with the drain capacitor. I believe to work best the
circuit is close to Class-E operation The drain capacitor determines the
optimum power level. Energy stored in the drain choke should flow into the
capacitor when the FET switches off. I am not sure of the design conditions
but this is the same effect as in my single-ended Clas-E PA experiments. The
best literature on these designs is by Nathan Sokal and Fritz Raab. Nat
published a paper in QEX about 3 years ago on Class-E designs. Raab has
published papers and books on alternative configurations, bothe single ended
and push-pull. Though there are "tuned circuits" in the high efficiency
stages they are NOT resonant at the operating frequency, but usually about
10% lower in frequency !!
As to the cores I think you are on your own there as each specimen will
probably be slightly different. I think it may be better to try to find some
large (not split) toroidal cores from old switch-mode PSUs and wind these to
the reactance formula once you find how the permiability of the cores
compares with 3C89s. 70uH should be about enough for the transformer
winding. That is about 60ohm at 137kHz, and the PA drive impedance is in the
10 ohm range. The problem may be the variability.
Cheers de Alan G3NYK
----- Original Message -----
From: "Dick" <[email protected]>
To: <[email protected]>
Sent: 10 May 2004 21:29
Subject: LF: TX-ouput transformer problems--part 2
Well, thanks all for your comments and tips.
It helped me, and I got some ideas about what
might be wrong.
I soldered out the transformer, and did some
measurements on it.
Dispite all precautions I found that apperantly the
2 core halves didn't make an ideal contact.
Pressing the two halves did gave a large difference
in inductance.........
To give you an idea, it changed from 70uH to over 130uH........
Now I have to find a creative way to put them under a
continue pressure, also when temperature changes, and
without making a short.........
I think this is the main disadvantage of using these cores.
Of course one clever guy can make an adjustable pressure
system, allowing to 'tune' the coil...........
I guess that the coil should be in resonance on 137kHz together
with the 10nF drain capacitors, which means roughly somewhere
around 126uH for each coil part.......
Hope to find some time this week to do this......but busy.....
73 Dick, pa4vhf
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