Filippo -
You should reconsider trying to PWM your RF switching amplifier. If
you are running Class-E, changing the duty of the drive signal will
not affect your output power significantly, but will destroy your
efficiency if you deviate from 50% duty.

With Class-D power amp, you can PWM the drive to control output power,
but it is very stressful to the MOSFET. However, there is a way to do
it, see US patent 7092269.

Good luck, keep us updated.

73 - Dave NM0S

On Jun 5, 11:32*am, spamhog wrote:
I'm playing with 600V ex-PSU MOSFETs (with very high Ciss) as HF
switching finals, with some success.

I'd like to put together a driver that takes sinusoidal input into a
fast voltage comparator, and produce a variable pulse-width low-Z
output that could be used to efficiently trim the output power.

With such a driver, a suitable slow data stream into a DAC could be
used to generate a variable control voltage that could in turn cause
the RF output to vary as to conveniently simulate ionospheric fading.
(Just kidding. I promise).

I read specs of many possible ICs types but I am at loss as to which
common and inexpensive types may be suitable, and found no examples of
specific types used in that specific role.

I had klutzed with DC-offsetting a sinusoidal signal into an ugly
nonlinear driver to that effect, but the idea of using a comparator is
from Paul Harden N5AN (who is CCed):

http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF2.pdf

Any suggestions?

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Also, did anyone press into HF service a standard suite of PWM and
MOSFET driver ICs used in automotive applications?

I saw operating frequencies in the order of 400kHz, but with driver
transition times in the 10-20ns range, which is already promising -
although perhaps I would not try them on a MOSFET totempole at 10MHz,
or at least not right away...

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TIA!

Filippo N1JPR/I2