Hi Chris,
To me, it doesn't sound like a good idea to combine two switching square wave
signals together...Might be okay if they were synchronized...Better to filter
them into sine waves first and then combine them...More expensive for double
the number of iron powder toroids though...
Jay is combining them ahead of the filter...That type of combiner must work
okay that way.
Cheers,
John VA3VVV
--------------------------------------------
On Sat, 4/8/17, Chris Wilson <[email protected]> wrote:
Subject: Re: LF: Wilkinson combiners
To: "Markus Vester" <[email protected]>
Received: Saturday, April 8, 2017, 12:42 PM
Hello Markus,
Thanks for the detailed reply,
let me see if i have this straight
though
please, sorry if I appear a bit dense, it's probably
because I
am.... :)
Right now I run one voltage mode Class D push
pull amp into a T type
low pass filter. All
works great, nice gate and drain wave forms, and
my Scopematch box shows perfect sinusoidal wave
forms for voltage and
current after the T
type filter. Modest output transformer warming,
each pair of FET's near as damn it cold.
I add another duplicate amp
and the Wilkinson combiner. The combiner
feeds the T type filter. With either one or
both amps running the gate
and drain wave
forms go to pot, the FET's run mad hot, as does the
output transformer (and the power input choke
gets hot, too, which it
doesn't with a
single amp straight into the filter).
Are you suggesting I build a duplicate T type
filter bank and put a
filter between each
amp and the combiner, with no filter on the output
of the combiner, just run the antenna co-ax to
the matching
transformer and loading coil
which are outside? I can do this, it
means
finding and buying some more big toroids, but it's
perfectly
possible....
Is the W1VD combiner what people call a
"broadband" combiner? And if I
built this instead of the Wilkinson one would I
be able to run it in
front of my single T
type filter? Or do I still need a pair of filters
between it and the two amp's outputs? To
remind, the W1VD one is here:
http://www.w1vd.com/137-500-500WCombiner.pdf
Thanks for your patience,
and thanks Graham for your input too, just
don't want to waste time and money
building things that are incorrect
due to my
ignorance!
> Hi Chris,
> the PI-type combiner per se is not the
problem. All I was trying to
> say is
that you have to prevent it from short-circuiting the
> harmonics directly at the output of a
(voltage-mode) class-D PA. In
> that
case, I would recommend a T-type low-pass filter between
each
> PA and combiner input, starting
with a series inductor rather than a shunt capacitor.
> Note that the desired
harmonic load impedance depends on the type
> of Class-D PA: Voltage-mode class D with
rectangular voltage and
> sinusoidal
current should see a high impedance termination for
> harmonics, i.e. an inductor or series
resonant circuit. On the other
> hand,
current-mode class D designs would have sinusoidal voltage
and
> rectangular current, and require a
low-impedance shunt for harmonics.
> Best 73,
> Markus
--
Best regards,
Chris
mailto:[email protected]
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