"spamhog" wrote in message
...

I built a Class D push-pull CW HF TX with 74HC240s providing
oscillation, buffering, an extra inverter on one side, and two stages
of current amplification, eventually driving a pair of IRF530
MOSFETs..

It looks like I struck a good balance even if there was a phase
offset.

This is what happened.

I turned ann inversting 8-section driver 74HC240 from a single-ended
oscillator-buffer-4Xamp (N7KSB style), into this push-pull monster:

| -inverter-2Xamp-16Xamp-...
VXO-buffer-buffer-|
|-------------2Xamp-16Xamp-...

From VXO to the 2+2 sections in push-pull current amplifier, all fits
into one 74HC240, fed at 5Vdc. Then, in a 8X equivalent fanout, I put
2 x 74HC240 on each side, fed at 8Vdc. 16 sections of current
amplification per side provide A LOT of current, already delivered
over 1W RF, and proved more than sufficient for driving a pair of
IRF530's in push pull with individually trimmed positive DC bias,
probably with room for further expansion.

The interesting fact is that there is one extra inverting section on
one side only. On that side the signal should have been delayed by
20-30ns, the propagation time for one section. At 10MHz this should
equal 70-110 degrees. In theory this should mean that both push-pull
sides should spend 110 degrees in high status (MOSFETs conducting),
and 110 degrees not conducting, leaving only 140 degrees for honest-to-
G_d pushpullery. :-(

Woe betide such a push-pull... BUT... with my scope having one
channel KO, I can't juxtapose the events - I can only combine the
results in the output transformer.

The result seems like a pretty symmetric waveform, with steep voltage
peaks on both sides (output balun primary still not tuned). Once
filtered into a sinusoid, I see about 60W DC going in and about 45-50W
RF coming out, with negligible second harmonic.

Is this normal? What did I get wrong? (I.e. got right, but just by
chance). Would I be wasting my time if I sought more push-pull symmety
by means of an unbal transformer? I mean somethign like this:

|-buffer-2Xamp-...
VXO-buffer-unbal
|-buffer-2Xamp-...

Any wisdom welcome!

[OK, I am going to have the scope's 2nd channel fixed anyway... :-) ]

Well, if the second harmonic is really low then you must have a good square
wave. On the output side you could determine the phase relationships by
looking at the symmetry of the square wave. Adjust the phase with an rc
network on the input of one of the parallel shortest delay stacks of 74hc's.
I have paralled several 74xx series of chips together before and found that
the propagation delays were within the spec's but varied a lot within those
specs. After you tune your primary you should get a really good sine wave
out of the amp. I would not go through the trouble of unbalancing the
transformer as this could result in some unusual spikes on the output due to
saturation of the core, or cross conduction of the Fets due to di/dvt
issues. One other thing comes to mind is that with a torrid core at high
current and a high frequency, then symmetry of the winding itself is
critical. The spacing of the winding should fill the whole core and be
spaced as uniform as you can make them. Having said that, then, match 2
IRF530's for rds on. (A few tenths of an ohm really makes a big difference
when you are throwing around a bunch of amps.) This will go a long ways in
preventing what looks like an unbalanced output transformer at 10 Mhz.

Sounds like your on the right track. Just a little bit of tweaking to go to
get there.


Rich
In Texas