Hi Everyone,
Apparently, there has been considerable discussion about drivers, driver chips and the like
on chat. Perhaps I'll join in, although it's more fun to talk about on the air!!!
Several folks are in the midst of, or are about to begin, building class E transmitters, and
are uncertain about what type of driver to use; conventional, transformer coupled circuits or
direct drive using one of the new ICs available for the purpose. The ICs, in my experiments and
when using them in a real transmitter, show great promise.
I believe this is the way to go - it's simpler, predictable, and probably costs less when you consider the TOTAL cost of building a
driver (including power supplies, control circuitry, MOSFETs, cores, etc. etc. etc.) that would
be elminated. There is also the savings of time, and the multi band capability of the IC based driver must
be considered as an advantage.
When Brent was visiting with me the other day, we raised the idea of phase delay between
modules in multi-module RF amplifiers. This has prompted considerable discussion.
To facilitate some answers, I made some real measurements on a real transmitter to see just
what the phase delay between modules really IS!
Now, my first swag on this would have been that there WILL be a necessary phase shift in a multi-module
class E transmitter because there is a finite and real inductance between modules in the output
stages, necessitating a similar phase delay in the input. I did not model this, nor did I
do calculations to prove the theory.
Now the actual measurements made at 4 mHz: On an 8 module transmitter, from the first module to the last, I measured a
20nS difference. Between "closer" modules, the delay was less 10ns, and almost 0 in come cases.
This seems to be a rather small delay, in proportion to the actual operating frequency of
4mHz which is 250nS.
Let's talk about it.
I think we should start with a defined scenario, from which we can expand the discussion. The proposed
"test" transmitter consists of 4 modules of 5 MOSFETs each. Each module consists of a single
driver IC, all of which are identical and we will assume the propagation delay through the ICs
is identical, and therefore not part of the equation. Each driver IC is connected by a 50 ohm terminated
(100 ohms to +5 and 100 ohms to ground) coaxially connected to a 74s NAND gate.
*IF* a phase delay *IS* necessary, there are a number of ways it could be achieved using standard
techniques. One simple method immediately comes to mind - using a coaxial delay line. Since there
will be coax cable between the RF sources and the driver ICs, simply make the lines longer
where phase delay is necessary.
A 2nd method involves a TTL delay line (there are ICs that do this). A 3rd method could involve an
R/C integrator followed by a schmidt trigger. R is adjustable, and sets the phase delay.
These are ideas just off the top of my head, and will need further consideration. They may prompt
a discussion and other ideas.
Note: All of this will be necessary ONLY if we actually need phase delay. If the delay is small (and it does
seem to be), coaxial delay lines are, by far, the way to go. Easy, reliable, never goes out of adjustment.
We need a
Real "test" RF amplifier to prove this out. Eventually, I am going to build
such an RF amplifier (with many more than 2 modules - I already have two modules - it works great!), however
if someone in the local area builds such an RF amplifier, I will lend assistance in debugging, analysis, and the like.
Comments?
Regards,
Steve
