I think I've figured out a good scheme, which may be the right way to
build the C-band antenna. It's a bit different than you guys have
been thinking.
You've been thinking about how to accomplish the phasing of the RF
signal, but ignoring the physical distribution of RF. Physical
distribution of C-band RF signal to 35 or so elements is
nontrivial. It could be done with splitters and cables and
connectors, or printed splitters and microstrip lines, but with any
of these schemes there will be amplitude mismatches and various phase
shifts associated with just the distribution. If we distribute RF,
we'll have to compensate for those amplitude and phase
variations. Can be done of course.
How about this. Instead of distributing RF, we distribute a much
lower reference frequency which is multiplied up by a PLL at each
antenna element. A PLL is one chip these days, so it certainly
doesn't cost much in dollars weight or power. We've already got a PA
and phase control stuff at each element after all. So maybe we
distribute 100 MHz or something easy. It is only gonna be used as a
reference, so we don't need to match amplitudes. We design the
reference distribution network for high isolation between the
outputs, so the individual element circuits have least chance to
couple. We don't need to match phases of the distributed reference
because we're gonna adjust the phase of each element under software
control anyway. (We'll build a calibration table which the software
will use as a term it adds into the total phase shift it specifies
for each antenna element.)
At each element we have a PLL to generate the RF frequency, followed
by a digitally controlled analog phase shifter chip, and a digitally
controlled attenuator chip (to adjust out variations in the phase
shifter chip vs control input), and a balanced mixer to generate the
180 degree phase shift of BPSK. (If the phase shift chip has a 180
degree input, we can leave out the balanced mixer.) Next, of course,
the signal goes to the local PA.
This entire circuit fits on a small board smaller than the antenna
element (ie patch) itself, and bolts to the back of the antenna element.
This design is highly modular. The only things distributed are
power, reference, data-to-be-modulated, and data to control the
individual elements. The controls are on/off, phase, and gain.
It is also possible to move the digital phase shifter to BEFORE the
PLL, at which point it may be possible to remove the digital
attenuator entirely. You would no longer need it to compensate for
the changing attenuation of the phase shifter vs control input, as
long as the PLL could handle the range of possible amplitudes. (On
the other hand you might still want the attenuator as an easy way to
balance the gain and/or power output level of the PAs. With the
digitally controlled phase shifter operating at a lower frequency it
would likely be more accurate and have less amplitude variation anyway.
This scheme works no matter what the geometry of the array. Software
just generates the phases, and sends 'em down some bus.
The thing I have described easily makes unfiltered BPSK or even
QPSK. Somebody was talking about possible filtering. That's
harder. I hope we don't need filtering. Distributed filtering is
not an easy thing.
What do you think?
_______________________________________________
Via the Eagle mailing list courtesy of AMSAT-NA