[eagle] Re: Eagle Microwave Antenna Arrays -- RF concepts
n1al at cds1.net
Mon Apr 2 11:30:54 PDT 2007
Good phase noise shouldn't be too hard. With a high reference frequency
you can use a wide loop bandwidth and basically get the same phase noise
as the reference +20log(Frf/Fref).
The other issue is that the phase shifter needs a full 360-degree
range. It would be easy to do at the reference frequency with an NCO.
On Mon, 2007-04-02 at 09:26, Robert McGwier wrote:
> The PLL would have to be really good to have sufficiently low phase
> noise. This will become the complex issue to be dealt with in this
> scheme. This sounds really good but some analysis of the expected noise
> floor will have to be done.
> Louis McFadin wrote:
> > This sounds like a scheme we could live with. It is very much like what
> > I thought we would need. I especially like the part about "This scheme
> > works no matter what the geometry of the array. Software just generates
> > the phases, and sends 'em down some bus". Being modular adds greatly to
> > the overall reliability.
> > I must remind everybody that power is very critical and will ultimately
> > determine the size and shape of the spacecraft. For the 170w average
> > power predicted at the San Diego meeting, it will require a 250w solar
> > power capability. This could be reduced by reducing the power during
> > eclipse or by relaxing the requirement for operating at all possible sun
> > angles.
> > Lou McFadin
> > W5DID
> > w5did at mac.com <mailto:w5did at mac.com>
> > On Apr 2, 2007, at 1:56 AM, Franklin Antonio wrote:
> >> 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?
> >> _______________________________________________
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> >> Eagle at amsat.org <mailto:Eagle at amsat.org>
> >> http://amsat.org/mailman/listinfo/eagle
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