[eagle] Re: Eagle Microwave Antenna Arrays -- RF concepts

Alan Bloom 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.

Alan



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.
> 
> Bob
> 
> 
> 
> 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?
> >>
> >> _______________________________________________
> >> Via the Eagle mailing list courtesy of AMSAT-NA
> >> Eagle at amsat.org <mailto:Eagle at amsat.org>
> >> http://amsat.org/mailman/listinfo/eagle
> > 
> > 
> > ------------------------------------------------------------------------
> > 
> > _______________________________________________
> > Via the Eagle mailing list courtesy of AMSAT-NA
> > Eagle at amsat.org
> > http://amsat.org/mailman/listinfo/eagle
> 



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