[eagle] Re: Eagle Microwave Antenna Arrays -- RF concepts
John B. Stephensen
kd6ozh at comcast.net
Mon Apr 2 11:54:55 PDT 2007
It shouldn't be hard at all for the tranmitter as the downlink will be
operating at about 2 Mbaud. VCO MMICs may work. Extendin this scheme to the
receiver would be harder but 1/2-inch square VCOs may be adequate.
----- Original Message -----
From: "Alan Bloom" <n1al at cds1.net>
To: "Robert McGwier" <rwmcgwier at gmail.com>
Cc: "AMSAT Eagle" <eagle at amsat.org>; "Louis McFadin" <w5did at amsat.org>
Sent: Monday, April 02, 2007 18:30 UTC
Subject: [eagle] Re: Eagle Microwave Antenna Arrays -- RF concepts
> 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?
>> >> _______________________________________________
>> >> 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
> Via the Eagle mailing list courtesy of AMSAT-NA
> Eagle at amsat.org
More information about the Eagle