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

John B. Stephensen kd6ozh at comcast.net
Mon Apr 2 12:45:44 PDT 2007


Ater thinking about this a bit more it seems that the use of multiple PLLs 
will be a problem. It will be almost impossible to prevent coupling and the 
VCOs will tend to lock to each other. In order to measure the phase noise of 
a phase-locked VCXO, I had to operate two copies on battery power with 
common-base amplifiers (very low S12), attenuators and ferrite beads on the 
cables to isolate them from each other. Each was in a shielded box so 
coupling paths were through the power supply, the DBM and backwards through 
amplifiers.

It should be possible to distribute PSK-modulated RF efficiently if the 
design is modular. If the antenna array is rectangular, it can be 
constructed as 9 modules with each containing 4 elements. Electronics for 
each module is on one PCB containing a 4-way splitter, 4 driver amplifiers, 
4 phase shifters and 4 power amplifiers. If the PCB can be made to fit on 
the back of the antenna module, RF could be distributed from the center and 
control could be distributed serially around the edges. There are then only 
9 RF cables and 9 DC power and control cables back to a common transmitter 
module.

The receiver array will be larger so the mechanical design could be similar. 
There is more space so that an RF attenuator can be included or the RF mixed 
down to a lower frequency and then applied to 4 vaiable-gain IF amplifiers 
before summing on the module.

73,

John
KD6OZH

----- Original Message ----- 
From: "Franklin Antonio" <antonio at qualcomm.com>
To: <K3IO at verizon.net>
Cc: "AMSAT Eagle" <Eagle at amsat.org>
Sent: Monday, April 02, 2007 05:56 UTC
Subject: [eagle] Eagle Microwave Antenna Arrays -- RF concepts


>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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