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

Alan Bloom n1al at cds1.net
Fri Mar 23 22:26:53 PST 2007


On Fri, 2007-03-23 at 13:05, John B. Stephensen wrote: 
> Good point. However I and Q signals may not be needed for each antenna. The 
> transmitter is BPSK so there can be one signal source at the final RF output 
> frequency. 
...

On Fri, 2007-03-23 at 18:39, Tom Clark, K3IO wrote:
...
> Franklin is right in noting that the element amplitude does not need
> to be tuned. We will always be in the mode of desiring maximum gain,
> and hence all individual PAs should be running at maximum efficiency.
...

I've been assuming that the BPSK modulation must be filtered to reduce
the bandwidth.  That means that the power amplifiers can't be run in
class C since that would clip the rise/fall waveform of the modulation
(unless we did some fancy pre-compensation).

But maybe unfiltered is OK.  The satellite signal will be quite weak by
the time it reaches earth, so maybe the unfiltered sin(x)/x sidebands
would be acceptable.

If so, that really simplifies things.  The modulator is simply a
180-degree phase shifter.  Something like a double-balanced mixer fed
with a bipolar digital signal (on the DC-coupled "IF" port).   The PAs
running in class C would have high efficiency and nice stable output
power levels even if the input drive level is not very constant.


Then the only problem left is to be able to vary the RF phase of each
element.  Does anyone know a clever technique to control the phase of a
microwave oscillator directly?  It's easy to do over a 90-degree or so
range using a PLL, but we need to do it over several cycles of phase.

Another method:
> ... It wouldn't even be necessary to do the phasing in DSP --
> programmable NCOs could be used.  ...

The NCOs would have to run at a lower RF frequency and then be
heterodyned up to C-band.  Still it wouldn't be too horrible a block
diagram.  Let's see if I can draw it in ASCII:

 ___________________
| C-band oscillator |
|___________________|
          |
 _________V_________
|         LO        |
| Balanced mixer  IF|<--BPSK modulation signal
|_________RF________|
          |
          |             ________
          |  Phase 1-->| RF NCO |
          |            |________|
          |                |
          |        ________V_________
          |----->| Mixer upconverter |---> Element 1 amplifier
          |      |___________________|
          |
          |             ________
          |  Phase 2-->| RF NCO |
          |            |________|
          |                |
          |       _________V_________
          |----->| Mixer upconverter |---> Element 2 amplifier
          |      |___________________|
          |
          |        ... etc.

The phases would be generated in real time by the DSP and downloaded to
the NCOs.  There would have to be at least a little filtering at the
upconverter outputs to reduce the image.  Also, the 36-way splitter at
the C-band oscillator output might get kind of interesting...

By the way, the same block diagram works even with filtered modulation
as long as all stages are linear.  Amplitude matching of the elements
would have to be dealt with somehow.

Also, the same block diagram works in reverse for the S-band receiver if
"BPSK modulation signal" is replaced with the proper LO to heterodyne
down to the final IF.

Alan

======================================================================

On Fri, 2007-03-23 at 18:39, Tom Clark, K3IO wrote:
> Let me try to answer a number of points. First, Alan wrote: 
> > Tom,
> > 
> > Have we given up on the idea of using DSP techniques to do the phasing? 
> > If each antenna/amplifier had its own RF generator controlled by
> > separate (I & Q) DACs, then it would be easy to control the phase of
> > each element precisely with "infinite" interpolation between steps.
> > 
> > The same goes for the amplitude.  So you could, for example, taper the
> > amplitudes of the elements near the edges to reduce sidelobes.  It
> > should be possible to get a fully-symmetrical beam pattern to eliminate
> > spin modulation.
> >   
> The full DSP approach has not been discounted, but it comes with a lot
> of pain. First of all, it would make sense to do the phasing at IF and
> then up-convert to microwave with a single, coherent LO. The problem
> is that we would need 40-ish D/A converters, and this would cost a lot
> of power. It wouldn't even be necessary to do the phasing in DSP --
> programmable NCOs could be used. If you think about it, the phase
> modulation pattern needed for all these LOs is a sinusoid, with the
> phase (with respect to the spin rate) and phase amplitude being a
> function of that particular patch's geometry. And yes, even if an
> array is off the spin-axis, the spin modulation can be removed too.
> 
> Franklin is right in noting that the element amplitude does not need
> to be tuned. We will always be in the mode of desiring maximum gain,
> and hence all individual PAs should be running at maximum efficiency.
> If we have too much TX power, we simply turn the unnecessary elements
> power off, saving valuable watts and making less heat. The presence of
> sidelobes is of no concern. We want sidelobe levels to be controlled,
> but that's just to prevent wasting power beaming it at the Venusians.
> 
> The problem with the phase shifters Jon mentioned is that their spec
> shows a nominal=4 dB, max=10 dB insertion loss. I can't find info in
> the spec sheet about power consumption. Again, if discrete phase
> shifters are to be used, doesn't it make sense to apply them at IF
> where power is of less concern?
> 
> Regardless (just speaking of the TX side), for N (a number like 40 ±4)
> patches in an array, we need N ~1 watt high efficiency amplifiers
> which will have at most 15 dB of gain. Somehow, we need to generate N
> drive signals thru some sort of power splitter. If we split at
> microwave, this will introduce at least 16 dB of loss, even if the
> splitter is perfect.  Hence we will need a few watts of RF just to get
> enough power to the PAs at the array elements. And if we use John's
> lossy phase shifters, we need even more watts in the driver.
> 
> Back on Feb 13, I circulated an idea for doing the phasing in
> something like a Butler Matrix (if you don't have that message, I can
> resend it -- the subject was "Eagle Microwave Antenna Concepts"). It
> suggested a way to do the power splitter and array phasing using a
> Butler matrix (or something like that). That concept included the
> concept that, if the Butler array had ports for all possible beams,
> then beam steering could be done with something like a crossbar
> switch. I ended that message with a set of 7 questions I am still
> pondering:
> > So here are some challenges to make something like this work:
> >      1. Is there some better way to do the phasing that using Butler
> >         matrices?
> >      2. The antenna array phase needed to point at a given location
> >         is a simple linear phase gradient. If we don't use a Butler
> >         combiner, is there a simple way to invoke a linear phase
> >         "tilt" on a bunch of elements.
> >      3. How complicated is it to fabricate the 2N Butler matrices
> >         that we will need in a microstrip structure?
> >      4. The current baseline calls for 36 elements, arranged as
> >         either a 6x6 square or as a 37 element "bee hive" hexagon.
> >         What do Butler-like matrices look like for the cases where
> >         the array does not have 2^N elements?
> >      5. Can we invent the microwave widget that allows us to
> >         linearly interpolate power between 2 (or preferably 4) of
> >         the (N-1)² beams?
> >      6. Remember that we need a similar structure for the S2
> >         receiver, with LNA's instead of PA's at each antenna.
> >      7. Have I gone off the deep end with these ideas?
> 73, Tom
> 
> 
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