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. Analog phase shifter ICs (see attachments) can be used to generate signals for individual power amplifiers at each antenna. The transmitter power amplifiers should be operated at saturation for best efficiency so the small amplitude variations in the phase shifters don't matter.
For the receiver, an LNA, mixer and LO phase shifter could be placed at each antenna. Amplitude variations in the LO have little effect as the LO input to the mixer is limiter. The mixers could be quadrature mixer ICs (such as an HMC597) if a low-IF receiver is used or single Gilbert-cell mixers (such as an LT5560) if the a high IF is to be used. A high IF probably uses less power and the conversion to I and Q baseband signals could occur after the outputs of all the mixers are summed. It might even be possible to sum the RF directly and use only an LNA, phase shifter and variable attenuator for each antenna element. The attenuators would compensate for phase shifter gain variation.
73,
John KD6OZH
----- Original Message ----- From: "Alan Bloom" [email protected] To: [email protected] Cc: "AMSAT Eagle" [email protected] Sent: Friday, March 23, 2007 19:42 UTC Subject: [eagle] Re: Eagle Microwave Antenna Arrays -- mechanical concepts
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.
This technique would allow full flexibility in antenna placement. The optimum phasings and amplitudes could be calculated before launch for all beam angles (every few degrees) and stored in a table. Software on the satellite would then interpolate between the table values.
Alan Bloom
On Fri, 2007-03-23 at 07:52, Tom Clark, K3IO wrote:
Grant Hodgson wrote:
Tom
Don't forget to claim back the expenses that you've incurred for these models...
More seriously - is the intention to have a separate phase shifter for each element?
Grant -- there are several basic ideas for doing the phasing: 1. A scheme which has been used in the past on electrically despun arrays is to have a discrete beam former with N beams and then discretely switch to the best of the beams as the s/c rotates. IMHO, this is a REALLY BAD :-P idea because there will be abrupt phase and amplitude discontinuities when switching from one beam to the next as the s/c spins. 2. A neat "zero click" adaptation of #1 can be done with a linear or square array. For this geometry, the "optimum" combiner is the Butler matrix which is the electrical realization of the Cooley-Tukey FFT. Assume that tap X is the beam now, and that Y is best for the next rotation step. If we use an in-phase variable power splitter that can linearly interpolate between the X & Y position, we can smoothly move the beam with no discontinuities. The interpolation is done in POWER with fractions [P] and [1-P] split between the X & Y taps. To build this for an NxN array, we build the combiner that makes NxN beams (of which [N-1]x[N-1] will be used -- we have no need to make use of the beam on the array's "horizon"). I've tested (in MATLAB) this idea for an 8x1 and 8x8 array. I haven't had a magic idea on a Butler-like matrix for hexagonal geometry. 3. We could devise some continuous phase shifter to be applied to each element. The required phase shift for any given pointing direction is a linear phase gradient across the aperture -- i.e. when viewed from the target (earth), we need to compensate for the geometrical phase offsets due to the plane of the array. [ Note: If we can generate the phase gradient easily, then we can free ourselves from any geometric constraints -- the elements an be located anywhere on the spacecraft.] Ideas are solicited!
73, Tom
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle