At 12:42 PM 3/23/2007, Alan Bloom wrote:
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.
That's a lot of DACs. 2 per antenna element. High speed DACs consume a lot of power.
A much better approach would be to use a digitally controlled phase shifter at the input of each PA. I believe that 3-bit phase shifters will prove adequate, but hey, you could use phase shifters with more bits if you want. Remember, all of these phase shifters don't jump at once as the spacecraft rotates. Therefore the phase discontinuity in the resulting composite signal is much smaller than the phase discontinuity at one element.
I don't think its necessary to do I & Q. Why not just put the phase shifter in the RF path right before the PA? That avoids a zillion mixers.
There are many possible solutions. The trick is to choose one that has a low power consumption and weight and complexity (hence high reliability).