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RE: variable phase delay / phased arrays




John wrote:

> At 5 GHz a one wavelength delay is 200 ps so I'd expect that
> 5 ps accuracy would
> be required for beamforming. Isn't this a bit fast for DSP?
>

If you were using the entire 5GHz of bandwidth (ie DC to 5 GHz), then yes,
you would need accuracy on that order and a DSP would not be able to keep
up, or at least not today.  ;-)

However, you are only interested in a teensy part of that spectrum, which
may be less than 1 MHz (including Doppler).  1 part in 5000 can be
considered narrowband.  In the time domain, let's say you spec'd an accuracy
of 5 ps.  At the single frequency of 5GHz (give or take a couple hundred
kHz), this time delay accuracy would be the equivalent of a phase accuracy
of 9 degrees.  Even though the carrier is at 5 GHz, the bandwidth of the
information is much lower.

For beamforming with a narrowband signal, you need to apply appropriate
phase shifts according to the frequency of the carrier, but you apply it to
a channel that need only be wide enough for the signal of interest.  If you
changed the carrier frequency to, say, 10 GHz, you would need to have a time
domain accuracy of 2.5 ps now, but you would still only need a 9 degree
accuracy in your DSP phase shifter.

I know that in some RF beamforming networks, PIN diode switched delay lines
are used, and they are called 'phase shifters', but they are really 'time
delays', since they are switching in different lengths of transmission line.
What I am referring to is a a true phase shift, or simply multiplying a
signal by e^(j*phi), where 'phi' is the phase shift I want to apply.  After
360 degrees, the cycle repeats.

Cheers,

George
VE3LQN


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