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Re: variable phase delay / phased arrays



I see what you are saying. A 10 degree phase shift at the RF frequency would be
translated to a 10 degree phase shift at the IF frequency by the mixing process.
The actual time delays may not be correct, but will always be off by multiples
of 360 degrees so the antenna pattern does not change. Any extra time delays
will cause some dispersion in the received signal but that will not matter for
narrowband communications.

73,

John
KD6OZH

----- Original Message -----
From: "George Kadziolka" <gkadziol@kaztek.com>
To: "'John B. Stephensen'" <kd6ozh@AMSAT.Org>; <amsat-bb@AMSAT.Org>
Sent: Friday, 14 November, 2003 07:11 UTC
Subject: RE: [amsat-bb] 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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>


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