[mmaimcal]Phase Repeatability of the Attenuators

[Bryan Butler]

by 'settle time' below, i meant the antenna mechanical settle
time, which is a few seconds, if i recall correctly.

	-bryan


On 2002.08.07 11:27 Bryan Butler wrote:
> 
> mark,
> 
> i think you've got the timescales too short here.  isn't the limit
> on switching bands something like 5-10 seconds (larry, correct me
> if i'm wrong here)?  certainly the settle time is more than 1 second,
> so you'll have to have many seconds on calibrator, and some multiple
> of that on source.
> 
> 
> 	-bryan
> 
> 
> On 2002.08.07 10:47 Mark Holdaway wrote:
> > 
> > Phase Repeatability of the Attenuators:
> > Requirements for Fast Switching
> > 
> > M.A. Holdaway
> > 7 Aug 2002
> > 
> > 
> > For fast switching phase calibration, a calibrator source will be
> > observed for about a second at 90 GHz, then the antennas will all slew
> > over to the target source (at the target frequency) about 1 degree
> > away, and the target source will be observed for several seconds.
> > Then, the cycle repeats, returning to the calibrator.  The phases
> > determined on the calibrator will be scaled to the target frequency
> > and interpolated in time (and perhaps position) to estimate the
> > atmospheric phase on the target source.
> > 
> > To achieve efficient digitization, different attenuator settings will
> > be used at the different frequency bands.  There can be phase jumps
> > due to the attenuators across the frequency bands, and even phase
> > drifts with frequency within each band.  If stable with time, these
> > effects can be removed by performing a bandpass calibration at the two
> > different bands.
> > 
> > How stable must the phases of the attenuators be?
> > 
> > Lets consider the worst case:  calibrator observations at 90 GHz
> > and target observations at 950 GHz.  
> > 
> > For fast switching phase calibration, we are aiming for residual phase
> > errors of about 15 - 20 degrees rms.  Lets take 20 for this argument
> > (at 950 GHz it will be exceedingly difficult to get 15 degrees rms
> > with fast switching).  This 20 degrees rms comes from two sources:
> > thermal noise manifesting as an imperfectly determined phase on the
> > calibrator scaled up to the target frequency, and the residual
> > atmospheric phase which differs from the calibrator and target source
> > observation times and positions.  Let's arbitrarily say we'll permit
> > one more degree of phase from the electronics:
> > 
> >     electronics = sqrt( 21^2 - 20^2 ) = 6.4 degrees
> > 
> > Now, we will be differencing two noisy numbers, so there WOULD be
> > a sqrt(2) in there, but the phase from the attenuator will be dominated
> > by the 90 GHz phase scaled up to 950 GHz, so we ignore the sqrt(2).
> > 
> > If the attenuators' phases are repeatable to about
> > 
> >     6.4 deg * ( 90/950 ) = 0.6 deg
> > 
> > then the effect on the fast switching residual phase will be minimal.
> > 
> > 
> > 
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