[mmaimcal]Phase Repeatability of the Attenuators

[Bryan Butler]

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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