[evlatests] Aliasing

[Claire Chandler]

Hi Barry,

Gustaaf has reported that the "up" or "down"-ness of the shape of the 
bandpasses is a function of IF (not just antenna).  How does this fit in 
with your analysis below, which has only one delay per antenna?

Claire

On Thu, 27 Mar 2008, Barry Clark wrote:

> Finally the penny dropped.
>
> The aliased power adds to the straight through power as
> 1 + v(f)exp(4*pi*f*Del)
> where f is the frequency in the bandpass, and v(f) the aliasing
> function of the digital filter, as provided by Mike, and Del is the
> differential delay to the two antennas involved introduced by the
> delay lines in the correlator.
>
> I have been assuming that the delay Del was just the adjustable part
> of the delay due to geometry, the w of (u,v,w).  It is not.  It also
> contains the compensation for the different fiber delays to the two
> antennas involved.
>
> The current fiber delays are given below
> ea01   W10     5214
> ea04   W18    12633
> ea11   E10     7129
> ea13   N16    12290
> ea14   E16    12004
> ea16   W6      3120
> ea17   W14     9144
> ea18   N12     9288
> ea19   W4      3943
> ea21   E2      5661
> ea23   E18    13493
> ea24   W12     6613
> ea25   N2      4941
> ea26   N6      6203
>
> So, in the observation Gustaaf recently took with w near zero, baselines
> in which these fiber delays are nearly the same will have the classic,
> rising to low frequency, profile - baseline 17-18 in particular, but also
> 1-25 and 13-14.  Baselines where the difference is near a microsecond will
> still be plunging down to low frequency, with a turnaround below the usable
> bandpass - 21-24, 11-26, 19-25 for instance.  Baselines where the difference
> is large will look all squiggly - (4, 13, 14)-(16, 19 25).
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