[evlatests] WIDAR/EVLA Birdies

[Michael Rupen]

                   Summary of Widar (and EVLA) Spurs
                           September 18, 2008

We have looked at enough data now to have noticed persistent birdies
generated in the antenna and, perhaps, in station board 2 which is
currently connected to ea23.  So far there is no evidence of birdies
generated in station board 1 (ea18).

The spurs seem for ea23 appear in the center channel of subbands
0, 2, and 6.  Subband 4 shows no obvious birdie in the central channel,
but there is a strong birdie around channel 290.  These birdies have
all been seen in both C and X bands, and do not move when the antenna LO
is shifted.  They must be generated in the sampler/DTS or in the station
board itself.

In X band there is large birdie seen at 8192 MHz in both antennas, and
also seen in Walter's autocorrelation plots.

In C band there is a large birdie seen at 4648 MHz in both antennas.
This moves in the wrong direction for a sky signal if the LO is retuned so it 
must be inserted between two mixers when the signal is lower sideband.  This 
is also seen in Walter's autocorrelation plots.

This table summarizes what we have seen.  The third column lists
the sky frquency for the X band test and the fourth column indicates
whether a birdie was seen at the same *channel* at C band.  The final
column indicates whether the line moved when the antenna LO was offset
at C band.

SB   Ch    Xband?  F_sky  Cband?    moves on retuning LO at C band?
0   1024     yes    8936    yes       no

1           nada            nada

2    320     no     ----    yes       no
     1024    yes     8680    yes      yes

3           nada            nada

4    290    yes     8470    yes       no
     1024    yes     8424    yes       no

5           nada            nada

6    640    yes     8192     no       --
     1024    yes     8168    yes       no

7           nada            nada

[Note: each subband is 128 MHz wide, and has 2048 channels.]

================================================================================