[evlatests] WIDAR/EVLA Birdies

[Jim Jackson]

There is a known birdie at 8192MHz  produced in the T304 
downconverter (probably the 4096 fixed LO x 2).  This will likely 
show up in all bands since the T304 is used for all bands. The 
location will probably move around depending on L302 settings.  I 
don't remember if this will also show up in the 2-4 GHz (3-bit) path 
or is limited to the 1-2GHz (8-bit) path where you are seeing it 
now.  We knocked it down as far as we could without a major redesign 
of the T304 circuit boards and at that time everyone seemed to be in 
agreement that it was too low to cause any major problems.  I think 
it was close to or even within the original spec of the module but 
it's been at least 3 years since we looked at it.  We'll take a quick 
look at it again but at this point in the game, we're probably beyond 
investing any major time or money into it.

Jim Jackson



At 11:49 AM 9/18/2008, Michael Rupen wrote:
>                    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.]
>
>================================================================================
>_______________________________________________
>evlatests mailing list
>evlatests at listmgr.cv.nrao.edu
>http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests