Note: the comb was coming from the azimuth 2 encoder (on the ch side). A new encoder had been installed on 02aug06. The old type encoder also has this comb (but a lot weaker). The old style encoder was reinstalled 05oct06 around 1pm.

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03oct06 Initial report of comb  (top)

    Dynamic spectra while sitting (.gif).

• This is the first 25 Mhz band centered at 322 Mhz. PolA and PolB have been averaged.
• The telescope was sitting while this data was taken.
• The data has been flattened in the frequency domain by fitting to a 25th order harmonic. It was flattened in the time domain by averaging over channels with no birdie.
• The points at 331 Mhz are the 60 second spaced birdies (more info).

• Both overlapping 25 Mhz bands have been over plotted here.
• dashed horizontal lines have been drawn at the spacing of the comb.

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04oct06 az, za and time dependence of comb  (top)

• Fig 1: az swing -90 to 270 and back (at .3 deg/sec)
• The dome sat at a za of 19 degrees. the ch was at stow.
• Two swings were done. Each is plotted in a different color.
• The 4 frames show 90 degree sections of the 360 deg spin,
• The two swings overlap exactly. This means that the birdie is outside the dome.
• Fig 2: za motion of dome  and carriage house (at .02 deg/sec). Azimuth sits at az=270 deg.
• Top: dome za moves from za=2.5 to 19 deg. The carriage house was sitting at 8.8 deg. This was done multiple times. Each pass is plotted in a separate color. The separate passes overlay each other pretty well. There is a sharp rise in the birdie strength at a za of 17 (this is close to where the beam spills over onto the ground screen). There is another peak around za=10 (the ring girder). A smaller one shows up at za=4
• The center plot is a blowup of the first plot showing za=16 to 20 degrees. You can see where the signal peaked.
• Bottom plot: The dome sat at 17.295 (a peak in the birdie). The carriage house was then moved from 2 to 19 degrees twice. This caused the strength of the birdie to vary. The variation was repeatable as the ch went over the same za a second time. This may be that the ch is tilting the platform causing the reflection of the birdie from dome to change as it scatters into the receiver. More likely it is the scattering path from the ch encoder into the dome receiver is changing as the ch move relative to the dome.
• Fig 3: Time variation of birdie and avg spectra. The dome was sitting at an az=270 and a za =18 degrees (the ch was at stow). 1 second samples were taken for about 4000 seconds.
• Top: time variation of birdie. Without changing anything, the birdie strength moved from tsys=14 to tsys 6 (although the tiedowns may have moved the platform a little).
• Center: The average spectra for the 316.418 Mhz birdie. This is for the 4000 seconds.
• Bottom: blowup of the average spectra. It looks like there are 3 peaks that are not quite resolved at 380 hz resolution. This FWHM is about 3 khz.

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05oct06 compare comb from old and new encoder.  (top)

The new az encoder was replaced with the old version on 05oct06. On 05oct06 data was taken with the old encoder back in. The az/za was 270/10 (it would have been better to do this at za=18 where the birdie is stronger but i didn't have control of the az/za). 382 seconds of data was taken in the same format as 04oct06.  The plots show the comparison of the new and old encoders (.ps) (.pdf).

• Top:  The 25 Mhz band (2048 channels) centered at 322 Mhz. The black traces are the old encoder. The red traces are the new encoder (taken 04oct06 at the same az,za). The old encoder has been offset vertically for display. The dashed green lines are at the comb frequencies.
• Center: a blowup of the top plot. You can see that most of the comb frequencies are no longer visible. The old encoder frequencies that still have birdies at the comb frequencies are: 324.408,  325., 329.32,330.549.
• Bottom: The high resolution spectra (395 hz resolution) with the new and old encoder over plotted. This comb element has completely gone away:

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Summary:  (top)

• There is a comb in the 327 receiver with a spacing of 614 khz.
• The comb goes away on load (so it is not in the IF).
• An individual  comb element is 1 to 3 khz wide.
• The comb shows a strong azimuth dependence so it is not coming from inside the dome.
• The comb gets at least 10 db   stronger when the dome is above 17 degrees.
• at az=270 a peak is at 17.295 (ch at stow)
• the strength of the comb also changes when the dome sits still and the carriage house is moved from za=2 to za=20 degrees.  With the dome at za=17.295 the comb peaks at ch=stow and ch=10degree. This probably the scattering path from the ch encoder into the dome.
• We have concentrated on the azimuth encoder, but there is probably birdies coming from the dome and ch encoders (although they are the "old" type).
• The comb was found to come from the azimuth encoder #2 (on the carriage house side). A newer type of encoder was installed in 02aug06.
• When the new encoder was installed we could see the comb and 430 Mhz and at 327 Mhz.
• replacing az2enc with an "old" encoder made most of the comb elements go away. The comb elements measured back in oct05 are still present. We should check if these are coming from the encoder by monitoring them while the encoders are shut off.