16khz comb in 327 receiver.

may05: Initial report
aug05: the 16Khz comb is coming from the alfa motor contoller.
01sep05 Grounding motor shield 29sep05 improved things but then deteriorated.

may05: Initial report

   A  16 khz comb was seen in the 327 receiver. It was reported by p1979 and A2057 in may05.
On 03jun05 data was taken with the 327 receiver  to see what may have been causing it.

03jun04  data:

    Data was taken with the interim correlator and the ri.
  • a 300 second integration with the 16 khz comb (.ps)  (.pdf) . The interim correlator was set to  190 hz/channel (after hanning smoothing) and 1 second dumps. The comb is strong in polA and very weak or non existent in polB.
  • high frequency resolution data set (.ps) (.pdf) was taken  by base band sampling a 200 kHz band for 48 seconds. The top plot shows the spectra smoothed to 1 hz resolution. The bottom plot is a blowup with .02 hz resolution. The hires spectral width  shows that the birdie has a width of around 1 hz (so it  is not a harmonic that is locked to the station clock).
  • Comb strength vs azimuth (.ps)  (.pdf) : The dome was placed at 18 degrees and then the azimuth was swung from 270 to 660 degs and back. The top plot shows the average of the comb channels (white, red) and the adjacent channels (tsys). The bottom plot is the comb strength (comb channels - offCombChannels). There is no apparent azimuth dependence in the strength of the comb.
  • We then did some test to see where the comb was coming from. We did the following:

    P1979 data (may05):

        P1979 used folding mode on the wapps with 1.56 Mhz Bw and 1024 lags. Typical integration times were 600 seconds. 4 separate frequency bands were taken and a frequency hop of about .5 Mhz was done between files. For each 600 second data file, all of the spectra were averaged for each frequency separately (4 freq x 2 hops). This data was taken between 21may05 and 31may05.
        The first set of plots are dynamic spectra images that were made  using the average spectra from each file. The vertical axis is data file. This is increasing in time but it is not evenly spaced since the data were taken on different days: One thing that stands out is that the 16 Khz comb is strongest when the 325 Mhz birdie is strongest (the red line on the left of each image shows the strength of the 325Mhz birdie.).

       The next set of plots shows the relative strengths of the birdies:



    aug05 the 16Khz comb is coming from the alfa motor contoller:  (top)

        The 16khz birdie in the 327 Mhz receiver was initialing reported in may05. Data taken in jun05 showed that the comb was about 1 hz wide. On 24aug05 we searched in the dome using a portable spectrum analyzer. The setup was:     Sitting at the equipment rack for the 327 receiver we saw a 2-3 db (above the noise floor) birdie at these two freq. After sniffing around it looked stronger at the cables that went over to alfa (this rack also contained the control cables that went to the alfa filter bank).
        The signal was stronger at alfa. We then opened the alfa motor controller rfi chassis and the signal increased.  When we turned off the amplifier/controller  the birdie went away. This chassis also contains the cryodynes for the alfa dewars. Turning these off made no difference.
        Down in the lab we setup the spare amp/controller in the screen room and looked at its rfi output. We saw the 16khz comb at 331.5903 (may 5db above the noise floor). Opening up the span we also saw a 40Mhz comb coming from the amp/controller. This comb goes  out beyond 1400 Mhz.  It was 20 to 30 db above the spectrum analyzer noise floor.
        Switching to a span of 50 Khz around 320Mhz (40*8) we saw the 40 mhz comb plus  the 16Khz comb centered on the 40 Mhz. The comb was a lot stronger than at 331.5903 Mhz where we had been looking. You could see the amplitude of the 16Khz comb fall off as you got farther away from 320Mhz.
        The specs of the amplifier/controller show that:     To verify that this all hung together, we went back on 25aug05 and took some data with the interim correlator. The setup was: The plots show the 40Mhz birdie and the 16 Khz birdie (.ps) (.pdf): The first page has a 195Khz span. The second page is blownup to a 40 Khz span (both of channel widths of 90 hz).     The encoder and motor cables that enter the rfi box are feed thrus. They do not have their shields tied to the box as they enter the box (even though the feed thru connector has connections for this). This may be causing the problem.
        To see whether any extra shielding works, we should concentrate on the 320Mhz comb since it is 25 db stronger than the 16Khz comb.

        I also took a look at all of the 40 Mhz comb members that were inside the lbw receiver. Using a 90 hz resolution and 60 second integration, i did not see the comb present.

    processing: x101/050825/doit.pro

    01sep05 Grounding motor shield 29sep05 improved things but then deteriorated.

        The alfa motor controller amplifier is causing a 16khz and 2khz comb in the 327 receiver. It is centered at multiples of 40 Mhz (the dsp clock?? of the motor controller). The motor power and encoder cables pass directly into the rfi shielded box that contains the amp. Their shields are not grounded to the box where they enter the box. On 29aug05 the shields for these cables were grounded to the rfi box. This decreased the signal by about 10db. On 01sep05 the 327 receiver was checked again. The grounded shields were no longer as effective as they were on 29aug05 when they were installed.
        The plots show the strength of the birdies around 320Mhz (.ps)  (.pdf):
        The units are Tsys. The resolution bandwidth is 190 hz (after hanning smoothing). The red lines are the initial birdies from 25aug05 with no shield grounding and the amps on. The dashed red lines show the locations of the 16Khz comb about 320Mhz.
    processing: x101/050901/doit.pro