Az, za encoder birdies.

Links to pages:
28oct05: 327 az swings showed comb with old encoders
03oct06: 614 Khz comb found at 327 Mhz.
04oct06: az,za spins show comb is outside the dome.
04oct06: A2125 saw comb at 430 MHz with new encoder
05cot06: compare comb strength new,old encoders using 327 Mhz.
10oct06 strengths of birdies from old encoder measured in the lab
31jul09: rfi from azimuth encoders


Intro:

    The vertex drive system (az, dome, carriage house) use encoders to determine the location of the axis.  They are located at: The encoders are connected to the vertex shelter via shielded cables.


History:

    We 05oct06 we discovered that the az,za encoders generate a comb at 614 Khz. Some of the comb elements had been seen before that time (but we didn't know where they were coming from).
  • oct05:  az swings 327:
  • azimuth swings were done to look at the 327 Mhz birdies. About 7 birdies had the same  az dependence. At the time we did not know that these birdies were coming from the az,za encoders. This was determined later when the comb frequencies were measured with the new encoder. The 7 birdies from oct05 all sat at one of the comb frequencies.
  • 02aug06: new azimuth encoder
  • The old encoders were getting water in them and failing from corrosion. A new waterproof encoder was purchased (same company). It had  a continuous metal case (that we thought would be better for rfi shielding).
  • sep06: 327 pulsar observations saw really bad interference.
  • 03oct06: data was taken with the 327 Mhz receiver. The comb was seen with a spacing of 614 Khz.
  • 04oct06: Using 327 Mhz, az spins and za motion showed that the birdie was not coming from inside the dome (since it had and az and za dependence).
  • 04oct06: The 614 khz comb in seen in the 430 receiver while new encoder is installed. A2125 did 430 mapping (with the dome) on 04oct06 when the new encoder was installed and again on 05oct06 (night) when the old encoder was back in. The 614 Khz come was seen when the new encoder was in. It may or may not have been seen with the old encoder back in (some comb elements were there).
  • 05oct06: We found that the comb was coming from the az2 encoder. We replaced the "new" az2 encoder with the "Old" encoder around 1pm.
  • 05oct06: after replacing the new encoder with the old encoder, data was taken to see how strong the 614 Khz comb is with just the old encoders. Most of the comb elements disappeared. The ones that had been seen back in oct05 remained.
  • 05oct06: In the lab
  • We saw that the new encoder had a 614 Khz comb. We opened the encoder casing and saw the comb go to > 1000 Mhz. At 327 Mhz the strongest comb element was at 326.269 Mhz. At 430 Mhz the strongest comb element was at 432.539 Mhz. When we closed the casing the 327 Mhz comb was strong (> 10 db). It was probably coming out of the cable.

    We opened the case of the old encoder and saw  a few elements of the comb were present. The strongest was at 324.415.  When we closed the encoder, we could still see the strong birdies (though not as strong as the new encoder.

    The new encoder has all the elements of the comb while the old encoder only has a few of these. There is an offset of about 4 khz between the birdies from the old encoder and the new encoder..



    161011: encoder rfi measured in screen room

        Azimuth encoder 1 failed on 10oct16. Before replacing the encoder, we checked the rfi coming from the encoder in the screen room:
    The setup was:

        The plots show the rfi measured (.ps) (.pdf):

    Summary:

    processing: x101/161011/encrfi.pro

    10oct06 strengths of birdies from old encoder measured in the lab

        On 10oct06 in the screened room we removed the case from one of the old encoders. Using the loop antenna (placed on top of the encoder wires as they leave the case) and the textronix spectrum analyzer (set to 1 Mhz span) we looked at the frequency range 300 Mhz to 1.2 Ghz. The procedure was to set the span to 25 Mhz, look for any birdies, then blowup  the individual birdies to 1 Mhz span. The spectrum analyzer was set to average 10 sweeps. The table below shows the strength o f birdies found. The amplitude is probably to to about 3db (the baselines were not that flat).
     
     
     
    "Old Encoder" birdies. In lab, cover open
    freq
    Mhz
    amp
    Db
    freq
    Mhz
    amp
    Db
    freq
    Mhz
    amp
    Db
    freq
    Mhz
    amp
    Db
    324.415
    25
    471.878
    15
    678.326
    24
    973.250
    20
    339.162
    17
    486.624
    14
    707.816
    15
    987.996
    18
    353.908
    17
    501.370
    22
    722.564
    20
    1002.742
    18
    368.654
    20
    516.116
    18
    752.056
    18
    1061.250
    18
    383.400
    15
    530.864
    22
    781.550
    20
    1076.472
    17
    397.650
    15
    560.356
    24
    796.294
    22
    1105.964
    15
    412.900
    22
    574.100
    15
    855.280
    18
    1135.458
    20
    427.640
    22
    634.086
    22
    870.026
    20
    1120.712
    15
    442.386
    15
    648.832
    20
    914.264
    20
       
    457.132
    15
    663.578
    22
    929.010
    17
       

        The amplitudes have not been corrected for the response of the small loop antenna used. When searching for possible birdies caused by the encoders, start at the strongest values  in the table.


    Summary:

    processing:

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