• encoder combs in 327 receiver
• 
  

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Intro:

• azimuth: The encoder rack gear runs around the inside of the platform ring girder. There are two azimuth encoders (azenc1, azenc2) that are mounted on the top of the azimuth arm and run in  the encoder rack gear. The encoders are fixed relative to the azimuth arm.  Azenc1 is mounted on the dome side of the azimuth arm, azenc2 is mounted on the carriage house side of the azimuth arm (next to the vertex shelter). The azimuth pointing uses azenc1. Azenc2  measures the bending in the azimuth arm.
• Gregorian: The elevation rack gear sits at the bottom of the azimuth arm. The gregorian encoder is mounted on the  gregorian trolleys and rides on the elevation rack gear. It is fixed relative to the dome.
• Carriage house: Same rack gear as the gregorian (but on the other side of the azimuth arm). Encoder mounted on the carriage house trolley.

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History:

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..

• 06-07: Old, new encoders, filters.
  
• at some point in time they stopped making the "Old" encoder (with the 614 KHz comb).
• The newer encoders had a 1.646 Mhz comb (or maybe 1/2 of that).
• We started putting filters in the communications lines to cut down on the rfi (which looked like it was coming out of the shielded cables running to the vertex shelter).
  
• 31jul09: measured combs from the encoders using the 327 receiver
  
• az enc1 had comb of 1.646 Mhz, azenc2 failed. it was replaced and had a strong comb of .614 Mhz..
• independently of what i wrote on the web page, i think we put an "Old" encoder in az enc2 and it didn't have a filter...
• 161011: az encoder 1 failed. before replacing it we measured the rfi in the screen room. This is a "new" encoder.

180605: turn off vertex encoders one at a time

    On 180605 we did a test to see which encoder was causing the .8391 Mhz comb. The  setup was:

• 430 Mhz rcvr
• telescope sitting at: az=227, za=11 deg
• The mock spectrometer. 8192 channels, 26.67Mhz bw, 1 second sampling, centered at 432 Mhz
• a 26.67/2 offset was added in the first mixing stage, and then removed digitally i the mocks to get rid of the junk at DC.

    Each encoder has a number of cables that go from the vertex shelter to the encoder (serial line, power, etc..)
We disconnected the power cable (at the vertex shelter) for each encoder (one by one).

The plot shows the average spectra with all of the encoders on (.ps) (.pdf) :

• The data comes from the first 150 seconds of the test (while all encoders were on).
• a 32 order harmonic fit was done and the used to normalize the bandpass.
• The red dashed lines are spaced every .8193 Mhz (the normal comb spacing for the new encoders).
  

The image shows the dynamic spectra for the test (.png)

•  After normalizing with the fit, the median value was removed from each spectra (to try and get rid of continuum sources passing through the beam).
• The dashed white lines show when an encoder was turned on or off (to within a second or 2).
• The green and blue lines at the bottom are a .8193 (blue) and 1.6386 (green) combs.
• The are positioned at the comb in the data
• The comb does not get weaker when we turn a single encoder on or off

SUMMARY:

• We see a .8193 Mhz comb in the data
• The comb does not change as we pull the power cable for a single encoder in the vertex shelter.
• This means either:
  
• we really weren't completely shutting off the encoder when we removed the cable... or
• the signal is coming from more than one encoder at a time .. or
• the comb has nothing to do with the encoders,

To do:

• take data with the same setup.
• Turn off the vertex shelter (the large lever).
• leave it off for awhile, then back on.. for awhile, then off, then on
• This will tell if the rfi has anything to do with signals coming from the vertex shelter (except for maybe what the ups holds up).

processing: x101/180605/enc_Tst.pro

180601: Encoder comb in 430MhzRcvr

• rf center freq 432 Mhz.
• mock setup: 32 Mhz bw, 8192 channels, 50 ms sampling, polA,B.
• Data was taken for 5 minutes on source then 5 minutes off source.

    Lots of rfi was seen in the online spectra. To investigate the source of the rfi:

• A 5 minute average spectra was computed using the off source data.
• PolA and polB were added together.
  
• A 43 order harmonic function was fit to the average bandpass.
• The bandpass was flattened with : (AvgSpc/fitSpc - 1).
• an fft was done on the flatten spectrum to look at the comb periods.

The plots show the results of the measurements (.ps) (.pdf)

• Page 1  average spectra and fit
  
• top: average spectrum and fit.
• Black is the 5 minute average spectra. the red line is the fit.
• the display was limited to 421-444 Mhz  (the rf filter cutoff).
• Middle: The flattened spectrum: (spcAvg/Fit - 1).
• The red dashed lines are spaced at .8193 Mhz.
• Bottom: blowup of  avgspc/yfit -1
• You see the low level alignment of with the comb.
  
• There are other rfi spikes present.
• Page 2: magnitude of : fft(flattened spectrum)
• This gives a clearer view of the comb. It shows the number of cycles in the 32 Mhz bw.
• Top: .8193 Mhz comb spacing. This hits the stronger cycles.
• bottom: Twice the comb spacing: 1.6885 Mhz.
  
• this includes the weaker comb elements.

Summary:

• A .8193 Mhz comb was seen in the 430 Mhz data.
• On 161011 the comb from the new encoders was measured in the screen room (before and filtering was done).
• the encoders had a .8195 Mhz Comb
• The 430 comb is coming from one of the az, ch, or dome encoders.
• The comb is probably also present in the 327 Mhz Rcvr.
  
• We need to do a test (turning them off one by one to see which are causing the problem.

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:

• Anritsu spectrum analyzer
• avg 5 sweeps
• 20 Mhz span, .1 Mhz rbw, cfr 316.2
• .1 MHz span, 1 KHz rbw, cfr 316.2
• The anritsu was not locked to the station clock (need to check the offset).
• probe: 6cm loop (901b H field probe)
• encoder rfi measured with encoder opened and closed.
• the cabling was:
• encoder->encoder cable -> rs485 to rs232 converter -> rs232 to usb -> computer usb port

• The probe was close to the cable as it exited the encoder.
  
• Top: Encoder open, no filter, all cables plugged in
• The blue dashed lines show the .819584 Mhz comb
• this is 1/2 the period seen in 2009
• the red dashed lines are blown up in the bottom frame
• Bottom:  .1 Mhz span, 1 KHz RBW.
• Black: encoder open, no filter, all cables connected
• Green: encoder closed, no filter, encoder -> rs485/rs232 converter connected but disconnected after that.
• Blue: encoder closed, filter installed, same cabling as green trace
• red:  encoder closed, no filter, but cable to rs485/rs232 converter is disconnected.

Summary:

• The comb is coming out the shielded cable. We did not see it around any other encoder opening.
• The strength of the rfi depends on the value being transmitted across the line.
• We looked at the SSI serial line with a scope.
• the system uses a grey code
• When there were more  transitions on a packet, the rfi was larger.
• The baud looked to be about 110 Usec (or maybe 1/2 of that).
• When we do rfi tests on the telescope, we should probably move the telescope (although you can't turn off the encoder and move..)
• If the cable from the encoder is not connected to the rs485 to rs232 converter, there is no rfi.
  
• The encoder probably doesn't send any data when the cable is disconnected.
• We did not have to be connected to the computer to see the rfi.
• closing the encoder gives 10db of isolation
• putting the filter in gives 10 more db of attenuation
• The comb lands at 316.206 MHz (this includes the anritsu offset at 326 MHz when unlocked. It's probably less than a few 100 hz.
• The comb spacing is Mhz
• The comb spacing was measured over the 20Mhz.
•  This was probably accurate to 100 KHzRbw/24.
  
• I then divided this number into 316.206 to get the fundamental
• I then rounded the fundamental to the closes integer.
• We also found that the cable shield is grounded to the encoder case (for the cable we were using).
• Need to check if the vertex cable has the shield connected to the ground on both ends.

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

    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.

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Summary:

• The 614 Khz comb is coming from the az,za encoders.
• The comb from the new encoder is about 1 to 3 khz wide.
• The comb in the new encoder (installed 02aug06) is a lot stronger than the comb in the old encoders. We can only see a few elements of the comb in the the older encoders.
• Back in oct 05 elements of the encoder comb were seen (but we didn't know they came from the encoders). We should verify that these are really coming from the old encoders by monitoring these frequencies while turning off all of the encoders.
• The new encoders comb is shifted about +5 Khz from the old encoder comb (as seen in the lab).
• In the lab, the strongest birdie from the new encoder is at 326.269 (327 band)  and at 432.539 (430 band). Using a log periodic antenna on the platform, the birdie was strongest at 324.408 (this is one of the strongest comb elements seen with the old encoders back in oct05).
• In the lab, the strongest birdies from the old encoder are at 324.415 (327 band). This is probably the same birdie as the 324.408. In the 430 band the strongest birdie is at 427.640. This was seen in the "oldEnd,newEnc" data taken at 430 Mhz.
• With the new encoders we've seen the comb at 327 and 430 Mhz using the telescope. With the old encoders we've seen it at 327 and maybe 430. In the lab the comb goes to > 1000 Mhz.
• The Az dependence of the various comb elements is repeatable at that frequency. Some comb elements show different azimuth dependence.
• 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 (or it could be the platform tilting and the scattering path from azenc2 into the dome changing).
• We have concentrated on the azimuth encoders, but there is probably birdies coming from the dome and ch encoders (although they are the "old" type).

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