encoder Combs in 327 receiver.
New encoder comb measured in screen room.
31jul09: az encoder combs. enc1,enc2 spacing is
03oct06 Initial report of comb: comb
spacing, dynamic spectra.
04oct06 az, za and time dependence of
05oct06: compare comb with old and
Note06: 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
31jul09: Az encoder combs
The 614 khz combs were seen in the 327 receiver.
Data was taken on 31jul09 with the 327 receiver and the mock
spectrometer to test the rfi.
The power to the encoders/vertex shelter were
then switched on /off while the data was being taken. The sequence
- az,za was stationary at 270,11.
- The mock spectrometer was used with 8192 channels over 34 MHz
(4 Khz resolution).
- data was taken at 10 hz sampling and then averaged to 1
- rec 1 : all on
- rec 182: enc1 off
- rec 286: enc12 off
- rec 470: vertex shelter power off (red lever)
- rec 512: vertex shelter power on (red lever)
- rec 668: enc1 on
- rec 809: enc12 on
- rec 1084: enc1 off
- rec 1158: enc12 off
- rec 1254: enc1 on
- rec 1344: enc12 on
A dynamic spectra was made for the 1500
seconds of data while the power was turned on,off (.gif):
- The .1 second 8k spectra were averaged to 1 second in time and
then decimated in frequency by 8 to give 1k channels. The
decimation took the peak value within each set of 8
- A baseline was fit to the average of the encoders off. It used
a linear polynomial and a 20th order harmonic fit. the baseline
was then divided into each 1 second spectra to remove the
- Each spectra was then normalized to it's median power (to
Overplotting the spectra showing the
birdie strength (.ps) (.pdf):
- The left had column labels what is current on/off. There are
dashed horizontal lines inserted where transitions occurred.
- 2on - rec 186 shows where encoder 1 was turned off. encoder 2
- 12off -rec 300 shows where encoder 2 was turned off (encoder
one remained off).
- Verpoff, verPon brackets where the vertex power was turned off
then back on. The az encoders were both unplugged before this
- 315.3, 339.2 birdies went away when this happened. They
could be coming from the ch,gr encoders (although we never
- polA records 1-960 were overplotted (the later recs had some
- top: frequencies 312-328
- bottom: frequencies 326-342.
- Vertical lines a the bottom of each plot:
- Red encoder 1 comb: 1.65 MHz
- green encoder 2 comb: 614 Khz.
- The dynamic spectra show that not all of the frequencies of
the combs went away when the encoders were turned off. These
must be coming from somewhere else.
- Solid vertical lines: these show the strongest comb elements
that went away when the encoder was turned off. These are
labeled at the top of the plot.
- The az encoder combs are at different spacings:
- azEnc1: 1.65 MHz
- azEnc2: 614 Khz.
- The strongest comb freq in 327 are:
- Az enc1:
- 316.2 : .2Tsys
- 321.1 : .15 Tsys
- 332.6: .18 Tsys
- Az enc2:
- 326.2 : .97 Tsys
- 327.5 : .39 Tsys
- 331.1 : .27 Tsys
- Not all comb elements are coming from the encoders.
- We moved to az=270, za=18 and took some more data. The comb
elements from the az encoders did not get appreciably
stronger (but some of the comb frequencies not coming from the
az encoders did).
A comb was seen in the 327 MHz receiver with a
614 Khz spacing. On 03oct06 data was taken with a 25 MHz bw, 2048
channels (24 kHz channel width after hanning smoothing), and one
second dumps while the telescope was sitting at az=229.25,
za=10 degrees. One 25 MHz band was centered at 322 MHz while the
second band was centered at 332 MHz.
Initial report of comb (top)
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
The largest spike in this freq. range is at 316.418 MHz. The birdie
at 313.5 is not part of the comb
- Both overlapping 25 MHz bands have been over plotted here.
- dashed horizontal lines have been drawn at the spacing of the
On 04oct06 the azimuth, zenith angle, and
time dependence of the 614 Khz comb in the 327 receiver was
measured. The comb element at 316.416 was used. A 395 Khz band with
2048 channels (385 hz resolution after hanning smoothing) was
centered on the 316.416 MHz comb element. One second sampling was
done. 3.8 khz about the center of the comb birdie was averaged and
04oct06 az, za and
time dependence of comb (top)
The plots show the
, and time variation of the birdie (.ps) (.pdf):
- 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.
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
of the new and old encoders (.ps) (.pdf).
Most of the comb elements are no longer seen with
the old encoder back in. There are a few that show up in the 25 MHz
band (with low resolution). We should see if these really belong to
the encoder by monitoring them (with high resolution) while the
encoders are shut off.
- 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:
- 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
- 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.