The xband receiver on the p12m telescope
was monitored for 7 hours on 08jun21. The setup was:
- The telescope tracked blank sky (ra 07:00:00 dec 16:00:00)
for 7 hours starting at 8:30.
- We were pointing south of zenith. The ra of
the sun was about 22deg dec.
- We used the xband receiver using 8050 to 9200 MHz
- Mock spectrometer used to record the data.
- 7 172 MHz bands centered at:
- Each band spaced by 164 MHz
- 2 pols, 4096 channels/pol, 42 KHz channel width
- spectra dumped once a second
- The datataking sequence was:
- 300secs data
- 10 sec cal on
- 10 sec cal off
- repeat sequence
- We covered elevations 10 to 87.6 degrees.
- We wanted to measure Tsys vs elevation using the cal
- We do not yet have accurate measurements of the cal
- So i used the enr of the diode and losses of the cables
and cal coupler.
- I ended up with a cal of 43K. This value was used across
all freq and both pols.
- -->These cal values are just an estimate. The Tsys
values computed from the cals should be take with a
grain of salt.
- During the test there were heavy clouds and 2 periods of
- What we ended up measuring were problems in the system.
- The total power was computed using the entire spectral
band. No rfi removal was attempted.
- The total power was normalized to data taken 11 to 12
hours (when there were fewer clouds).
- So the units would be Tsys.
- The total power vs time for each frequency band was
normalized to the average of the first 1000 points
taken (so the units become Tsys).
Plotting the data
The first plot shows the
total power vs hour of day (.ps) (.pdf)
- Top: total power vs hour of day for polA.
- Each color is a separate 172Mhz frequency band
- The different bands responded similarly.
- The narrow spikes are the 10sec calon, caloff
- The power at 12.5 and 13.5 to 15:00 are
- Middle:total power vs hour of day for polB.
- The cal input to polA and polB has similar power.
- The polB caldeflection being smaller than polA -->
that tsys PolB is higher than polA
- During the power increases at 12.5 and 13.5 ->15
(rain,,clouds) the cal deflections are not seen.
- Bottom. Elevation vs hour of day
- This shows the elevation we tracked.
The second plot includes the rainfall and size
of the cal deflection (.ps) (.pdf)
- Page 1: total power and rainfall vs hour of day
- Top: total power vs hour of day and rainfall
- The uses the 8624 Mhz band for the total power.
- Black:polA , red:PolB
- Green is the rainRate in inches/second
- Blue is the cumulative rainfall for the day (in
- The rain monitor is located at the lidar lab (a
few 1000 ft from the 12 meter)
- Bottom: blowup showing the period of rain.
- The large increase in Tsys coming from clouds and rain
- For polA Tsys is more than doubling.
- for polB you see no cal deflection during the rainfall
- The purple dashed line shows that polB power never
got above this level.
- Page 2: tsys in kelvins and the size of the cal
- Top: Tsys vs hour of day
- I used 43K for the cal
- polB is much higher than polA (although Tcal needs to
be measured more accurately).
- During the rainfall, Tsys goes off scale. Since the
cal deflection goes to 0 , tsys will get really large.
- Middle: caldeflection vs hour of day.
- The units are spectrometer counts. The attenuation was
not changed during this period.
- Changes in the electronic gain of the system will
cause the values to change.
- During the rain periods
- PolA cal deflection gets noisy. This is expected
since tsys is probably changing.
- PolB cal deflection goes toward 0 during the rain
- Bottom: Cal deflection vs Tsys value cal off.
- The horizontal axis is the cal off tsys value in
- PolA values are smaller that polB.. part of this is
that the electronic gains were different.
- The vertical axis is the cal deflection in
- polA has a slight downward trend with
- Polb caldeflection goes to 0 as the tsys power
- The polB chain is going into saturation. It is
actually hitting the voltage rail of an amp somewhere.
- We were not able to measure Tsys vs elevation because of
- We found that polB is going into total saturation
with the increase in temp from clouds/rain..
- PolA increased by 3db. so the temp for polB was probably
- We need to find out where the saturation is occurring.
power levels shows the power levels that the
system runs under (although it was a clear day:)
- fiber optics rcver output : -27,-31 dbm . well below
the 10dbm saturation of the link
- lna: output around -35dbm
- amp in hub : around -15dbm
- input to postamp chassis: -26dbm
- after first amp in postamp chassis: +2dbm (2 db
- filter,2nd amp output: -8.3dbm
- diplexor input: -10dbm
- All of these values are way below saturation.
- One thing we've noticed is that most of the power coming
out of the lna is
around 4.5 GHz.
- we need to check if this can actually get
through the waveguide cutoff (probably the
circular part, especially with the dielectric rod
in the center.
- The plot shows polB, but polA has a similar output.
- We need to figure out where this extra power is coming
- If this is the problem, why is polB saturating
and not polA?
- It would be a great help if we could purchase a cal kit
for the xband waveguide so we could use the transitions to
see what is going on inside the waveguide . Unfortunately
we've been told there are no funds to do this.. so i guess
we'll have to continue messing around trying indirect tests.