Lbw response below 1100 Mhz
Measuring the SEFD around 1100 Mhz
Looking at the rfi
The response of the lbw system was tested on 12feb07
to see if it could be used for galaxies below 1100 Mhz.
Measuring the SEFD around 1100 Mhz:
The standard configuration for lbw has:
To test below 1130 Mhz the setup was:
bandpass filter low frequency falloff around 1130 Mhz
pol Box with amplifier
The wave guide starts to cutoff around 1040 Mhz. A 500
Mhz bandpass centered on 1100 Mhz (via the spectrum analyzer) showed a
6 db difference between 1100 Mhz and the noise floor at 1000 Mhz. The noise
floor was being set by the 26 db amp (2 db noise figure) rather than
PolB - from the dewar go to a 1200 Mhz low pass filter and then a 26 db
amp (2 db noise figure). Then into the if/lo system.
PolA - from the dewar go to to 25 Mhz bandwidth tuneable filter and then
a 26 db amp. Then into the if/lo system. The tuneable filter was
tuned to 1100 Mhz.
Taking the data:
On, off position switching using the source B0038+32
(3C19) was used to measure the system performance. It has a flux of 4.09
Jy at 1100 Mhz. The telescope was at za=15.1 degrees when the measurements
were made. 2 1 min on/offs were done. The interim correlator was
used with 25 Mhz bands centered at: 1040, 1060, 1080, and 1100 Mhz.
The sefd was then computed as:
This becomes Tsys/Gain with units of Janskies.
show the system performance (.ps) (.pdf)
Page 1 Bandpass shape: The first 4 plots show the bandpass shape
for each of the 25 Mhz bands. Black is polB, red is polA. Only band 3 has
appreciable power in polA (because of the tunable filter). Each of the
25 Mhz bands have different attenuation. The bottom plot places the 4 bands
on the same power scale so you can see the power drop off with frequency.
The power starts to rise around 1090 Mhz probably from an increase in the
system temperature. It turns over and starts coming down around 1070. This
may be the omt starting to cutoff the frequency. The power drops
by 7 db between 1070 and 1040 Mhz. The actual drop may be greater since
Tsys is probably increasing over this range.
Page 2 SEFD below 1100 Mhz: This shows the sefd for the system.
4.09 Jy was used for the source strength. Both on/offs are over plotted.
Black is polB, red is polA. Only the 1100 Mhz band for
PolA (red) is higher than polB (black). This probably because the tuneable
filter has a larger noise figure than the 1200 Mhz low pass filter.
The SEFD is 4-4.5 till around 1080. It is 5 and 1060 and then shoots up
rapidly. There is probably not much sky getting through the omt below 1050
Things to think about:
The sefd looks pretty good down to 1080 Mhz (4. vs 3.5 at 1175).
Filter edges can be less stable than the flat portions. We haven't looked
very hard to see if there are any omt resonances in this frequency range.
The rfi needs to be looked at closer.
This setup will probably not be the final configuration for use (since
it required manual intervention to set it up).
rfi near 1100 Mhz was monitored with LBW and the interim
correlator on 12feb07. A 1200 Mhz low pass filter was used instead of the
standard 1120 to 1750 Mhz bandpass filter. 500 1 second dumps were taken
with each data set. The frequency channel resolution was 50 Khz (after
hanning smoothing). The data sets were:
Looking at the rfi around 1100 Mhz with
The plot shows the
rms/mean (by frequency channel) (.ps) (.pdf)
60 Mhz of data centered at 1100 Mhz. The 60 Mhz narrow band filter was
placed after the 1st mixer to cut down on IF harmonics of the aerostat
radar. The telescope was sitting at az=270,za=18 degrees.
80 Mhz centered at 1070 Mhz. Unfortunately this data was taken while tracking
the 4 Jy source above (so Tsys was about doubled).
Dynamic spectrum for each 25 Mhz band show the time variation of the rfi
for the 500 seconds.
The black line is the 80 Mhz centered at 1070. The red line is the 60 Mhz
centered at 1100 (it has been offset for display).
The first 4 images are less sensitive than the last 3 since they were
taken while tracking a 4 Jy source (doubling Tsys) (you can see this by
comparing the two 1080 Mhz bands).
The worst rfi is around 1090 Mhz. It gets up to 1.5*Tsys in a 1 second
integration and covers 4-5 Mhz.