sbn Noise floor with 35MHz filter
Changes to sbn:
Changes to sbn receiver:
- 08feb18: A 35 MHz bandwidth rf filter was inserted in place of
the 120 MHz filter.
- Using the 750 narrow band mixer:
- On the spectrum analyzer in the control room, we saw
a 10 to 12db step between the noise floor of the 35MHz
filter and the noise floor of the 60 MHz IF filter after the
- a rough schematic of this is:
- 09feb18: the 15 db pad was removed.
Plots of the sbn bandpass (.ps)
- Data was taken on 12feb18 with the 15 db pad removed.
- The 750nb mixer was used.
- the mock spectrometer took 172MHz bw data centered at 2380 MHz
- 3 sets of spectra were taken:
- rf attn =0db
- rf attn = 3 db
- rf attn = 10db
- The spectra were scaled to the median value across the 35 MHz
- The plots are db below this level
- The if Atten (after the mixer) had 7db inserted (the 750
mixer polB has 7db more gain than polA).
- The colors show the different runs.
- Top : 172 MHz about 2380 MHz.
- bottom: blowup 2350 to 2410 MHz
- The 60MHz noise floor does not change when we change the rf
attenuator by 10db
- This shows that this noise floor is in upstream from the
- The rfi at 2350-2360 is the at&t rfi that the filter was
to cut off (it was 30db above the noise floor with the 120MHz
- You can start to see the 60MHz noise floor around 2360
and 2400 MHz. It is 15 db down from the center of the 35 MHz
- The right edge is lower since the 2355-2360 rfi is not
- The 35 MHz rf filter was installed on 08feb18.
- It cut down the 2355-2360 rfi from 30db above the noise floor
, to 1 or 2 db above the noise floor.
- With the 35 MHz filter in, we can see the noise floor of the
- With the 15db pad in, the floor is only 10 to 12 db below the
35 MHz noise level.
- a 10db step would say that 10% of Tsys would be coming from
the source of the 60MHz bw floor.
- Changing the rf attenuator by 10db did not change the
60MHz bw noise floor
- This implies that it is coming from before the rf
- Removing the 15 db pad changed the 60MHz noise floor to about
- The 10 and 15 db levels were measured with different methods
(spectrum analyzer at 750 MHz and baseband with the mock
- We need to further investigate why there is only a 15db step
between the 60MHz filter and the 35 mhz filter.
- This difference was probably always there, we just couldn't
see it until we inserted the 35 mhz filter.