sbn Noise floor with 35MHz filter

Changes to sbn:

• 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 750 mixer.
• a rough schematic of this is:

• 09feb18: the 15 db pad was removed.
  

• 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 Filter
• 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 attenuator.
  
• 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 filter).
• 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 filter
• The right edge is lower since the 2355-2360 rfi is not present.
  

Summary:

• 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 60MHz filter.
• 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 attenuator.
• Removing the 15 db pad changed the 60MHz noise floor to about 15 db.
• The 10 and 15 db levels were measured with different methods (spectrum analyzer at 750 MHz and baseband with the mock spectrometers.
• 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.