[evlatests] P band, 4 band and power levels

[Terry Cotter]

Ken Sowinski wrote:

> This is a more rambling than necessary summary of what was
> learned last week by looking at sampler spectra and trying
> to get P band to work.  The quick summary is that we need
> to do a better job of matching spectral power density
> between the two bands.
> 
> 
> While I was debugging P band support last week Chuck was
> able to capture sampler spectra for all the EVLA antennas
> at P band.  Antennas 13, 16 and 19 were not working at 
> P band at the time.  Antenna 24 may have worked, but its
> L304 does not properly report band code which confuses the
> spectrum gathering/plotting program.  Antenna 23 has no 
> 4 meter receiver package.
> 
Antenna 24 has an M301 for band selection

> All IFs have spectra with similar features.  There is a broad
> pedestal from about 25 MHz to 120 MHz on top of which is the 
> 4 MHz "spike" defined the the 74/4 filter in the T301.  P band
> is visible as a 50 MHz passband from 300 MHz to 350 MHz.  The
> noise floor is usually 20 to 25 dB below the 75 MHz pedestal
> and the P band passband.  In many cases the 75 MHz pedestal
> power density is the same or larger than the P band bandpass,
> and because it is also twice as wide, in most T304s the total
> power seen at the detectors is dominated by the 75 MHz noise.
> This is a surprising result, but it is not obvious to me that
> it matters.
> 
> At the VHF bands the T304 attenuators will be set the same for
> both 4 and P bands and the FIR gain parameter in the deformatter
> module used to match the 50 MHz output to the input power level
> required by the VLA T5 module.  This will work only if there is
> only a small difference in power density between the two passbands.
> as seen in the T5.
> 
> In the first table below I list the measured power density for
> each of the features, for all IFs to show the large variation
> amongst IFs and antennas.  The most critical thing is that the
> P band and 4 meter power densities be about the same in each IF, 
> so the second table below shows the difference in dB between the 
> power density in the 4 meter band and P band.  Differences between
> IFs and antennas are unimportant.  Except for antenna 18 all the
> 4 meter bandpasses are about 15 dB above P band bandpasses.  Was
> the T301 in antenna 18 built differently or is the solar attenuator
> permanently inserted?
> 
> 
> Table 1
>         IFA        IFB        IFC        IFD
>      Ped 4  P   Ped  4  P  Ped  4  P  Ped  4  P
> 14   25 42 24   25 46 25   24 37 31   20 33 30
> 17   30 42 28   25 36 21   26 37 30   25 34 25
> 18   26 38 35   19 29 27   31 42 35   20 31 28
> 21   25 43 26   17 34 18   26 43 29   19 34 20
> 23         37         32         36         33
> 24      ??         ??         ??         ??
> 
> 
> Table 2
>           PD_4 - PD_P
>       A    B    C    D
> 14   18   21    6    3
> 16   14   15    7    9
> 18    3    2    7    3
> 21   17   16   14   14
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