[evlatests] 3-bit update

[Keith Morris]

I will add that the necessary equalizer settings were, in general, 
fairly well clustered around the default setting of 10.  50% of the 
cases were between 9 and 11 (on a 0 - 15 scale).  See attached histogram 
plot.  Ea22-A1 and ea22-C1 required lower-valued eq settings in all cases.

The "phenomenon" was visible in most cases.  See attached 
evla_ea22_A_ka2.png


The file evla_ea12_A_ka1.png shows the uncorrectable slope in ea12-A1.

Rick and I will be visiting the antennas Friday morning to perform the 
attenuator installation and cable/label swapping.  I will inspect the 
T304-A output to see whether the signal degradation is occurring before 
the sampler.  If not, we will replace D301 with an 8-bit only version to 
permit lab investigation of the slope/level problem.


Michael Rupen wrote:
> We spent most of the day on 3-bit testing.
> 
> 1- We confirmed that RCP from the antenna is being labeled LCP by the time
>    the data reach the correlator.
>    - Rich will go out tomorrow (Friday) morning and switch the A1/C1 cables
>      into the 3-bit samplers to fix this.
> 
> 2- Hichem & Mike sorted out a version confusion for the deformatters,
>    so the monitor points now correctly report the rms from the 3-bit samplers.
> 
> 3- We tested the Station Board software which fits a line to the
>    wideband correlator (full baseband) output, and found that
>    (1) the data to which that line is fit do not agree with those
>      reported by the correlator's Real Time Data Display (RTDD), and
>      by Todd's bandpass tool.
>    (2) the fitting routine correctly fits those incorrect data and reports
>      accurate parameters for that fit.
>    Bruce is looking into the data problem.  The aim is to enable auto-setting
>    of the gain-slope equalizers.
> 
> 4- Barry showed Michael how to keep the attenuators steady within a 3-bit
>    observing script.
> 
> 5- The sampler for ea12 was returned to the array.
>    - We now have fringes for all 3-bit baselines and IFs.  We didn't
>      bother to set the 3-bit delays.
> 
>    - The 8-bit sampler for IF A now looks fine.
> 
>    - The 3-bit samplers for ea12 are a bit odd:
>      * the input power required to achieve the desired rms (~1.7) of the
>        state counts is by far the highest for A1 (-22 dBm, vs. next highest
>        3-bit sampler is -33 dBm), while that required for C1 is slightly
>        lower than for any other 3-bit sampler (-42 dBm, vs. -40 dBm for the
>        next lowest).
>      * the gain slope equalizer can be set to give a flat bandpass for all
>        3-bit samplers on the array *except* ea12-A1.
> 
> 6- Keith & Michael spent the afternoon testing whether the gain slope
>    equalizers can flatten the bandpass at a range of "problematic"
>    frequencies:
>      C  band  5-7 GHz
>      K  band 19-21 GHz
>      Q  band 44-46 GHz
>      Ka band 35-37 GHz, 38-40 GHz
>    I will send out a detailed summary this evening, but in general
>    we were able to flatten the bandpass for all samplers except ea12-A1,
>    with gain slope equalizer settings between 3 and 12.  The *best* setting
>    for ea12-A1 still left a 12-15dB slope across the 2 GHz band.
> 
>    - The input power required to achieve the desired rms from the 3-bit
>      samplers (~1.7) ranged from -33 to -40 dBm (excluding ea12; see above),
>      compared to the standard 8-bit setting of -30 dBm.  Keith has some
>      theories which the detailed summary I'll send this evening may help
>      sort out.
>    - Based on this 15 dB attenuators will be installed before the 3-bit
>      samplers, to allow us to have enough power going into the T304s.
>      If this works that would allow us to remove one of the 15 dB amplifiers
>      from the 3-bit sampler boards, which would make many of the engineers
>      happy.
> 
> More details (including state-count histograms) later --
> 
>            Michael
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-- 
Keith Morris
National Radio Astronomy Observatory
1003 Lopezville Rd.
Socorro, NM 87801
575-835-7060 (phone)
575-835-7027 (fax)
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