[evlatests] EA14 polarization and VLITE

[jjackson]

It was always known we would need additional gain on the input of the 
VLITE module.  Until the digitizer board was built and characterized, we 
just weren't sure of exactly how much.   The high isolation amp in the 
module has only 15 dB of gain but can provide the drive level needed for 
full scale on the digitizer. It's the same series of amp we used with 
the EVLA digitizers. Once Mike had the VLITE digitizer board up and 
running in the lab, through measurement and experimentation we 
determined we needed about 20dB of additional gain.  Some brand new 
ZFL-1000LN's were on hand and seemed to fit the bill. Those amplifiers 
are about 23dB of gain as connected to +15VDC in antenna 14.  When we 
move them to the internal +12VDC power of the V-Lite module (which we 
are doing on the next antenna) that drops to exactly 20dB.  Looking at 
the specs of both amplifiers, the internal amplifier driving the 
digitizer should to go into compression at least 5dB before the ZFL-1000LN.

As installed in Antenna 14, the module seems to be setting up at 
reasonable digitized signal levels with reasonable digital attenuator 
settings.

Cheers,
Jim

Jim Jackson
Project/Lead Hardware Engineer
Very Large Array(VLA) / Very Long Baseline Array(VLBA) radio telescopes
National Radio Astronomy Observatory
1003 Lopezville Rd.
P.O. Box O
Socorro, NM 87801
(575) 835-7132 (W)
(575) 835-0158 (H)
jjackson at aoc.nrao.edu

On 6/26/2014 5:53 PM, Paul Harden wrote:
> We swapped the polarization Thursday on VLITE antenna 14 and should now
> be correct.
>
> Here's the problem.  Nomenclature for the low band linear polarization
> has been standardized such that:
> Horizontal antenna = RCP
> Vertical antenna = LCP
>
> The input connectors on the VLITE module are labled "Ch. 1" and "Ch. 2."
>    There is no indication what polarization is assigned to what channel.
>    The 50/50 choice used was obviously wrong.
>
> What polarization is assigned to Ch.1 and Ch.2?
>
> When installed, RCP was connected to Ch. 1 and LCP to Ch. 2, which
> caused the reported swapped polarization.
>
> LCP now goes to Ch. 1
> RCP now goes to Ch. 2
>
> This channel assignment needs to be standardized, and perhaps some "LCP"
> and "RCP" labels made to eliminate the channel references.
>
> Also, there are two 30dB coaxial amplifiers on the Ch.1 and Ch.2 inputs
> on the VLITE module, amplifying the low band receiver output by 30dB.
> Typical LBR receiver output is about -75 dBm/MHz on the cold sky,
> meaning about -45 dBm/MHz input to VLITE.  I don't know how much
> internal amplification there, the ADC range, or the compression points,
> but seems this much amplification would seriously limit the dynamic
> range and gain headroom of the VLITE module.
>
> Paul
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