[evlatests] WIDAR P-band status update: 13SEP2014

[Barry Clark]

I presume this means the power at the correlator, and therefore
after the level setting in the T304.  So it means a really low
power where the attenuators in the T304 can't compensate, or it
means a really screwed up bandpass (or birdies) in the 1 GHz
bandpass that the detector in the T304 sees.

On 09/17/2014 12:36 PM, Huib Intema (NRAO) wrote:
> Hi Paul,
>
> The P-value is the measured relative power with respect to the median.
> So 0.35 means that the measured antenna power is 35 percent of the
> median, or 65 percent lower than the median.
>
> In principle, healthy antennas with a moderately lower power level are
> not a problem in themselves. Over the commissioning period, I have seen
> that healthy antennas have P-values in the approximate range 0.8 - 1.2.
> I have also seen that antennas with P-values < 0.7 tend to have issues,
> like strong bandpass ripples, polarization issues, variable gains, or
> other issues. Antennas with P-values < 0.5 are almost all problem cases.
>
> -- Huib
>
>
>
> On 17-Sep-14 12:09, Paul Harden wrote:
>>
>> On 9/16/2014 2:38 PM, Huib Intema (NRAO) wrote:
>>
>>> * Antenna power levels (determined from gain calibration):
>>> Most antennas have similar (healthy) power levels, within ~30 percent of
>>> the median, except:
>>> - EA04 RCP: P = 0.05
>>> - EA04 LCP: P = 0.40
>>> - EA14 RCP: P = 0.35
>>> - EA16 RCP: P = 0.35
>>> - EA18 RCP: P = 0.45
>>> - EA19 RCP: P = 0.45
>>> (P = power level relative to median)
>>
>> I want to ensure I understand this correctly.  For example, on EA16
>> P=.35, this means the power is 35% lower than your mean/median?  That
>> would equate to about 4.5dB less than the median (probably marginally
>> OK - see below).  On EA04 P=.05 would be -13dB (clearly bad).
>>
>> Specs for most of the modules calls for LCP and RCP powers to be
>> within 2dB of each other.  I know the low band receiver output powers
>> never exceed 2dB with 1-1.5dB typical (when they leave the lab).
>> However, this 1-2dB LCP-RCP difference is also probably typical of the
>> T301 4/P Converter and the T302 LSC Converter.  If the LCP channels
>> (for example) in these modules are all low by 1-2dB, this could easily
>> accumulate to 4-5dB difference by the time it gets to the T304s ...
>> but shouldn't the T304 automatic leveling make up for that?  (WIDAR
>> path, not VLITE). Thus, I would think 4-5dB accumulative difference
>> might be the norm, and within the EVLA system specs.
>>
>> On some of the reported low antennas, Dan and I can check the power as
>> it trickles through the IF system (once it stops raining) to see where
>> this difference is originating, or if an accumulative effect.
>>
>> Bottom line: What would be the maximum power difference between LCP
>> and RCP in the system you'd like to see?  Then, we can try to figure
>> out how to equalize the powers if ~ 4dB difference is not acceptable.
>> Might require adding coaxial attenuators between modules to equalize
>> the power (at the expense of a few dB loss of sensitivity in one
>> channel).
>>
>> Paul
>>
>>
>>
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