[evlatests] L-band 'Set and Forget' Mode?

[Bryan Butler]

a table lookup is more than an 'inconvenience'.

i'd advocate rather trying to use sub-bands with no interference to do 
the 'set' part.

	-bryan


rperley at nrao.edu wrote, On 4/26/11 19:27 PM:
> Numerous reports attest to the fact that the L-band attenuators are often
> set to the wrong level, often by many dB.  The L-band data from my 3C273
> data taken a couple weeks ago gives support to the cause being the RFI
> environment.
>
> The data were taken in 16 subbands, each 64 MHz wide.  There is of course
> switched power available for each.  A review of the PDif and PSum for each
> of the 16 subbands is very interesting.
>
> For only about 6 of these subbands is there 'normal and correct' switched
> power -- the band between 1370 and 1500 MHz, and between 1800 and 2000
> MHz.  All the others show large disturbances, of varying nature.
>
> For most of these the variation in total power is typically a factor of 2.
>   For some of the subbands, it's clear the power fluctuations are
> quasi-random (clearly the case in the aeronautical bands), in others it's
> obvious the switched power is being greatly manipulated by pulsing (as in
> radars) emission.
>
> But the worst cases are those subbands covering the satellite transmission
> bands, where the range of power seen exceeds a factor of 50.  There are
> two of these, so even if we divide these by 16, the effective power range
> over the whole 1 GHz subbands is at least a factor of 6 or so (4 dB).
> Such variations are the rule, not the exception.
>
> The receivers themselves are far more stable than this.  It is both risky
> and foolish to use the current 'set and remember' algorithm for setting
> the T304 attenuators -- from the distribution of the output power one
> easily sees that errors of many dB are possible -- even likely.
>
> I advocate a 'set and forget' strategy, as in 'set it once, and remember
> it forever'.  We can establish, for each IF on each antenna, the typical
> power level on cold sky, and enter the appropriate T304 attenuator levels
> in a lookup table, which is occasionally updated when receivers are
> changed out, or other changes occur.  Although I realize that managing a
> table system is unpopular, I think the risks of the current strategy
> outweigh the inconveniences of a table lookup.
>
> Probably we'll need this at S-band also, given the digital satellite radio
> bands.
>
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