[evlatests] Solar EVLA Discussion today

[Rick Perley]

    All:

   Reminder:  Discussion of implementing solar mode capabilities on the 
EVLA, in Rm 317 at 1PM today.

    Agenda:

    a) Tim Bastian will review requirements for solar observing. 

     b) Q& A and Discussion of  
needs/capabilities/timescales/budgets/priorities.

All interested parties are welcome.  Circulate this reminder as needed.

*******************************************************

 Tim has provided the following background. 

  
  

Points of reference -

Memo 70 considers the impact of the quiet Sun, and active regions, and 
flares on the EVLA system. For the EVLA bands, the quiet Sun elevates 
Tsys by 18 dB (Q band) to 33 dB (L band). Loosely speaking, the presence 
of active regions adds another 5-6 dB. It is estimated that roughly 1% 
of flares observed will elevate Tsys by 50-60 dB; 5% by 40-50 dB; and 
25% by 30-40 dB.

Memo 72 sketches out a possible EVLA solar mode (suggested by Paul 
Lilie). It involves routing the quiet Sun signal through “an unmodified 
EVLA receiver” (LNA) while active Sun signals would be routed through a 
second path with a high-compression amplifier (HNA). A variable cal 
signal would need to be made available to both paths. Calibration of the 
HNA relative to the LNA path needed. Barry Clark suggests a possible 
approach.

Comment: Ideally, we want variable attenuation working in tandem with a 
variable cals, and for the two to operate independently for each 
polarization channel.

Additional refinement to specifications needed:

• The L band Tsys measurement needs to be updated at the dump time under 
some circumstances. I gather the default minimum dump time is 100 ms. My 
understanding is that by trading channels and/or baselines, it may be 
possible to reduce it to 10 ms.
• It is probably OK to make the Tsys measurement at 100 ms or even 1 s 
in the higher frequency bands although S band needs some additional 
consideration.
• Depending on the implementation details, it may be acceptable to limit 
the number of antennas with solar cals.

Open issues/questions

• Is a two path hi/lo scheme the best approach?
o If so, are the two paths needed at all bands?
o Are two paths in any case needed for all bands?
o How would the hi path be calibrated relative to the lo?
• A 20 dB attenuator has already been built into the LSC converter 
module; I gather it’s possible to do the same with the UK converter, but 
is it sufficient?
• A variable attenuator scheme is desireable – 5 dB steps?
• It appears feasible to add external cal modules a la the current VLA 
scheme; what about variable cals to keep up with variable Tsys (and 
variable stepped attenuators)?
• Are there significant complications in the M&C of variable attenuators 
and variable cals?
• By what feedback mechanism should control be imposed?