Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.naic.edu/~phil/talks/fast10/talk_2_tsys.html
Äàòà èçìåíåíèÿ: Wed Jul 28 21:15:06 2010
Äàòà èíäåêñèðîâàíèÿ: Sun Sep 12 01:34:43 2010
Êîäèðîâêà:
Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ï ï ï ï ï ï ï ï

fast2010_page2

Antenna performance: Tsys

Measure Tsys with a calibration diode.

Diode power injected via a directional coupler before the first amplifier.

For most receivers diode injected after polarizer.
For alfa receiver diode is injected before the polarizer (at the horn).

Turning the diode on and off gives Tsys in Units of the diode temperature.
Diode needs to be temperature stabilized.

All drifts in electronic gain can be removed with the cal diode (assuming the diode is stable).
Gain drift and correction Examples:

No cal correction:

total power a2010(alfalfa) showing electronic gain drift (.pdf)

telescope sitting still, sky is drifting by.
Each line is 1 days worth of data
Cal is sampled once every 10 minutes.
Dotted line is 5% change in Tsys.

total power with cal correction (.pdf)

Some variation occurs because sky temp can change a little.

Measure diode temperature using hot and cold loads

Hot load: absorber that fits over the mouth of the horn.

absorber should not have moisture/condensation on it.

Cold load: blank sky

Need cold sky temperature as well as the scattered radiation (from the ground) (or leakage thru the reflector in the case of FAST).
You could also use liquid nitrogen as the cold load

Stability of Tsys:

Tsys measurements for 2010 lband wide at 1405 Mhz (.pdf)
for cooled receivers you should also monitor dewar temperature.
Jumps in Tsys are often loose cables, switches going bad, etc..

Install transfer switches in electronics path so you can switch the path the polA, polB takes. This helps in debugging where the problem is.

Misc:

We have ground radiation leaking through the reflector panels giving a polarized component to Tsys (.1 to .5 degK). Panels are 1 by 2 meters. The long direction has more leakage. (More info)

More info:
    cals
    Dewar temperatures
    Tsys

next previous