The concept Provide a small (2 K) difference signal to allow continuous receiver gain measurement to...
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Transcript of The concept Provide a small (2 K) difference signal to allow continuous receiver gain measurement to...
The concept
Provide a small (2 K) difference signal to allow continuous receiver gain measurement to 1%
Alternate between three loads: 300K, 400K, sky (~2% coupling to beam)
50ms switching time (3 or 6 Hz cycles on BIMA) – faster than sky variations
Broadband system – use with all receivers S/N of 300 in 10 s (continuum)
Other pieces of the puzzle Knowledge of the antenna gain (measure once) Real-time measurement of the atmospheric opacity
System overview
A stable calibration signalAntenna 6, 221 GHz, 2002 Jun 28 (00:00 UT)
-1
0
1
2
3
4
5
6
7
8
0 50 100 150 200 250 300
Time (min)
Vo
ltag
e
total power on calwheel
total power on sky
difference signal on sky (x 80)
difference signal on calwheel (x 80)
Measuring the coupling coefficient
Liquid nitrogen and ambient loads in large foam boxes (difficult to hold in place)
automatic box placement under computer control
Integrate “chopped” signal for several minutes (S/N)
At 1mm – more stable receiver
Measuring the coupling coefficient
box at 5° angle to beam; thus reflections (1%) are to ambient in cabin
ambient
LN2
Coupling versus frequency
1.0%
1.2%
1.4%
1.6%
1.8%
2.0%
2.2%
2.4%
220.5 221 221.5 222 222.5 223 223.5
Frequency (GHz)
Cou
plin
g co
effic
ient 20.1
20
19.9
19.8
19.7
19.6
19.5
19.4
focus position
Coupling vs focus (some frequencies in the band)
1.0%
1.2%
1.4%
1.6%
1.8%
2.0%
2.2%
2.4%
19.3 19.4 19.5 19.6 19.7 19.8 19.9 20 20.1 20.2
Focus position (mm)
Cou
plin
g co
effic
ient
periodic with focus standing wave of some kind thermal effects will dominate!