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Calibration

The original version of the SRT was shipped with a motorized vane calibration system. However, the current version includes only a noise calibrator.


To use the electronic noise calibrator you need:

1. The latest java code from the SRT Software Download page.

2. Add the keyword NOISECAL followed by the noisecal's temperature (around 1000) to the SRT.CAT file.

3. The latest stamp code in ground control MTRNEW4.BAS

4. A noise source unit constructed and mounted at vertex of the dish.

5. Connect the electronic noise source to the same cable as currently used for the vane calibrator motor.
The correct polarity is to connect the wire from pin 3 to the +ve terminal of the noise circuit (this goes through a 1k ohm resistor to the cathode of the noise diode) and the wire from pin 6 to the -ve terminal of the electronic noise source.
To use both motorized vane calibrator and the electronic calibrator:
It is is possible to have both the electronic noise and the vane calibrator connected in parallel so they can both be used. However, if both are connected, the voltage applied to the noise source (and hence the current through the noise diode) will depend on the motor current, as there is a 200 ohm series resistor in the ground control.

There are several solutions to this problem: the 200 ohm resistor can be moved so that it is only in series with the motor or another 200 ohm resistor can be put in series with the motor and the resistor in series with the noise diode can be reduced to to give the 5-10 ma current through the noise diode at the reduced voltage due to the load of the motor. (In this case it would be good to add a zener diode or other voltage regulator to make the current though the noise diode largely independent of the external applied voltage)
[we leave this as an exercise for the student.]

With both the motorized vane and the electronic noise - a positive voltage on pin 3 relative to pin 6 turns on the electronic noise and moves the vane out of the way. A negative voltage on pin 3 relative to pin 6 moves the vane over the feed.
To calibrate the electronic noise source. Several methods are possible:
a) Use manual or motorized vane calibration and adjust the noisecal temperature to give the same system temperature.

b) Measure the system temperature by comparing the noise power with the antenna pointed at the ground or a large absorber and make the noisecal temperature consistent with this measurement.

c) Use a flux density measurement on the Sun from another instrument to place a temperature scale on the radiometer and transfer this scale to the noisecal to get its equivalent temperature.

d) Use a manual vane calibration - which is supported by the software by placing the keyword MANCAL 1 in the srt.cat file. When you click on Vane you will be given instructions to place a load in front of the feed.

 

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