Difference between revisions of "System Gain Calibration"

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(Calibrating Front End Power Detectors)
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Revision as of 15:54, 22 September 2016

Gain Control "Knobs"

EOVSA Gain Control "Knobs"
Non-solar radio interferometers can make the assumption that the system noise is dominated by the relatively uniform sky, but this is not at all valid for the Sun--the Sun dominates the system noise, and can be highly variable, especially during flares and other radio outbursts. This is a main reason why it is necessary to design solar-dedicated instruments for observing the Sun. In order to cope with the high and variable noise from the Sun, EOVSA is equipped with a series of attenuators, two RF attenuators in the frontend, and an IF attenuator in the analog downconverter. In addition, it is possible to change the gain via parameters (ADC Attenuation, FFT Shift, and Equalizer Coefficients) in the digital correlator. The table above lists the various gain control points, their purpose, and other relevant information.

Calibrating Front End Power Detectors

There are power detectors in each of the two channels in each front end, just before the optical link, which measure a voltage proportional to the RF power level (integrated over the full 2.5-18 GHz range). To convert these voltage to power measurements, in dBm (decibel-milliwatts), the input of each front end is terminated with a room temperature 50-ohm load, and the output just before the optical link is connected to the E4418B power meter. There is a LabVIEW vi called "E4418B Measurement.vi," to be run on the Win1 computer, that steps the attenuation and performs the measurements both with and without the ND turned on, to range over a wide range of voltages and powers. The vi then writes the result into two text files named, for example,

Antenna 8 HPOL 412016 193307UT.txt
Antenna 8 VPOL 412016 193307UT.txt

There are many more measurements in the text files than needed, and there is no close synchronization between the switched state of the attenuators and the measurement, so one must then edit the file to remove all measurements except one for a given state. Here is an example of an edited file:

Figure 1: Example plot from fem_cal.py showing the voltage to power curves (symbols) for the H and V channels of Antenna 8 front end, overplotted with 4th-degree polynomial fits.
HPOWER ND HATTN1 HATTN2 HVOLT VATTN1 VATTN2 VVOLT
10.629 1.000 0.000 0.000 2.280 0.000 0.000 3.406
9.774 1.000 1.000 0.000 1.917 1.000 0.000 2.981
8.982 1.000 2.000 0.000 1.624 2.000 0.000 2.620
7.063 1.000 4.000 0.000 1.086 4.000 0.000 1.880
3.231 1.000 8.000 0.000 0.576 8.000 0.000 0.879
-4.953 1.000 16.000 0.000 0.173 16.000 0.000 0.271
9.791 1.000 0.000 1.000 1.924 0.000 1.000 2.998
8.960 1.000 0.000 2.000 1.616 0.000 2.000 2.617
7.044 1.000 0.000 4.000 1.082 0.000 4.000 1.868
3.160 1.000 0.000 8.000 0.571 0.000 8.000 0.876
-5.139 1.000 0.000 16.000 0.168 0.000 16.000 0.266
6.886 0.000 0.000 0.000 1.045 0.000 0.000 1.841
5.813 0.000 1.000 0.000 0.859 1.000 0.000 1.487
4.881 0.000 2.000 0.000 0.737 2.000 0.000 1.223
2.777 0.000 4.000 0.000 0.540 4.000 0.000 0.825
-1.246 0.000 8.000 0.000 0.303 8.000 0.000 0.457
-9.454 0.000 16.000 0.000 0.081 16.000 0.000 0.137
5.837 0.000 0.000 1.000 0.862 0.000 1.000 1.501
4.861 0.000 0.000 2.000 0.735 0.000 2.000 1.221
2.752 0.000 0.000 4.000 0.537 0.000 4.000 0.820
-1.343 0.000 0.000 8.000 0.300 0.000 8.000 0.452
-9.886 0.000 0.000 16.000 0.076 0.000 16.000 0.127

When the edited files are ready, the Python script fem_cal.py is run to create the plot shown in Figure 1 for antenna 8, which includes the measured points and a 4th-degree polynomial fits. The parameters of the fit are printed to the terminal, which for the example in Figure 1 are:

HPOL.c0 =  6.6138626
HPOL.c1 =  5.6355898
HPOL.c2 = -1.0031312
HPOL.c3 = -0.1882171
HPOL.c4 =  0.0348016
VPOL.c0 =  5.5092565
VPOL.c1 =  5.4037776
VPOL.c2 = -0.9535324
VPOL.c3 =  0.3611041
VPOL.c4 =  0.2671788

These lines are then entered into the corresponding crio.ini file, which is located in the crio's /ni-rt/startup folder. The new values will take effect on the next reboot of the crio, or in response to a sync command.

Gain Calibration Procedure