System Gain Calibration

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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 Voltage Detectors

Figure 1:
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 a power meter. Then the attenuation is stepped both with and without the ND turned on, to range over a wide range of voltages and powers. Both voltage and power are measured in this lab setting, and the measurements, shown in Figure 1 for antenna 8, are fitted with a 4th-degree polynomial using the Python script fem_cal.py. The parameters of the fit are printed to the terminal, which for the example in Figure 1 is:

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 would be 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