Calian SatService sat-nms Manuel de l'utilisateur - Page 14

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5 Theory of Operation

The sat-nms PowerSensor consists of the following four major parts:

In the analog input section a microwave detector provides a DC voltage which is depending on the input

power range either proportional to the input power or proportional to the input voltage of the input power at

the power sensor. The relationship between input power level and the DC voltage to be measured in the last

instance by the analog to digital converter is not necessarily to be known exactly, as this is measured and

calibrated during the in-factory calibration of each individual sensor.

The DC voltage coming from microwave power detector gets amplified in an operational amplifier. Via a

switchable resistor network the gain of the amplifier is controlled by the microcontroller, which allows a so

called high and low sensitivity mode. The power sensor software will control this switch and either set the

gain to a fixed high or low-sensitivity value or in 'auto' mode take full control about the sensitivity control

and depending on the analog to digital converter (ADC) output value switch the sensitivity automatically.

At the output of the operational amplifier the DC voltage which is a measure of the RF input power is

provided as input value to the analog to digital converter. The sat-nms PowerSensor uses a 16 bit ADC, this

provides enough resolution to always guarantee the 0,01dB resolution in power level output via the web

server.

The complete power sensor is operated by a 24V DC voltage source. This has the advantage that the already

existing 24V supplies in a satellite ground station or an M&C System can be used to provide the supply

voltage of the power sensor. The 24V supply voltage is routed via a EMV coupled inductor and a

transsorber diode which switches off at voltages greater than 36V. This together with a SMD fuse protects

the power sensor from to high input voltages or EMV disturbances introduced via the 24V supply line.

After this input and safety section a DC/DC converter provides the 5V DC voltage necessary for the

microprocessor and digital part of the power sensor. Parallel to this a linear voltage regulator provides a so

called analogue 5V which is independent from the microprocessors digital 5V supply and therefore more

stable and less noise which is better for the analog to digital converter and its analog input section.

Beside the detector diode and the RF matching circuitry the RF sensor module contains a temperature

sensor. The temperature registered by this sensor is used by software in the M&C module to perform the

temperature compensation of the measured power.

5.1 Linearity and Temperature Compensation

The microprocessor module measures the RF power by means of the A/D converter, measures the

temperature and controls the input amplifier sensivity. Using this data, the software calculates the true RF

input power by interpolating in calibration tables.

During the in house commissioning each power sensor individually gets calibrated at several temperatures

over the whole dynamic range. The calibration tables derived from this are stored on the power sensor.

During operation, the power sensor interpolates in these tables to evaluate the true measured power from the

raw ADC value and the temperature reading.

www.satnms.com

PS-UM-2002 Page 13/16