Rimski korshakov

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DIGITAL TRANSMITTERS AND LINKS POWER AND SIGNAL LEVEL: MEASUREMENT TECHNIQUES

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frequent question and, at the same time, a common mistake made by technicians with a mostly analogbackground, concerns the appropriate digital measurement techniques of signal/power levels, both for terrestrial broadcasting (such as DVB-T) and for microwave

T e c h n i c aNoteo t e l TechnicalNoteN Technical

POWER MEASUREMENTS
The power of a digitally modulated radio frequency signal is its RMS value, and it is usually referred to as thermal power. An appropriate instrument for suchmeasurements is the bolometer: a thermal Power Meter. Although bolometers are extremely precise, they are not selective, and they combine all signals at their input, without considering their frequency andbandwidth. Normally, Power Meters have two disadvantages: the maximum power they can measure is around 100mW (+20dBm), whereas the minimum level they can measure, to obtain a quite precise result, isaround –30dBm. To accurately measure the output level of power equipment, an appropriately rated and calibrated attenuator should be inserted between the instrument and the transmitter. (Attenuatorsup to few kilowatts are available on the market.) A further explanation: most Power Meters are provided with a diode sensor (not a thermal detector). It is necessary, therefore, to have a “ThermalPower Meter,” so as to avoid significant measurement errors. Besides bolometers with an attenuator, other specific instruments can be used to measure digital transmitters’ power (e.g., Wattmetersspecified for digital modulations, calorimeters, etc.) that ensure a correct measurement of digitally modulated signals. Keep in mind that normal wattmeters, suitable for measuring analog signals, cannot beused: the error that can be easily made could be over 3dB, that is less than half, or more than double the real value!

SIGNAL LEVEL ANALYZERS

MEASUREMENT

WITH

SPECTRUM

When a spectrum...
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