DIGITAL TRANSMITTERS AND LINKS POWER AND SIGNAL LEVEL

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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 analog background, concerns the appropriate digital measurement techniques of signal/power levels, both for terrestrial broadcasting (such as DVB-T) and for microwave

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 such measurements 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 and bandwidth. 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, is around –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. (Attenuators up 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 “Thermal Power 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., Wattmeters specified 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 be used: 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

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SPECTRUM

When a spectrum analyzer is used to measure the level of a digitally modulated signal, some important aspects should be taken into consideration. First of all, when a radio frequency carrier is digitally modulated according to the most common schemes (QPSK, QAM, OFDM, just to name a few), it “distributes” its energy in the occupied frequency band (channel). The spectrum display will be similar to one of thermal noise (the spectrum of a digital modulation is, indeed, said to be “noise like”). Thus, the signal level displayed by a spectrum analyzer will vary according to the employed IF resolution filter (RBW – Resolution Band Width); in other words: if with a given digital signal and 10KHz Resolution Bandwidth the level displayed by the spectrum analyzer is –30dBm, with the same signal, but with 100KHz Resolution Bandwidth the level displayed will be –20dBm. In fact, the real signal level is neither –20dBm, nor –30dBm; the correct level could be measured only by employing an IF resolution filter, as broad as the frequency band (channel) occupied by the digital modulation.


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DIGITAL TRANSMITTERS AND LINKS POWER AND SIGNAL LEVEL by MonitoR magazine - Issuu