DIGITAL TRANSMITTERS AND LINKS POWER AND SIGNAL LEVEL

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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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.

The correct level can be approximately determined by making a simple calculation. Suppose you wish to measure a DVB-T digital terrestrial broadcasting signal (8MHz nominal bandwidth) with 100KHz RBW (Resolution filter) and to read a –20dBm level on the spectrum analyzer. In fact, the total bandwidth occupied by the DVB-T signal is 7.6MHz, that is 76 times the bandwidth of the resolution filter employed in reading the –20dBm level.

By converting the value 76 into decibel (dB = 10 x log10 76), the result is 18.8dB. This has to be added to the value measured, so as to obtain the total signal level, which is, therefore, -1.2dBm.

The IF 100KHz resolution bandwidth of the spectrum analyzer is obtained by using a filter whose amplitude/frequency characteristic is not rectangular, but more like a Gaussian curve, where 100KHz represents the –6dB bandwidth. Thus, a correction should be made, according to the shape of the IF filter of the spectrum analyzer. Moreover, the detector of the analyzer is not thermal and, as it was said about Power Meters, this will imply other errors.

In addition to the above mentioned aspects, there are the normal precision limits of a spectrum analyzer (inaccuracies related to amplitude/frequency response, IF bandwidth, logarithmic scale, calibration, etc.) that can result in errors of a few dB.

Fig. 1 – Spectrum of a OFDM signal (DVB-T) with 10KHz resolution filter (RBW) and an appropriate video filter, suitable to display the medium track.

© ABE Elettronica SpA 03/2005 ● All specifications contained in this document may be changed without prior notice.

Although it is closer to the real value, the normalized level is still approximate, since other factors must be kept in consideration.

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Fig. 2 – Spectrum of the same OFDM signal (DVB-T) of Fig. 1, measured with a 100KHz resolution filter (RBW): in comparison with the preceding figure, the level is 10dB higher (from –30dBm to –20dBm), not because the signal varied, but because the IF resolution bandwidth of the spectrum analyzer was different. By using a 7.6Mhz resolution band (as broad as the signal spectrum), the total signal power and, therefore, its level should be –1.2dBm. In fact, the level of the same signal, measured with a precise thermal power meter, turned out to be +0.5dBm: 1.7dB higher than the calculated value (see the text for further explanation).

According to the above illustrated points, a normal spectrum analyzer doesn’t ensure the precise level of a digitally modulated radio frequency signal. Though corrections can help normalize the IF broadband filter, the measurement will be approximate, since an error of a few decibels can occur.

© ABE Elettronica SpA 03/2005 ● All specifications contained in this document may be changed without prior notice.

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