Page 88 - Complete Wireless Design

Modulation

Modulation 87

Figure 2.41 The RF2703 IC in demodulator configuration with support components.

2.6.2 Common digital tests and measurements
Measuring digital signal power. Accurately measuring the power of wideband
digital signals can be much more difficult than measuring the output power of
most analog signals.
Normal analog CW wattmeters can be very inaccurate when utilized to mea-
sure the power of digitally modulated signals, which can have a peak-to-aver-
age power ratio of 10 dB or more. Most analog modulation power meters were
not calibrated to measure these types of wideband signals.
As the symbol rate of a digital signal increases, the bandwidth also increas-
es, so the power in a digital signal is stretched across a very wide frequency
range, and not localized around the center frequency of the carrier as in AM or
FM modulation. This means that a single power measurement, at a single
location within the digital channel, taken with a spectrum analyzer will give
a deceptively low power output measurement. This is because the spectrum
analyzer’s maximum resolution bandwidth (RBW), its internal IF filtering, is
actually narrower than the digital signal’s own total bandwidth. Any accurate
method of measuring the output power on a spectrum analyzer would involve
taking many discrete average power measurements of the digital signal over

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