Introduction

Fig. 1. shows a typical modern communications signal chain. The signal to be transmitted is generated at baseband by a digital-to-analog converter (DAC). This signal is then mixed up to an intermediate frequency (IF) where it is filtered before being mixed up to the final radio frequency (RF). On the receive side the inverse transformation takes place. The received signal is amplified, mixed down to IF and filtered before being mixed down to baseband where it is digitized.

While signal chains such as these come in all shapes and sizes, there is a universal need in communications applications for measurement and control of signal power. On the transmit side we must ensure that the power amplifier (PA) is meeting regulatory emissions requirements. We must also ensure that the PA is not transmitting above some power level that will cause it damage by overheating.

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