Transmitter Output Power Measurements In Digital Broadcast Systems

By Tim Holt, Bird

Introduction

The measurement of transmitter output power has always been an important consideration in the operation of broadcast transmission systems. As broadcast network systems are planned and integrated, coverage predictions, as well as the prediction of the possibility of co-channel interference are based upon several factors, including geographical terrain, antenna gain and directionality, and transmitter output power. The introduction of new digital modulation formats into broadcast settings has necessitated a re-thinking of the methods that are used for the measurement of transmitter power. The accuracy and reliability in which these measurements may be made is related to our understanding of the limitations of conventional power measurement methods, as well as to our understanding of the proven techniques that have been developed for use with digital broadcast systems. In this paper, we will review some of the characteristics of conventional measurement methodologies, as well as to develop a foundation of understanding of newer techniques. We will also discuss “first principle” power measurement methods, and present data as to how various measurement methodologies compare with measurements developed using first principle techniques.

Review of Conventional Transmitter Power Measurement Techniques - Instruments used through the years for the measurement of transmitter output power may be categorized as follows:

• In-line or Thruline Instruments
• Terminating Power Meter / Directional
• Coupler Based Instruments
• Radio Frequency Calorimeters

Of the above categories, in-line power meters have been the most popular instruments, owing to their simplicity, ease of use, and their ability to measure both forward and reflected power. First generation instruments of this class were developed in the 1950’s, uses simple point contact diode detectors. Within the past five years, versions of these instruments have been developed using up to date diode devices and low noise amplifiers, more appropriate for the measurement of signals incorporating complex modulation.

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