Transmitter power
Aug 1, 2007 12:00 PM, BY TIM HOLT
Bird Technologies’ square-law-based diode meter and thermal power meter provide accurate measurement.
Broadcast network systems are planned and integrated, and predictions of coverage and cochannel interference are made based on several factors, including geographical terrain, antenna gain and directionality, and transmitter output power. The measurement of transmitter output power has always been an important consideration in the operation of broadcast transmission systems. However, new digital modulation formats necessitate rethinking the methods used to measure 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 article, we will review some of the characteristics of conventional measurement methodologies and develop a foundation for understanding new techniques.
Conventional techniques
Instruments used through the years for the measurement of transmitter output power can be categorized as follows:
- In-line power meters
These have been the most popular instruments, owing to their simplicity, ease of use and ability to measure both forward and reflected power. First-generation instruments of this class were developed in the 1950s and use simple point contact diode detectors. Within the past five years, versions have been developed using up-to-date diode devices and low-noise amplifiers, more appropriate for the measurement of signals incorporating complex modulation.
- Terminating power meters and their associated directional couplers
Also used extensively, power measurement techniques developed around these instruments are adaptations of power meters designed for laboratory use. They can provide high-quality measurements in broadcast applications when paired with the appropriate directional coupler.
- Radio frequency calorimeters
These provide measurements that truly represent heating power, as their definition would imply. These devices also provide the advantage of responding to the aggregate power presented to their input, as they are typically broadband devices.
One might argue that terminating-type laboratory power meters would also provide this advantage, in that these instruments are also typically broadband in nature, but they are limited to measuring low power levels and must be used with a directional coupler. These couplers are useful only over a relatively narrow band.
First-generation in-line power meters
These power meters are comprised of a short length of precision transmission line fitted with either a single or a dual directional coupler. The output of the directional coupler is typically 40dB to 60dB below the main transmission line level. The coupler output is connected to a simple diode detector and then scaled and displayed on a meter movement. (See Figure 1.)
Figure 1.
Most of these power meters measure the peak power of the signal while the meter scale is calibrated in average power. While this approach has served the broadcast industry for many years, the use of simple in-line power meters in complex modulated signal systems is limited by the inability of simple diode detectors to respond to signals with high peak to average power characteristics common to digital modulation formats.
Diode detectors in conventional in-line power meters are operated largely over the nonlinear portion of their dynamic range with their accompanying meter scales calibrated to read average power, even with the diode operating in a nonlinear fashion. This approach works fine, so long as the power meter is used to measure a single defined waveform or a closely related signal, such as FM or CW modulation.
In-line power meters with square-law detectors
This latest generation of in-line power meters is configured in much the same manner as the first-generation instruments, with the important difference in the detector technology. (See Figure 2.) An alternative approach is to operate detector diodes below -20dBm in an area known as the square-law region of the diode's dynamic range. This works well in systems carrying complex modulation. In the square-law region, diode detectors behave in much the same manner as thermal detection devices at low signal levels.
Figure 2.
The diode's rectified output is a function of the square of the root mean square input voltage. The transfer function for a full-wave square-law diode detector is about V
This relationship holds as long as the total excursion of the signal is contained within the diode's square-law region. The theoretical bounds for this range are from about -20dBm on the high side to the noise floor as determined by the bandwidth of the measurement at the lower end. Measurement ranges of 50dB are possible in most systems.
Terminating power meter and directional coupler
These wide frequency and dynamic range instruments, generally used for laboratory applications, may be used in conjunction with high-power directional couplers for making high-power measurements. (See Figure 3.) They may use either thermal converter technology or diode detector measurement approaches to power detection. They are generally more difficult to use, as they require frequent calibration and are more expensive than the above choices. Like the square-law-based instruments, they work well in cases of complex modulation, as they respond to the heating power of the signal.
Figure 3.
The error analysis of a typical implementation for this power measurement approach appears in Table 1. While the analysis is fairly self-explanatory, there are a few notable points:
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The accuracy of power meters in this class are dependent on many factors, one of which is the accuracy of the instrument's internal reference. Also, the internal reference should operate at a single frequency and power level.
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Operation of the power meter at frequencies other than the internal reference frequency requires the use of calibration offsets. These offsets carry their own uncertainties.
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The effects of mismatch uncertainty between the input to the power sensor and the output of the directional coupler are significant. Because the VSWR characteristics of the sensor input and the coupler output change with frequency, the magnitude of the mismatch uncertainty will also change with frequency.
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