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 metersThese 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 couplersAlso 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 calorimetersThese 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.)
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.
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 Vout=(Vin/5.77)2, where all voltages are in millivolts.
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.
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:
- 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.
- 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.
- 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.
RF calorimeters
These meters have formed the foundation for high-power measurements for many years. This power measurement method remains in use today as the means by which the National Institute of Standards and Technology (NIST) establishes primary RF measurement standards. As mentioned above, calorimetric systems measure the true heating power of a signal, including the fundamental frequency, all harmonics and sidebands, and other modulation related contributions.
The calorimeter measures the total aggregate power contained in the signal. It responds to heat and measures the heating power of a low frequency (50Hz or 60Hz) or DC energy in exactly the same manner in which the calorimeter responds to RF signals. This characteristic enables the calorimetric system to be highly accurate, as the low-frequency AC or DC energy used to calibrate the calorimeter may be known precisely.
This calibrating energy is also useful in the establishment of a path back to NIST primary standards. Typical field calorimetric system accuracy is ±4 percent, but accuracies of ±1 percent are possible using the substitution calibration methodology. Although calorimetric power measurement methods yield highly accurate results, calorimetric systems have limitations. These include:
- Calorimeters are generally difficult to use. This is especially true in field settings, with typically uncontrolled environments.
- Best results with calorimetric methods are obtained with highly trained operators.
- Calorimeters are terminating devices and are not suitable for directional power measurements leading to antenna match measurements.
A diagram shown by a typical calorimetric system is described in Figure 4. In this system, a water-cooled, high-power RF termination is used as a means to convert radio frequency energy into heat, with the constraint that this must be done in a highly efficient manner so as to capture the majority of the energy dissipated in the load.
Load efficiency is also important for proper calibration, as the heat flux from the load in areas other than the coolant path cannot be easily captured and will also behave as a function of the ambient temperature. In other words, if the calorimeter is calibrated at 25°C and the ambient temperature changes to 15°C, this additional gradient will result in more heat escaping from the load in areas other than the coolant path. This will shift the calibration point of the calorimeter.
Such a calorimeter must also be able to measure the mass flow rate. While spinning fan-type flow meters have been used in field calorimeter instruments, more precise turbine-type instruments are available.
Finally, the system contains two temperature-sensing elements, one placed at the input to the RF load and the other placed at the output. Most modern systems use thermocouples or thermistors because of their improved accuracy and repeatability.
Calorimetric systems measure power in accordance with the following equation: Power(kW) = 0.263 × ΔT × Flow, where temperature measurements are in degrees centigrade, and the flow rate is in gallons per minute. While this formula will provide an indication of the power dissipated in the load, it is necessary in most cases to compensate for the physical changes to the coolant used in the system, both in terms of changes due to temperature, as well as coolant mixtures such as ethylene glycol and water.
For example, the specific heat of pure water has a value of 1.0 at a temperature of 15°C, but this value drops to 0.998 at a temperature of 35°C. Modern calorimetric instruments will automatically compensate for these changes.
The measurement process
As mentioned above, one important attribute of the calorimetric system is that the system will respond essentially the same for DC or low-frequency AC energy as for RF energy. This “substitution” calibration procedure may be characterized as follows:
- Low-frequency power referenceThis reference measures the actual power used for calibration. Low-frequency energy is used for calibration, so inexpensive, highly accurate instruments are available. Inexpensive digital multimeters, are typically accurate to within ±1 percent for low-frequency voltage and current measurements.
- Low-frequency sourceIn many cases, 60Hz energy may be used. A primary consideration is the stability of the energy source.
- Perform calibrationThe calibration should be performed at or near the power level where the RF measurement will be made in order to avoid linearity errors. Connect the low-frequency source to the calorimeter, along with the reference standard, and calibrate.
- Perform substitutionConnect the RF source to be measured to the calorimeter in place of the low-frequency source, and perform the measurement.
Digital modulation
The measurement of RF power in digitally modulated signals presents a challenge due to high peak to average power ratios (crest factor) found in 8VSB, COFDM and similar signals.
In general, the average power of signals using complex modulation is constant, whereas the peak power is data dependent. In practice, crest factor values of 7dB are typical for these systems, with crest factor values as high as 12dB, especially in multiple carrier settings. Conventional diode detector power meters, being peak reading instruments, tend to follow the envelope established by the peak power value of the signal.
Conclusion
While there are several ways to measure transmitter output power, a best choice often comes down to a trade between cost and accuracy. Few broadcasters need a laboratory-grade calorimeter to adjust the output power of their DTV transmitter. Likewise, that 25-year in-line power meter that has served well on an analog transmitter may not be the best choice when it comes to measuring today's 8-VSB signal.
The bottom line is that a square-law-based diode power meter and the thermal power meter/directional coupler combination can accurately measure 8-VSB transmitter power. In fact, when properly calibrated, these devices provide accuracy that approaches the more complex (and expensive) calorimetric power measurement.
Tim Holt is director of applications and systems engineering for Bird Technologies Group.
