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.
RF calorimeters
Table 1. Click to enlarge.
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:
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Calorimeters are generally difficult to use. This is especially true in field settings, with typically uncontrolled environments.
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Best results with calorimetric methods are obtained with highly trained operators.
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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.
Figure 4.
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 reference
This 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 source
In many cases, 60Hz energy may be used. A primary consideration is the stability of the energy source.
- Perform calibration
The 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 substitution
Connect 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.
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