1.2 Why Measure Power?

The first question is- why measure power at all, rather than voltage? While it is true that very accurate and traceable voltage measurements can be performed at DC, this becomes more difficult with AC. At audio and low RF frequencies (below about 10 MHz), it can be practical to individually measure the current and voltage of a signal. As frequency increases, this becomes more difficult, and a power measurement is a simpler and more accurate method of measuring a signal’s amplitude.

As RF signals approach microwave frequencies, the propagation wavelength in conductors becomes much smaller, and signal reflections, standing waves, and impedance mismatch can all become very significant error sources. A properly designed power detector can minimize these effects and allow accurate, repeatable amplitude measurements. For these reasons, power has been adopted as the primary amplitude measurement quantity of any RF or microwave signal.

There are many reasons it may be necessary to measure RF power. The most common needs are for proof-of-design, regulatory, safety, system efficiency, and component protection purposes, but there are thousands of unique applications for which RF power measurement is required or helpful.

In the communication and wireless industries, there are usually a number of regulatory specifications that must be met by any transmitting device, and maximum transmitted power is almost always near the top of the list. The FCC and other regulatory agencies responsible for wireless transmissions place strict limits on how much power may be radiated in specific bands to ensure that devices do not cause unacceptable interference to others. Although the real need is usually to limit the actual radiated energy, the more common and practical regulatory requirement is to specify the maximum power which may be delivered to the transmitting antenna.

Figure 1.2.1 Transmission interference

In addition to the regulatory issues, transmitter power needs to be limited in many communication systems to allow optimum use of wireless spectral and geographical space. If two transmitting devices are operating in the same frequency band and physical proximity, receivers can have a more difficult time discriminating the signals if one signal is much too large relative to the other. Even in commercial broadcast, the transmitting power of each broadcast site is licensed and must be constantly monitored to ensure that operators do not interfere with other stations occupying the same or nearby frequencies in neighboring cities.

Controlling transmission power is particularly necessary in modern cellular networks, where operators constantly strive to maximize system capacity and throughput. Many modern wireless protocols use some form of multiplexing, in which multiple mobile transmitters (ex: cellular handsets) must simultaneously transmit data to a common base station. In these situations, it is necessary to carefully monitor and control the transmitted power of each device so that their signals arrive at the base station with approximately equal amplitudes. If one device on a channel has too much power, it will “step on” the transmission of other devices sharing that channel, and make it impossible for the base station to decode those signals.

Another power control issue in cellular systems is due to the close proximity of base stations in congested areas. If a device is transmitting with too much power, it will not only interfere with signals in its own cell, but can possibly interfere with the transmissions of devices in neighboring cells. Mobile devices for these systems typically implement both open-loop and closed-loop, real-time power control of their transmitters. Without accurate power control of every single device within range of a base station, cellular network capacity can be severely degraded.

Too much power has other dangers as well. For higher power systems, too much RF power can present biological hazards to personnel and animals. Safety limits are often placed on transmitted power to protect users and bystanders from the dangers of high-power RF radiation. A good example of the potential dangers of RF energy is a common microwave oven, which can severely burn human flesh just as easily as it can heat a meal. Radio and RADAR transmitters operate at still higher power levels, and present their own special hazards. It is hypothesized that even low-power RF transmitting devices such as cell phones may have potential to cause lasting biological effects. In all of these cases, there will be times when the actual power present must be monitored to ensure compliance with safety standards or guidelines.

Measuring power is important for circuit designers as well. Any electronic device can be overloaded or damaged by too high a signal. Too much steady-state power can cause heating effects and destroy passive and active components alike. Too much instantaneous (“peak”) power can overstress semiconductor devices, or cause dielectric breakdown or arcing in passive components, connectors, and cables.

But even at power levels well below the damage threshold of the circuit components, excessive power can cause overload of system, clipping, distortion, data loss or a number of other adverse effects. Similarly, insufficient power can cause a signal to fall below the noise floor of a transmission system, again resulting in signal degradation or loss.

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