Home Networking Basis: Transmission Environments and Wired/Wireless Protocols

4.3.1 FCC Regulation

The emission from electrical in-house wiring caused by a home networking system also needs to be below the limits set forth by the Office of Engineering Technology (OET) under the Federal Communications Commission. There are some specific definitions about using the electrical wiring for the purpose of transmission either over the wiring or through the air. Definitions for carrier current system, incidental radiator, intentional radiator, and unintentional radiator can be found in the text of Code of Federal Regulation title 47, Section 15.3 [7] and are of interest to anyone planning to implement an in-house electrical wiring based home networking system.

According to Section 15.3, a carrier current system transmits radio frequency energy by conduction over the electric power lines. A carrier current system can be designed such that the signals are received by conduction directly from connection to the electric power lines (unintentional radiator) or reception through the air as a result of radiation of the radio frequency signals from the electric power lines (intentional radiator). An in-house wiring-based home networking system fits the definition of a carrier current system.

An incidental radiator is a device that generates radio frequency energy during the course of its operation although the device is not intentionally designed to generate or emit radio frequency energy. Examples of incidental radiators are DC motors and mechanical light switches. An intentional radiator is a device that intentionally generates and emits radio frequency energy by radiation or induction. An unintentional radiator is a device that intentionally generates radio frequency energy for use within the device, or that sends radio frequency signals by conduction to associated equipment via connecting wiring, but it is not intended to emit RF energy by radiation or induction. An in-house wiring-based home networking system also fits the definition of an unintentional radiator.

With these terms defined, the related FCC regulation in Section 15.109(e) says that carrier current systems used as unintentional radiators or unintentional radiators that are designed to conduct their radio frequency emissions via connecting wires or cables and that operate in the frequency range of 9 kHz to 30 MHz shall comply with the radiated emission limits for intentional radiators provided in Section 15.209 for the frequency range of 9 kHz to 30 MHz. The related radiated emission limit provided in Section 15.209 is shown in Table 4.5. This same limit has been applied to the emission from twisted pair cables.

4.3.2 Emission Measurement

The measurement standards are also recommended by the text of Code of Federal Regulation title 47, Section 15.31. It recommends a procedure defined in American National Standards Institute (ANSI) C63.4-1992 [8]. Some technical details such as the use of a quasi-peak detector or a spectrum analyzer are also specified in CISPR 16-1 [9]. Care should be taken when using a spectrum analyzer such that signal saturation should be avoided, a bandwidth defined at 6-dB corner points should be observed, and leakage should be prevented. Section 15.31(d) further states that field strength measurements shall be made, to the extent possible, on an open field site. Test sites other than open field sites may be employed if they are properly calibrated so that the measurement results correspond to what would be obtained from an open field site. In the case of equipment for which measurements can be performed only at the installation site, such as a carrier current system, measurements for verification or for obtaining a grant of equipment authorization shall be performed at a minimum of three installations that can be demonstrated to be representative of typical installation sites.

Section 15.31(f) also discusses those certain circumstances where the measurement distance can be less than 30 m. It says that at frequencies below 30 MHz, measurements may be performed at a distance closer than that specified in the regulations; however, an attempt should be made to avoid making measurements in the near field. Pending the development of an appropriate measurement procedure for measurements performed below 30 MHz, when performing measurements at a distance closer than that specified, the results shall be extrapolated to the specified distance either by making measurements at a minimum of two distances on at least one radial to determine the proper extrapolation factor or by using the square of an inverse linear distance extrapolation factor (40 dB/decade).

Following these guidelines, the measurement of leakage emission from a power line based transmission system can be summarized with the aid of Figure 4.22. We first assume that the power lines are wired around the perimeters of a house and that it is likely that a transmitter of a home networking system can be connected very close to the perimeters of the house. Based on these assumptions, the emission pick-up antenna should be located in an open field and 30 m away from the house. Different types of antennas can be used to pick up the emission in general. However, the FCC requires a loop antenna. Effects of a particular antenna resulting from its orientation and attenuation should be compensated by including the antenna factor provided by antenna suppliers. When dealing with a broadband emission, a 9-kHz bandwidth filter whose corner frequencies are defined as being 6 dB off the passband should be used before the calculation of emission strength. A peak detector can be used to calculate emission strength for estimation purposes. A quasi-peak detector is recommended by these regulations. A quasi-peak detector weighs signals according to their repetition rate, which is a way of measuring their annoyance factor. For a continuous wave, the peak and quasi-peak are the same. For a random Gaussian signal from an advanced home network transmission system, we usually measure the signal strength on the power line with Power Spectrum Density. The peak of a Gaussian signal can be as high as five to six times that of the voltage level corresponding to the power measurement. However, very high peaks occur rarely, and they would probably never be caught by the measurement device. For 96% of the time, peaks of a Gaussian signal would not exceed twice the voltage level corresponding to the power measurement. The readings of a quasi-peak detector depend on the signal characteristics of a particular home networking system and the circuit specifications of the detector manufacturer.

Equipment manufacturers, such as Agilent, have measurement devices designed according to these regulations with built-in qualified quasi-detectors. In fact, the use of some special purpose electromagnetic interference (EMI) analyzers can simplify not only the rule interpretation process but also the real-time measuring procedure. Figure 4.23 shows an example using Agilent E7400A for the emission measurement. This EMI analyzer has built-in filters and detectors according to related regulations.

Emissions are caused by the home networking transmission system. We can use a prototype transceiver and connect it to the power line as a source for emission measurement. Alternatively, we can use a continuous wave (CW) signal generator or sweeper as a source of emission. These signal generators usually have only a 50-ohm grounded output interface. A Line Impedance Stabilization Network (LISN) is normally used to connect a conventional signal generator to the power line. For accuracy, the PSD on the power line induced by a signal source also needs to be measured and extrapolated in accordance with the intended transmit signal strength.

The emission limit of 30 dBµV/meter needs to be adjusted for the Antenna Factor and filter attenuations according to the following expression:

Equation 4.31

where V_dB is measured in decibel microvolts (dBµV), AF is the antenna factor measured in decibels per meter (dB/m), and Loss is for filter and cable attenuations measured in decibels. This calibration process is usually automatically included in these special-purpose EMI measurement devices.

4.3.3 Power Spectrum Density

Once the home networking induced electromagnetic field strength is measured and compared with those of FCC limits, the allowed Power Spectrum Density can be estimated. The effect of the field strength caused by the in-house electrical wiring is similar to that caused by an antenna. The effect of an antenna is called the Antenna Factor (AF), and the effect of in-house electrical wiring is called the Coupling Factor (CF). Based on the CF, we can estimate the allowed PSD. The PSD level is usually expressed in decibel millwatts per hertz (dBm/Hz), while the field strength is expressed in decibel microvolts per meter (dBµV/m). We need to find some conversion factors to relate these different terms. We first find the relationship between power (dBm) and voltage (dBµV). For a terminal impedance of 50 ohms, the equal power conversion factor between terms of dBm and dBµV is expressed as

Equation 4.32

In the conventional case, an antenna converts the applied voltage to a corresponding field strength. We define this voltage-to-field strength conversion as the AF, or the CF in a more general sense, which applies to the field strength caused by voltage on in-house electrical wiring. We further define the 0 dB AF or CF as a 0 dBµV voltage to a 0 dBµV/m field strength conversion. Now we can relate a power on the in-house electrical wiring to the field strength using

Equation 4.33

where CF is the Coupling Factor. Since the FCC limits are defined for a bandwidth of 9 kHz, the PSD can be related to the field strength using

Equation 4.34

By imposing a field strength of 30 dBµV/m, the allowed PSD can be estimated according to

Equation 4.35

Some field measurements have shown that CFs are in the range of between 65 and 45 dB at a distance of about 30 m. Therefore, for the worst case, the PSD is limited at 72 dBm/Hz.