Comms rides power lines via optical AFE

A new approach to the analog front end used in broadband-over-power-line (BPL) applications is built on optical coupling technology, which is designed to significantly improve power efficiency, system robustness and overall solution cost.

Designers intended that BPL networking technologies provide networking and Internet access at DSL or cable modem speeds over electrical-power wiring. "Home BPL" operates over the 120/240-Vac low-voltage distribution wires, while "Access BPL" runs on the medium-voltage (1- to 40-kV) power distribution network over longer distances. BPL carrier frequencies are in the 1-MHz to 80-MHz high-frequency range.

While digital signal-processing techniques have significantly improved communication performance, the problem of how to couple the analog-modulated signals from a power-line modem (PLM) to and from electrical wiring has not seen any major developments. The analog signal-processing circuitry, together with other components required to couple the modulated signal on and off the power line, is commonly given the collective name of analog front end.

The AFE is called upon to provide voltage and current signal amplification. It must also provide galvanic isolation to protect the user and equipment being interfaced from both the nominal 120/240 volts and the transient voltages that are common to power wiring. It is traditionally a complex circuit combining many discrete components with bulky isolation transformers.

In a typical power-line communication modem for use in home-automation applications, the modulated signal is generated with a digital lookup table and a D/A converter. Demodulation is also carried out digitally, by first converting the received analog signal into the digital domain with an A/D converter and then by applying subsequent digital processing. These two functions are typically integrated into a single modulator/ demodulator (modem) IC fabricated using a low-voltage CMOS process.

The use of CMOS makes it possible to optimize die size, cost and power dissipation. However, it makes the integration of analog circuitry problematic; generally some, if not all, of the analog signal-processing circuitry must be provided externally. Although the modulator/demodulator topology used in the digital IC has a significant impact on overall communication performance, it is fair to say that in many cases the performance of the AFE has an equal, if not greater, influence on the overall performance of the power-line modem.

Optically coupled AFEs

The traditional method used in power-line modems to provide safe insulation is an appropriately constructed and certified isolation transformer for coupling the modulated signal to and from the power line. A transformer can inherently propagate signals in either direction, while optical coupling methods use a unidirectional combination of optical emitter and detector, and can only propagate signals in one direction. To achieve bidirectional communication in an optical AFE, one transformer has to be replaced with two optical channels.

Packaged inside an integrated optical PLM AFE are four discrete semiconductor elements: two LEDs and two BiCMOS ICs (IC1 and IC2), ground-referenced to the low-voltage circuitry and the power line, respectively.

With respect to the transmit optical path, IC1 includes a transconductance amplifier, which drives LED1 with a forward current equal to the sum of a dc biasing current and a current that is directly proportional to the analog input signal. The type of LED used has a very linear current-to-light transfer characteristic to minimize harmonic distortion.

On the power-line side of the isolation boundary, a photodiode and photodetector amplifier are integrated into IC2. The circuit converts captured photons to an output voltage that is linearly related to the light emission of LED1.

The output of the photodetector amplifier is coupled to the output stage via an external ac coupling circuit that removes residual dc offset voltage from the output of the photodetector amplifier. This coupling circuit may also include filtering or frequency shaping to remove out-of-band harmonics generated in the modulator or optical channel.

The receiver optical path is similar to the transmitter path, with the optical emitter and detector functions integrated in IC2 and IC1, respectively.

In addition to the basic function of coupling transmitted and received signals, other important features are integrated into the optical AFE. The first is the implementation of a highly linear line driver circuit, which is capable of delivering up to 1 amp peak to peak. This high-current capability ensures that an adequate transmit signal level is coupled onto the power line even in the case of very low power-line impedance. The low-distortion specification ensures that the generation of out-of-band harmonics is minimized.

The optical AFE also includes an additional gain stage in the receive function, which may be used to amplify attenuated signals, ensuring optimal link integrity even in worst-case signaling environments.

The optical AFE incorporates integrated control functions. To ensure that the line driver does not unnecessarily attenuate received signals, it is important that the line driver be switched to a high-impedance state while the PLM is receiving signals. Normally, this would require the use of an additional isolated communication channel to couple the transmit-enable signal from the modulator control circuit to the line driver. To minimize packaging complexities, the optical AFE allows digital control signals to be simultaneously multiplexed with the analog signals, allowing a single optical channel to simultaneously transmit analog and digital control signals. One of these digital signals is the transmit-enable signal.

Since the output stage of the PLM is totally isolated from the appliance, in some cases the appliance's control circuitry would have no way of determining if the Vcc2 power-supply voltage is present and correct. To prevent such a scenario, an undervoltage detection circuit is included on IC2, which monitors Vcc2 and transmits an indicator signal across the isolation boundary in a manner similar to that used for the transmit-enable signal.

One of the first and most obvious advantages of an optical solution is the ability to construct a low-profile SMD component that contains not only the isolated coupling mechanism but also much of the analog signal-processing components. This results in a large reduction in the number of components required, which in turn reduces the complexity and cost of a PLM.

Unlike magnetic transformers, the optical-signal coupling devices do not require close physical coupling to achieve satisfactory performance. Theoretically, optically coupled signal paths can operate through an optical fiber at distances of hundreds of kilometers, making the common-mode impedance infinitely high. In practical terms, optically integrated devices in standard SMD packages typically exhibit coupling capacitance on the order of 1.3 pF. This is a very significant improvement over magnetic transformers, which typically exhibit parasitic capacitances in the range of 30 to 100 pF.

Magnetic-signal transformers are capable of not just transmitting and receiving communication signals; they are also capable of coupling high-voltage surge transients. Conversely, optical coupling devices are only inherently capable of coupling low-power optical signals. Transient-voltage surges are very effectively blocked from reaching the low-voltage appliance circuitry. From a practical point of view, this means the overall robustness of an electrical appliance need not be degraded by inclusion of a PLM.

Patrick Sullivan is senior R&D engineer at Agilent Technologies Deutschland GmbH (Boeblingen, Germany).

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