Powerline communication modems (PLMs) are a popular communication medium used in many AMR applications. Although it is a relatively old technology in terms of basic concept, successful implementation in AMR applications is dependent on the ability of the PLM to deal with the unique technical challenges of this particular medium – noise and signal attenuation.
In this respect the implementation of PLMs has benefited from continuous technical developments. The HCPL-800J is a new integrated circuit developed to enhance the performance of the analog conditioning circuitry commonly used in a typical PLM.
The circuit of a typical powerline modem can be partitioned into two key blocks – the digital portion, which at the very minimum provides the modulation/demodulation function, and the analog front end (AFE) which provides signal condition and coupling of the signals on and off the power line. Implementation of advanced digital signal processing techniques in PLMs has helped to improve communication performance in the digital block. However, up to now there has been little or no improvement in the performance of the AFE circuit.
Although attempts have been made to integrate the AFE with the digital modulator/demodulator function using mixed signal technology, in the majority of cases this has been limited to the ADC and DAC. Reasons for this are varied, but one of the most compelling is the relentless shift to lower voltage, smaller, geometry IC processes. This technological shift reduces the power consumption, IC size and cost, but makes the implementation of the necessary analog circuitry more and more difficult.
The consequence is that the majority of PLM solutions today implement a large portion of the AFE circuitry off chip, using a complex discrete circuitry which often requires as many as 60 individual components, including bulky isolation transformers. Furthermore the desire to keep the number of components as low as possible often results in significant compromises on circuit performance.
OPTICAL PLM AFE
The HCPL-800J is based on optical isolation technology, as opposed to the magnetic technology commonly used in most traditional AFE implementations. One of the most obvious advantages of an optical solution is the ability to construct a low profile, surface-mount component that contains both the isolated coupling mechanism and much of the additional analog signal processing components.
An optical solution also adds significant performance benefits. The first can be observed when we consider operation during power line surges. Transformers inherently have the ability not only to pass the modulated communication signals but also to couple power surges into the isolated low voltage circuitry. Opto-isolation, on the other hand, is incapable of transferring sub-optical frequencies, which results in a high level of protection for the low voltage system circuitry.
The second isolation benefit results from the low parasitics of optical isolation technology – in particular the leakage capacitance across the isolation device. In the case of a PLM transformer, the primary-secondary leakage capacitance is typically in the order of 30 to 100pF. This is a consequence of the number of windings and their close proximity, so an improvement on this parasitic capacitance can only be achieved at the expense of a reduction in the magnetic coupling efficiency.
In contrast opto-isolation technologies benefit from relatively large galvanic separation distance and very small areas of adjacent coupling planes. The result is a parasitic capacitance of less than 1.5pF.
To consider the benefits of higher isolation, we have to consider the propagation characteristic and influence of noise on the PLM receiver circuit. Noise on the power line is predominantly made up of bursts of high amplitude noise interleaved with periods of low-level noise.
To enhance receiver performance the received signal is typically filtered to reduce out-of-band noise. Furthermore the negative impact of high levels of in-band noise bursts is reduced via digital error correcting techniques such as FEC and bit interleaving. However none of these techniques can reduce the negative impact of in-band noise in the relatively quiet periods between noise bursts. It is this noise level which places a finite limit on the communication distance.
This residual problematic noise is made up of two components – differential noise and common mode noise – although the level of observed differential noise at the receiver will depend on the relative position of the noise source to the receiver and transmitter nodes. As a general observation, the level of differential noise measured at the receiver tends to have at least some correlation with the attenuation level experienced by the transmitted signal. In other words, a heavy attenuation signal would tend to be accompanied with lower differential noise level, and a relatively un-attenuated signal would tend to be accompanied by a relatively high differential noise level. This effect, although perhaps obvious, is particularly fortuitous.
Common mode noise is, however, not attenuated by propagation effects such as line length and load impedances in the same way as differential noise. In fact, the longer the length of the power cable, the more common mode noise is picked up. Since the common mode noise is propagated to the receiver via the leakage capacitance of the isolation, minimising the leakage capacitance is key to maintaining high common mode noise rejection.
LINE DRIVER REQUIREMENTS
Another factor affecting communication distance capabilities is the level of transmitted signal injected onto the power line. In the AMR environment the power line impedance is typically very low, so the actual injected signal level is more often than not dependent on the current capabilities of the line driver circuit. Furthermore the amount of power available to supply the line driver is not limitless, so efficiency is important.
To meet these requirements the HCPL-800J line driver is able to supply output currents in excess of 1A. Furthermore energy efficiency is assured by its capacity to drive undistorted signals to within 0.4V of its supply rails.
The need to meet many international RF emission regulations places tight limits on the amplitude of out-of-band harmonics. To enable EMI compliance, it is commonplace with many traditional discrete based line drivers to use a finely tuned coupling circuit to remove out-of-band distortion harmonics. Unfortunately component tolerances in this type of coupling circuit, and the unpredictable load condition on the power line, can result in significant signal insertion losses. To eliminate this insertion loss, the line driver used in the HCPL-800J is extremely linear, reducing distortion harmonics to levels which require no external filtering.
Flexibility was a key design criterion in the development of the HCPL-800J, allowing it to perfectly match the interface requirements of a wide range of PLM transceivers, from proprietary PLM transceiver ICs right through to low cost generic DSP processors. The future trend is towards system-on-chip (SOC) solutions, which are capable of carrying out additional control functions such as user interfaces and even measurement functions. It is therefore not improbable that future developments will position the PL communication function as just another integrated serial communication interface. In this instance the HPCL-800J could be considered analogous to today’s widespread use of RS232 transceiver ICs with microprocessor SCI interfaces.