A signal processing ASIC for an all digital spread spectrum modem for power line communications

Through the use of spread spectrum techniques low voltage electrical power distribution networks become reliable and universal digital data links. After a short review of appropriate signalling schemes the paper will outline the basic ideas leading to the construction of an ASIC. It is shown that this step has to be taken because the use of general purpose digital signal processors (DSP) is neither feasible nor cost-effective. The design of the transmitter section into an ASIC is relatively easy. An inexpensive counter with a reset-to-zero feature operates as the core element for precise and fast signal synthesis. Signal selection and timing control are performed by an on-board microcontroller. The solution for receiver signal processing is more complicated. A fast signal processing front end is needed which does correlation operations based on a MAC structure (multiply and accumulate). The mentioned on-board microcontroller performs bit decision, serial data transfer as well as timing and control functions. Investigations revealed that a word length of 8 bit is sufficient in the transmitter as well as in the receiver section of a typical power line modem. Finally the ASIC has to contain the following functional blocks: an 8 bit MAC unit with at least four separate 32 bit accumulators, a microcontroller interface, a 9 bit programmable counter, and a clocking and control unit. ASICs according to the outlined scheme were designed and fabricated in spring 1993 using a 0.8 μm BiCMOS full custom process. Testing revealed a power consumption of less than 5 mW at the full operating speed of 12 MHz. Half-duplex modems are under construction with different signalling schemes for data rates up to 2400 bits/s. First measurements revealed a bit error rate at typical indoor installations in the range of 10^-5. Further work will concentrate on outdoor applications such as remote meter reading as well as control and supervision of consumer´s premises