A digital wattmeter employing a trapezoidal compensation approach

An improved accuracy digital wattmeter for distorted signals is implemented using a Motorola 68000 microprocessor with associated peripherals. The power computation algorithm utilizes a trapezoidal compensation method, described in the literature, to compensate for nonsynchronous sampling effects which can occur with a fired sampling interval. Unlike the software interpolation used in the literature, a simple hardware approach is used to determine the fraction of a sampling interval which will result in synchronous sampling. The trapezoidal rule is applied to the product of simultaneously sampled voltage and current signals over an integral number of cycles of the signal fundamental, taking into account the trapezoidal compensation. The result is displayed on an LCD unit. The unit enables 2 voltage-current pairs to be sampled simultaneously and a keypad is used for selecting the various display formats and channel selections.