A low-cost method for rapid transfer function measurements with direct application to biological impedance analysis.

The measurement of the linear transfer function of a biological system has found wide use in the characterization of the system's input/output properties, and in the separation and measurement of the properties of the different subelements that make up the system. Unfortunately sophisticated and expensive instrumentation has traditionally been required to make these measurements. In this paper, we present detailed design specifications of a low-cost instrument that is capable of yielding transfer function measurements with a high degree of accuracy and speed. The instrument is comprised of a pseudo-random binary sequence signal generator with precise date acquisition synchronization circuits, interfaced to a general-purpose mini-or microcomputer system common to many laboratory environments. The instrument is capable of measuring the transfer function of an arbitrary biological system up to a bandwidth of 8.3 KHz, with a frequency resolution of 425 points. In cases where the biological measurements are not contaminated with experimental noise, the transfer function can be determined in as little as 47 ms of data collection. In the case where experimental noise is present in the biological measurements, a simple averaging method is described which results in an effective increase in the signal-to-noise ratio, thereby yielding accurate transfer function estimates. The instrument is especially well suited to the measurement of transfer functions of biological systems, where experimental noise is a problem and where only limited time is available to acquire stable measurements.