Coupling Circuitry: Understanding the Functions of Different Components 1

The design of coupling and de-coupling circuitry is often neglected at the expense of reduced signal levels and increased noise levels. This paper summarises the fact that coupling capacitors or de-coupling inductors don’t merely pass or block signals, but that their filtering characteristics are quite dependent on the loads into which the waveforms terminate. The functioning of a coupling transformer is also investigated, and it is shown that the leakage inductance, together with a series capacitor, forms a seriesresonant band-pass filter that can be modelled as a simple LC-R circuit. Design equations are given and verified by measuring the transfer function of the coupling circuit for different values of L, C and R. 1. Filtering concepts Filtering (ability to discriminate between different frequencies) forms the basis of most coupling and decoupling circuits. De-coupling filters are typically used for network conditioning whereas coupling filters feed the communication signal to and from the power line. Other applications of filtering include the filtering of noise, filtering to improve phase distortion and filtering to prevent incoming and outgoing EMI. It is obvious from the above examples that proper filter design is i) essential for minimum signal attenuation when coupling to the power line and ii) can drastically improve signal to noise ratios.