For many years, the ARRL Handbook has included a circuit for an audio phase shift network designed by HA5WH. I have not located the original reference for this network. The Handbook claims that the circuit gives approximately 60dB of opposite sideband suppression using 10 percent tolerance components. This flies in the face of the usual result that you need 1 percent components to get around 40dB suppression. In this article, I will analyze and give design equations for this type of network. Unfortunately, this analysis shows that using 10 percent tolerance components can lead to poor sideband suppression. With ideal components the network can give excellent performance, and by using either high tolerance components, or well matched lower tolerance components, the network still can give good performance.
In section 2, I give the general formula for the sideband suppression in terms of the phase and amplitude errors in the phasing network. In section 3, I derive an efficient method of analyzing a general network of the HA5WH type. Section 4 gives the analysis of an ideal realization of the network. Section 5 describes the optimization of the network in terms of easily calculated elliptic functions, and section 6 gives the effects of component tolerances. The result is a set of simple design equations for the ideal network and an estimate of the sensitivity to component tolerances. The Appendix contains a listing of a set of Fortran programs that implement the methods described.