The use of multi-pole bandpass filters and other multiple resonator circuitry as oscillator frequency stabilization elements

This paper describes the design considerations and tradeoffs involved in the use of multi-pole bandpass filters and other cascaded resonator structures as oscillator frequency stabilization elements. Oscillator signal near-carrier noise is minimized when the oscillator loop group delay is large. A comparison has been made between single-pole and multi-pole bandpass filters and other multiple resonator circuits with regard to the most efficient way of obtaining maximum, net group delay at a given insertion loss. As an example, at 12 dB net insertion loss, a two-pole filter provides approximately the same group delay as either a higher order filter or an isolated cascade of two, 6 dB insertion loss, single-pole filters. Calculated results for variable order (number of poles), fixed loss filters are provided in terms of normalized group delay, amplitude response bandwidth, and group delay "flatness" bandwidth.