Tracking Filters for Rejection of Harmonic and Spurious Responses in Automatic Test Systems

Narrow band YIG tuned filters are ideal for filtering out harmonics, spurious responses, and other unwanted signals from signal sources such as sweepers and synthesizers. They are also extremely useful in ATE used for spectrum analysis and receiver testing. Unfortunately various tuning errors make it difficult to accurately track these devices to the desired signal. Readjustment is needed whenever the setup is changed or equipment or tracking filter component aging causes detrackinq. In ATE this means either manual " tweaking" or a computer controlled calibration algorithm that is time consuming and expensive. A closed-loop stabilizing technique is shown which ensures extremely accurate tracking of the YIG filter to the desired signal without separate computer control. Applications to signal source cleanup, spectrum analysis, and mixer/receiver testing will be shown. The various tuning errors in standard open-loop tracking filters, and their effect upon ATE measurements will be explained as well as how these are overcome by the closed-loop technique.