EEL 3XXX --- Wireless Circuits and Systems

Laboratory 7 - Distributed Element Filter

Objectives/Background

This lab focuses on the design, fabrication and measurement of a 2.4 GHz band-pass filter. The filter is used as the ``preselector'' in a pager's receiving unit. It is placed after the RF amplifier and before the first mixer in the system block diagram below.

As we learned in Laboratory #5, the parasitic capacitance and inductance of lumped elements becomes important at this frequency and can negatively affect filter performance. Therefore, in this experiment we will use distributed circuit elements such as the $\lambda$/2 resonator studied in Laboratory #6. Using filter synthesis software, we will design a filter consisting of multiple resonators in series. After fabrication and measurement, the filter will be modeled using the CAE circuit simulator to compare predicted and experimental results.

Equipment

Personal computer, 6 GHz vector network analyzer, 2.9 GHz spectrum analyzer and tracking generator, circuit board milling machine; filter synthesis software (ParFil) and CAE circuit simulator.

Reference Material

Reference Data for Engineers, SAMS, Ch. 8

Parallel-Coupled Transmission-Line-Resonator Filters, S. B. Cohn, IRE Transactions on MTT, pp. 223-231, April 1958

Active and Passive Analog Filter Design - An Introduction, L. P. Huelsman, Ch. 4, McGraw-Hill, 1993

ParFil (TM) User's Manual

Tutorial

1. Stepped-impedance low-pass filter (simple model): describe direct correlation between circuit model and distributed circuit layout, i.e. series inductor and high impedance transmission line, and shunt capacitor and low impedance transmission line.
2. CAE circuit simulator: analyze pre-defined lumped-element low-pass filter and then replace the capacitor with a low impedance transmission line; compare results and observe that in-band response is the same but out-of-band response changes due to electrical length of transmission line section (students follow along at their own work-station using the PC).
3. Technique for parallel-coupled transmission line filter design (more complicated model): describe circuit model and discuss the dependence of filter performance on the coupling strength between resonators and on the number of filter elements.
4. Filter demonstration: demonstrate a pre-fabricated 2.4 GHz band-pass filter using the vector network analyzer; compare measured and simulated results using the CAE circuit simulator (students follow along at their own work-station using the PC and obtain the measured data file through the network).

Procedure

1st Period

Design a 2.4~GHz band-pass filter to meet given specifications, using the filter synthesis software; CAD files from each group will then be transferred to a common file for the class and the milling machine will be utilized for fabrication (performed by a teaching assistant).

2nd Period

Set up the CAE circuit simulation and study the predicted response, answering questions pertaining to bandwidth and out-of-band rejection; characterize the filters using the spectrum analyzer/tracking generator setup; compare results to the predicted values; measure filter using the vector network analyzer (assisted by teaching assistant).

Results and Discussion

Summarize the measurements and the comparison between predicted and measured results and try to explain the discrepancies. Discuss the insertion loss and spurious responses, and how they might affect overall system performance.