GW Instek GRF-3300 Series Student Manual - Manual (Page 8)

GRF-3300 Manual
3 316
4. Find the L, C values according to the standard filter theory.
5. Convert the L, C to microstrip lines.
6. Construct and simulate the filters.
7. Fine tune and modify the design until the goal is met.
8. Fabricate the filters.
When fabricating the RF circuit on the PCB, the substrate material is a key factor
for the success. Usually the FR4 material is not suited for frequencies above 1GHz
because of the following reasons: the dielectric constant is not uniform over the whole
material, the dielectric constant also varies according to frequency change, most circuit
board suppliers can guarantee dielectric constant within a range but not at a precise value,
etc. In practice, microwave substrate PCB such as ceramic and duroid
“
substrates is the
better choice for microwave applications. In our experiment, FR4 is chosen because it is
easier to obtain. The results may contain a margin of error, but the basic concept can
definitely be established.
The main theory reference of this chapter is the following textbook: ‘Microwave
Engineering’, by David, M. Pozar, Addison-Wesley Publish Company, Inc..
16-2. Stepped –Impedance Low Pass Filter
A common way to implement a low pass filter is to use alternating sections of very
high and very low characteristic impedance lines, which is referred to as stepped
–impedance, or hi-Z / low-Z filter.
16-2-1. Design Example
Design a stepped-impedance type Butterworth low pass filter with cutoff
frequency at 2GHz and > 20dB attenuation at 3GHz. The input and output impedance are
both 50
:. The PCB material is FR4 with dielectric constant 4.2 and thickness 1.2mm.