GW Instek GRF-3300 Series Teacher Manual - Manual (Page 21)

16 Microstrip Line Filters
3 329
Questions
Question 1:
What is the possible factor, other than the design of the filter itself, that affects
implement the transmission line filter?
Answer: Connectors. In the experiments, two connectors, not included in the
design, are used at the input and output terminals. Using high quality connectors will limit
their negative effect to the minimum. On the other hand, when the filter is designed as
part of the circuit, connectors are unnecessary but then in-circuit measurement technique
becomes another issue to be considered.
Question 2:
Describe how to limit the tolerance caused by connectors.
Answer: Adding an attenuator pad can improve the mismatch between the
connector and filter. One 3dB pad will improve 6dB of return loss. In practice, adding a
pad to improve mismatch is a common method in microwave circuitry design.
Question 3:
Describe how to choose the right PCB for implementing microstrip line filters.
Answer: The first issue is the frequency. As explained in section 16-1, PCB made
of FR4 is not suitable for high frequencies. The second is the dielectric constant
Hr.
Higher
Hr can reduce the filter size but increases the loss. In addition, the small gap in
couple-line type filter and the narrow line width in Hi-Z LPF are difficult for fabrication.