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Measurement and Simulation
Lab Assignment 3: Filters, Matching Network Design, and Resonant Circuits Characterisation
In the previous labs, you have modelled transmission lines which were mostly matched and are theoretically frequency-independent. This lab focuses on the design and analysis of frequency-dependent and resonant components including basic resonators, matching networks, and filters.
You will learn:
1. How to design matching networks for complex impedance loads.
2. Observe the frequency limitations of different matching topologies.
3. Design and understand the operation limits of filters, particularly in practical systems.
The scheduled lab sessions are an opportunity to familiarise yourself with the software and ask questions. You should spend time outside the lab carrying out the exercise and writing your report.
Refer to lab 1 for software installation and getting started instructions.
1. Assignment Tasks:
Your test PCBs included non-50 Ohm transmission lines, e.g. Line 2 and Line 4. In addition, Line 3 exhibits a periodic behaviour with filtering properties.
You will work with the modelled and measured s-parameters of these devices to design matching networks and filters. In the labs, you can start by measuring Line 3 on your PCB and observing the provided touchstone files on Moodle.
Start by measuring the response of Line 3, and compare your NanoVNA measurements to the provided s-parameters
Present the following in your report and discussions:
Matching Networks Design:
1. Impedance matching design:
a. Design an input matching network for the CPW line on your board (line 2) to achieve an input impedance of 50 Ohms at 550 MHz. Use both the ADS CPW model and the provided measured s-parameters on Moodle.
i. Present an LC matching network design.
ii. Implement a practical version of this matching network using real L/C parts.
1. You are free to import or use whichever model is available for the LC elements, but the parasitic values must be discussed, and the accuracy of this approach should be critically evaluated.
iii. Present the S11 and S21 response of the line before and after your matching network is included.
1. Present the results for both the closed-form. model and the measured touchstones files.
b. Repeat your design process for 5 GHz.
i. Present the s-parameters of your newly matched line.
ii. In your report, compare and discuss the differences between the matching network design at 550 MHz and 5 GHz.
1. Useful points to consider include the fractional bandwidth, matching quality |S11|, component values.
Multi-Port Devices Design:
Exploiting the periodic behaviour in Line 3 could be used to realise very basic band-stop filters. Building upon your models from lab 1:
1. Using the microstrip line models, in the same PCB process as your test boards, design a two- way balanced power divider with a centre frequency of 1 GHz.
a. Present the input and output matching parameters including the 10 dB return loss bandwidth of the power divider.
i. Make sure to clearly show the insertion losses and isolation.
ii. Include the dimensions of your lines and explain how they were calculated.
iii. Discuss any deviations from the expected line length in order to achieve a
1.5 GHz power splitter.
b. Compare the isolation between the output ports with and without resistors.
Another common example of multi-port devices is adiplexer:
2. Use your knowledge of the band-stop filters to design a two-way diplexer with the following specifications:
a. The input needs to maintain an VSWR<3.
b. The output ports need to pass signals (with under -3 dB) loss, at 1 and 2 GHz, each.
c. The out-of-band rejection for the other signal should be over 20 dB.
d. Hint: refer to Line 3 on your board for an example of a band-stop filter.
Conclusions and Discussions:
Justify the component choice in your matching network, comment on practical layout considerations, and discuss alternative technologies for implementing the matching network. You should also comment on the suitability of measured vs. modelled s-parameter files in designing matching networks. For the passive multi-port network design, you should discuss the practical limitations of your model, design error sources, potential discrepancies between simulation and measurements (using Line 3 on your PCB), and highlight the limits governing the specifications for filters, dividers, and diplexers. The impact of the performance of such devices in a microwave frontend (including active devices such as PAs/LNAs) should be discussed. Careful discussions area large part of this assignment and you should make use of external references and resources to present a complete discussion.