Design Project #1: Coupled Line Couplers
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Due date: March 28 th, 2022
READING MATERIAL: CHAPTER 7: MICROWAVE ENGINEERING, DAVID POZAR,
PAGES 383 397. PLEASE CHECK EXAMPLE 7.8 ON MULTI-SECTION COUPLER
DESIGN AND PERFORMANCE
project SCOPE
Design a coupled-line coupler with the following specifications: Number of sections 3& 5 Center frequency 1 GHz Coupling -1 2 dB Port impedance 50 Frequency response Maximally Flat PROJECT TASKS:
A. Design
1) Determine the odd and even mode impedances for each of the 3/ 5 sections. The coupling coefficients for each section of this multi-section coupler are: 3 – sections coupler 5 – sections coupler
c 1 =
c 2 =
c^3 =^
c 1 =
c 2 =
c 3 =
c 4 =
c^5 =^
The odd and even mode impedances of each section are thus: 3 – sections coupler 5 – sections coupler
section (^) () () 1 2 3 section (^) () ()
1
2
3
4
5
B. Implementation Based on Ideal Coupled Lines
2) Implement both designs (3 and 5 sections) in Keysight ADS. We will start with the ideal behavior using ideal coupled lines (CLIN). Instructions on how to use ADS is at the end of this document.
3) Plot S 11 , S 21 , S 31 and S 41 (in dB) from 0 to 2 GHz, using a vertical scale from – 50 dB to 0 dB for both couplers for plotting S 21 & S 31 , and a vertical scale of – dB to -300dB for S 11 and S 41.
(Insert figures here)
Q1: Do these results indicate that your design is correct? Explain why you think so. Give specific numerical examples from each plot.
(Provide your specific, detailed answer here)
4) Use the markers to determine the bandwidth of your design, given that the coupling must be numerically less than 15 dB to satisfy specifications (i.e., a 3 dB bandwidth).
The bandwidth of this coupler has been determined in ADS to be:
Coupler Bandwidth = (Insert value here) GHz
(Attach accompanying figure(s) here)
C. Signal Flow Graph Analysis 5) Draw an exact signal flow graph of this (4-port) directional coupler. In other words, a signal flow graph of the form below, where c is the specific coupling coefficient of this coupler at the design frequency.
(Write signal flow graph here)
6) Reduce this signal flow graph for the case where ports 2, 3, and 4 are terminated in matched loads ( L2 = L3 = L4 ) and determine in decibels the numeric values of S 11 , S 21 , S 31 and S 41 , at the design frequency.
The reduced signal flow graph for this coupler circuit at the design frequency of 1 GHz is:
(Show reduced signal flow graph here)
From these results, the values of S 11 , S 21 , S 31 and S 41 (at 1 GHz) were determined to be (in decibels):
Q2: Do these values precisely match those provided by the ADS analysis? Why or why not?
(Give your explicit, detailed answer here)
7) Now attach a short circuit ( L4 = – 1 ) to port 4 of the coupler signal flow graph (with ports 2 and 3 again terminated in matched loads). Reduce this graph and determine in decibels the numeric values of |S 11 |^2 , |S 21 |^2 , |S 31 |^2 , at the design frequency.
The reduced signal flow graph for this coupler circuit at the design frequency of 1 GHz is:
S 11 (dB)=
S 21 (dB)=
S 31 (dB)=
S 41 (dB)=
(Show reduced signal flow graph here)
From these results, the values of S 11 , S 21 , and S 31 (at 1 GHz) were determined to be (in decibels):
8) Likewise place a short circuit on port 4 of your ADS designyou now have a 3-port device! Replot S 11 , S 21 , and S 31 (in dB) from 0 to 2 GHz, using the same vertical scale as before. Do not plot S 41!
(Attach accompanying figure(s) here)
Q3: How do these new results compare to the case where port 4 is terminated in a matched load (i.e., tasks 2 and 5)? Use your knowledge of the physical behavior of coupled-line couplersincluding any physical insight provided by the signal flow graph of task 7to explain why you get this result. What physically happens to a wave incident on port 1, once it is inside the coupler?
(Provide your specific, detailed answer here)
9) Now attach a short circuit ( L2 = – 1 ) to port 2 of the coupler signal flow graph (with ports 2 and 4 terminated in matched loads). Reduce this graph and determine in decibels the numeric values of S 11 , S 31 , S 41 , at the design frequency.
The reduced signal flow graph for this coupler circuit at the design frequency of 1 GHz is:
(Show reduced signal flow graph here)
S 11 (dB)=
S 21 (dB)=
S 31 (dB)=
From these results, the values of S 11 , S 31 and S 41 (at 1 GHz) were determined to be (in decibels):
10) Likewise place a short circuit on port 2 of your ADS designyou now have a 3-port device! Replot S 11 , S 31 and S 41 (in dB) from 0 to 2 GHz, using the same vertical scale as before. Do not plot S 21
(Attach accompanying figure(s) here)
Q4: How do these new results compare to the case where port 2 is terminated in a matched load (i.e., tasks 2 and 5)? Use your knowledge of the physical behavior of coupled-line couplersincluding any physical insight provided by the signal flow graph of task 9to explain why you get this result. What physically happens to a wave incident on port 1, once it is inside the coupler?
(Provide your specific, detailed answer here)
D. Actual Microstrip Implementation
****Please watch the educational video using the following link
D.1) Using ADS, implement the 3 sections and 5 sections coupler using microstrip coupled lines (MCLIN). Please note that several steps might be needed to achieve convergence when using linecalc since the line spacing might be small to converge. Also note that to connect different pairs of coupled lines, you will need to use some forms of bends or tapers, these can be found in the library as (MSOBND_MDS; MTAPER; MCURVE2)
S 11 (dB) =
S 31 (dB) =
S 41 (dB) =
For board material, assume FR Thickness: H =1.57mm Relative Dielectric Const. r= 4.4 (note: r has frequency dependence) Relative Permeability: Mur= Dielectric loss Tangent: TanD=0. Copper Conductivity: 6e7 s/m Metal Thickness: T: 17.5um (for oz copper)
D.2) Discuss the design challenges of the 5 sections compared to the 3 sections? Please note that in the implementation, when you add transitions going from one section to the other, these transitions will change your design parameters. You should be able to see that during EM simulations. Build the whole design using the layout tool and perform full electromagnetic simulations on the structure.
ADS INFORMATION
- You will need to use four ADS Term elements (one for each coupler port).
- You will need five CLIN elements, which are the ideal coupled transmission lines found in the TLines-Ideal element category.
- To attach a short circuit to a coupler port, simply change the characteristic impedance of the Term element at that port (only at that port) to a value of 1 (the smallest for ADS ). This is not exactly a short circuit but it is a valid approximation. If you remove the Term element from a coupler port and then connect that port directly to a ground terminal, it is also possible. But, if you remove the Term element from a port, then ADS will renumber the remaining ports.
PROJECT GRADING
- report organization and completeness (/25)
- Meeting technical specifications (/20)
- Design Strategy (use of correct equations and attaching calculations) (1/15)
- Design analysis (correct analysis, answer to questions, clear and correct plots from ADS) (/40)