Experiment5—CouplerDesign.

1 experiment5 H Matzner, S. Levy and D. Prelab Exercise23 Theoretical Coupled Line Directional Coupler .. RequiredEquipment .. Single Section Half Octave Edge Coupled Line CSTS imulationofaMicrostripCoupler .. Calculating the Even and Odd Modes Measurement .. Measurement .. 2711 ObjectivesUpon completion of this study, the student will become familiar with thefollowing topics:1. Measuring the basic parameters of Understanding the role of couplers in a Prelab Exercise1. For a20dBsingle section edge coupled line directional coupler, con-structed as a stripline with ground plane, spacing ofB= ,dielectric constant of r= ,T= ,tan = ,character-istic impedance of50 ,and center frequency of1 GHz.

2 Assuming alossless component and a perfect termination: Write down the S-parameters matrix of the coupler. Calculate itsZOeandZOo. UseADSLineCalc,setthecomponenttypeto CPWCPL2 ,andfind the dimensions:W- width of the lines,S- separation of the lines,L-lengthofthecoupled For a20dBsingle section edge coupled line directional coupler, con-structed as a microstrip, height of substratehs= , dielectricconstant of r= ,T= ,tan = ,characteristic im-pedance of50 ,and center frequency of1 GHz. Assuming a losslesscomponent and a perfect termination: UseADSLineCalc,setthecomponenttypeto MCLIN ,C_DB= 20,E_Eff=90 deg,andfind the dimensions:W- width of the lines,S- separation of the lines,l- length of the coupledlines (ZOeandZOoremain the same as the previous question).

3 23. For a20dBthree sections edge coupled line directional coupler, con-structed as a stripline with ground plane, spacing ofB= ,dielectric constant of r= ,T= ,tan = ,character-istic impedance of50 ,and center frequency of1 GHz. Assuming alossless component and a perfect termination: CalculateZoeandZOoof a edge coupled line with a coupling of20dB, center frequency1 GHzand characteristic impedance of50 ,in a stripline with a ground plane. Using ADS LineCalc,find the dimensions:W- width of the lines,S- separation of the lines,l- length of the coupledlines, for each Theoretical BackgroundA very commonly used basic element in microwave system is the directionalcoupler. Its basic function is to sample the forward and reverse travellingwaves through a transmission line or a waveguide.

4 The common use of thiselement is to measure the power level of a transmitted or received model of a directional coupler is shown in Figure waveSampled waveThrough waveIsolated wave1234 Figure 1 - Directional coupler seen in thefigure, the coupler is a four-ports device. The forwardtravelling wave goes into port 1 and exit from port 2. A small fraction of itgoes out through port 4. In a perfect coupler, no signal appears in port the coupler is a lossless passive element, the sum of the signals powerat ports 1 and 2 equals to the input signal power. The reverse travelling wavegoes into port 2 and out of port 1. A small fraction of it goes out throughport 3. In a perfect coupler, no signal appears in port directional coupler S-parameters matrix is:S= 00 j 1 k2k00k j 1 k2 j 1 k2k00k j 1 k200 (1)Wherekis the coupling factor (a linear value).

5 One popular realization technique of the directional coupler is the coupled-lines directional coupler; two quarter wavelength line are placed close to eachother. The wave travelling through one line is coupled to the other line. Sucha coupler is shown in Figure waveSampled waveThrough waveIsolated wave1234 Figure 2 - Coupled lines based directional there is no ideal coupler available, some of the forward travellingwave is coupled into port 3. This mean that we may think that there isa reverse travelling wave when there isn t. This is very critical in applica-tion where the directional coupler is used to measure the return loss of thedevice. By calculating20 log(S31/S41)we canfind the return loss of the de-vice connected to port 2.

6 If out coupler has no perfect directivity then outmeasurement is not are few simple parameters to describe the functionality of a coupler: Insertion Loss:20 log(S21)or10 log(1 k2). Return Loss:20 log(S11). Coupling:20 log(S31)or20 log(k). Directivity:20 log(S31) 20 log(S41). Non - Directional CouplersIn some applications, the directivity of the coupler is not important. Forinstance, if we know there is only forward travelling wave then we may usea non-directional coupler. One possible realization is using a simple resistordivider as shown in Figure waveSampled waveZ0Z0Z0R2R1 Figure 3 - Resistor divider non-directional transmission lines are not electromagnetically coupled and the cou-pling equations are derived from the lumped circuit calculation of the Coupled Line Directional A Single Section CouplerThere are two modes of currentflow in an electromagnetic situation.

7 Thefirstis one currentflowing down one conductor with a contra-flow current backup the other conductor caused by displacement current coupling betweenthe two conductors. This is termed the odd mode current, and it has anassociated odd mode characteristic impedance, by displacement current betweeneach center conductor carrying the same polarity, and the ground that iscommon between them. Hence this is called the even mode current, and ithas an associated even mode characteristic impedance, the polarity of the lines of each a single section coupler the even and odd mode characteristic im-pedances are defined as:6Z0e=Z0r1+C1 C,(2)Z0o=Z0r1 C1+CWhereC<1is the voltage coupling factor of the coupler (a linear value).

8 IIII+V+V+V-VEven modeOdd modeFigure 4 - Even and odd characteristic Multi - Sections CouplerFor the case of three sections coupler, the coupling equation is:C=C1sin 3 +(C2 C1)sin (3)WhereC1is the coupling of thefirst and third section, whileC2is the couplingof the second section. In order to solve the coupling equation, one may derivetwice equation3as:d2Cd 2| = /2=0andfinallyfindZOeandZOofor each section using picture of the dimensions of a stripline coupler is shown in Figure 5 - The dimensions of a stripline coupled line top view of a three sections coupled lines coupler is shown in Figure 6 - Top view of a three sections microstrip coupled line of the symmetry of the structure, all reflection coefficients willbe identical as well as several transmission Experiment Required Equipment1.

9 Network Type N calibration type N accessory Signal Spectrum Power DC power Single Section Half Octave Edge Coupled Line ADS Simulation1. Simulate a single section edge coupled line with the dimensions youfound in Prelab Exercise question 1,as shown in Figure "SSub1"SSUBSSub1 TanD= +50T= mmB= cmMur = 1Er= MHzStop=1500 MHzStart=500 MHzS-PARAMETERSTermTerm3Z=50 OhmNum=3 TermTerm4Z= 50 O hmNum =4 TermTerm1Z=50 OhmNum =1 TermTerm2Z=50 OhmNum =2 Figure 1 - Single section edge coupled lines Draw the following graphs: Coupling (dB). Directivity (dB). Insertion Loss (dB). VSWR primary and secondary line. Frequency sensitivity (of the primary line), frequency range800 MHz the CST Simulation of a Microstrip Coupler1.

10 Choose the "Coupler (Planar, Microstrip, cpw)" template. When thistemplate is used, the background material is defined as vacuum, theunits are changed tomm,GHzandnsec, and the boundary conditionsare set to "electric". Furthermore, the mesh settings are changed toaccount for the planar Define the parameters, as shown in Table +30 Ground x dimensionygS+2*w+30 Ground y of substratewWidth of stripSSpacing between the linesLLength of coupled linesTable 1 - Parameters DefinitionFor the missing values, use the dimensions you found in Prelab Exercise question Create the substrate break, as shown in Figure 2 - Building the substrate with a new Define thefirst strip, as shown in Figure 3 - Defining thefirst Zoom in to the end of the strip, as shown in Figure 4 - Zooming in to the end of the Pick the face of the end of the strip, as shown in Figure Choose Rotate.