RF/Microwave Circuits I Baluns - ECS

Surface Mount BalunsImportant terms: Amplitude balance – how closely matched are the amplitudes of the two outputsignals Phase balance – how well does the phase of one output track the phase of thesecond output Insertion loss – (we know this one)Output 1input(ground)groundOutput 2Surface Mount BalunsBaluns are commonly made using the center-tapped transformer below Each output is terminated by N 2 Z 1 /2 The center tap (nodes 4,5) is grounded This provides a 180-degree phase difference between nodes 3 and 6(+)(gnd)(-)•5

Surface Mount BalunsTypical equivalent circuit with parasitic elements (these should looksomewhat familiar…)Surface Mount BalunsDo we need anything other than surface mount baluns?Answer = of course, sometimes Non-planar components may not be ideal Frequency range may be limited Integrated designs are sometime required Configuration may not work in architecture for other reasons•6

Distributed Balun DesignsA simple example is a transition between a microstrip line (unbalanced) anda stripline (balanced) A stripline is similar to microstrip, except both conductors have the samedimension Various techniques for tapering the dimensions of the lower conductor havebeen developedGroundTaper sectionSignalLowerconductorcontinuesunderneathMicrostripStriplineDistributed Balun DesignsCoupled microstrip lines can also be used to realize a balun:Analyze at f c , whenthe length = λ/41243If a short circuit is placed at port 2:SS3141−=( Y0,odd−Y0,even)( Y + Y )0, odd0, evenWe want this ratio to be -1 over a wide bandwidth, which requiresS 11 is zero if Z o,odd = Z o /√2Z 0,even= ∞•7

Distributed Balun Designs1243Achieving a high even-mode impedance (when both strips are at the samepotential) means the ground plane has little effect Thick, low dielectric constant substrates Very thin conductorsAchieving this in practice can be difficult, so this type of balun is not alwaysthe best solutionDistributed Balun DesignsPlanar Transmission Line Baluns – consist of two sections The first section divides the signal into two signals with equal amplitude andphase The second section provides -90 degrees and +90 degrees phase shift for thesignals, so the total phase difference between outputs is 180 degreesA Wilkinson divider is often used for the first section; multi-sectionWilkinson dividers can provide very wide bandwidthTLINTL1E=90CLINTL5E=90TermTerm2Num=2Z=50 OhmTermTerm1Num=1Z=50 OhmRR1R=100 OhmCoupled line sections(Lange couplers often used)WilkinsonTLINTL4E=90CLINTL3E=90TermTerm3Num=3Z=50 Ohm•8

Distributed Balun Designs0⎢⎣−j− j⎡ ⎤[ ] = ⎥ ⎦S open0TermTerm1Num=1Z=50 OhmCLINTL6E=90TermTerm2Num=2Z=50 Ohm180 degree phase difference0⎢⎣ j⎡ ⎤[ ] = ⎥ ⎦S shortj0TermTerm1Num=1Z=50 OhmCLINTL6E=90TermTerm2Num=2Z=50 Ohm•9