Permittivity Measurement By Short-circuit Method On Microstrip

Impedance control and signal integrity are always the key problem incircuit design and analysis, and the permittivity is one of the most importantmaterial characteristic. In some occasions, the traditional impedance precisioncontrol of+/-10%has been unable to guarantee the consistency of productperformance; there have been+/-7%and even+/-5%accuracy requirements.As we know, in addition to the inherent factors in material manufacturingprocess, the alternation of storage and usage environment will also affect thepermittivity. So the original material parameters provided by manufacturerhave been unable to meet the requirement of impedance precision control.Thus we must take measurements for material before actual design andmanufacturing to ensure the accuracy of impedance control.There are already lots of researches on permittivity measurement. A lotsof different methods are also presented. We mainly discuss the method tosolve permittivity by using microstrip line on PCB, more than the waveguidemethod. Within the lots of permittivity measurement methods by usingmicrostrip line on PCB, there are two methods commonly used. One is1/2wavelength series coupled resonant method, the other one is1/4wavelengthopen-short branch resonant method. The first method takes advantage of openterminal coupling in series structure, by means of signal coupling through theline gap at high frequency, to obtain the resonant frequency. Then we couldsolve the effective permittivity by using the empirical formulas. While thelatter one practice the principle of1/4wavelength conversion, i.e. theimpedance of one end is infinite and the other one is short. During the signaltransform by using microstrip line, when the length of open-short branchequals to1/4wavelength of signal, the insertion loss of signal transform will become very large. Counting for both the3dB frequency measured by NVAand the1/4wavelength of open-short branch, we could obtain the effectivepermittivity by empirical formulas. From above, both two methods usesopen-circuit structure to realize resonance, the short-circuit methods areseldom used in permittivity measurement for much difficulties in short-circuitstructure realization within a wide range of frequency. Accounting for therealistic short-circuit structure in waveguide, we could solve the effectivepermittivity by using short-circuits structure on PCB if we could realize thesame structure on PCB.Based on the imagination, this paper focus on permittivity measurementby short-circuits method on microstrip line of PCB. By the short-circuit at theend of microstrip line, we could get the reflection coefficient of single port byVNA, then we would have the final solution on relative permittivity at the aidof several formulas. So we start discuss from the theoretical analysis in ADS,and then perform the3-D modeling and simulation in Ansoft HFSS. Inaddition to the practical measurement on PCB, we could process thecomparison between the simulation result and practical result. Thus weverified the feasibility of short-circuit method.The short-circuit method is applicable from100MHz to5GHz due to therestriction both on PCB realization and phase velocity.