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Unwanted Coupling in Millimetre-Wave Multilayer Circuits
Keywords: Multilayer Structures, Millimeter -wave, Unwanted Coupling
New data are presented on the effects of coupling between conductors in a highly integrated, multilayer circuit working at frequencies up to 100 GHz. Whilst multilayer circuits offer many advantages for the microwave circuit designer, unwanted coupling due to dense integration may detract from the electrical performance of the multilayer package. Using a combination of electromagnetic simulation and practical measurement we have established criteria for the optimum spacing of conductors in multilayer ceramic packages. The work was based around the use of photoimageable thick-film conductors and dielectrics; the dielectric had a relative permittivity of 3.9 and the conductors were silver. Two situations were considered in detail, firstly the effect of the crossover of conductors separated by dielectric, and secondly the effect of coupling between parallel conductors on different layers. Both of these geometries were modelled using an electromagnetic simulator (CST – Microwave Studio®), and the results of simulation were confirmed by practical measurement. The practical circuits were fabricated on alumina, with the crossover and coupled lines built up with successive layers of photoimageable thick-film paste. It was found that the input match of a crossover structure, and the unwanted coupling, varied non- linearly with the thickness (h) of the separating dielectric layer. For a 90 crossover at 20GHz, and 50μm of dielectric, the input return loss (S11) was 13dB, with a coupling (S31) between the conductors of 12dB. A dramatic improvement was observed when the dielectric thickness was increased: with h=200μm, S11=22dB and S31=18dB. This was a reasonable input match, together with a tolerable level of coupling. With h200μm, there was relatively little improvement by increasing the value of h. Using the experimental data, general design graphs have been developed to summarize the results and provide guidance to the circuit designer on the minimum spacing between conductors in a multilayer package.
Anne D. Abeygunasekera, Research Student
University of Surrey
Guildford GU2 7XH,

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