Dielectric material characterization using rough surface transmission lines

Abstract

A transmission line characterization method based on S-parameters is presented and validated. As an example, the method was applied to a microstrip line to capture its frequency-dependent loss, dielectric constant, and loss tangent of substrate material. Also, the surface roughness effects on conductor loss and eventually on the loss tangent were studied. The results showed that the surface roughness effects are not negligible at high frequencies and affect tie value of loss tangent significantly. Therefore, the knowledge of RMS values of conductor surface RMS is required for any dielectric characterization methods using PCB samples with low loss tangent material. The validation of the method was achieved by comparing the new method to a direct measurement with Agilent 4291B operating below 1 GHz. For the direct measurement, a thin dielectric sheet made of the same substrate material was measured for both dielectric constant and loss tangent; therefore, there is no error associated with conductor loss or surface roughness. For the new method, two microstrip lines were designed on a substrate and the Agilent 8510C was used to acquire the S-parameters of the two lines. From the S-parameters, initially, the dielectric constant over a 45 MHz to 5 GHz range was extracted. Secondly, the conductor loss for perfectly smooth conductor was evaluated from the Ansoft HFSS with the dielectric constant that has been found. Thirdly, the surface roughness was accounted for using a empirical equation to come up with a total conductor loss. Finally, pure dielectric loss or loss tangent was found from the evaluated total loss and conductor loss. The comparison between two measurements at low frequencies is in very good agreement, indicating the validity of the method. The extended frequency sweep shows that the material has very desirable frequency response; i.e., constant dielectric constant and loss tangent over the frequency range.