Determination of Dielectric Constant （介电常数的确定）.pdfVIP

Determination of Dielectric Constant Of Printed Circuit Boards Sam Wetterlin 1/24/10 Introduction The dielectric constant of PCB material is important in determining the trace width required to produce a characteristic impedance of 50 ohms, or any other desired impedance. The most commonly used material, FR-4, has a dielectric constant that may vary widely between manufacturers or batches, and also varies over frequency. It is useful to have a method to determine the dielectric constant for a particular board in order to properly use that board. Technically, we are seeking the relative dielectric constant (ε ); i.e. the value relative to the r dielectric constant of a vacuum. But we here just refer to it as the dielectric constant (ε). The dielectric constant is sometimes referred to in the literature as E , K or Dk. r A related quantity, relative permittivity, contains a real and imaginary part, the real part of which is the dielectric constant and the imaginary part of which represents a loss factor. The ratio of that loss factor to the dielectric constant is the dissipation factor, also known as tan(δ). Here we are concerned primarily with the dielectric constant, though the dissipation factor, which has an effect on the Q of resonance, also has some relevance. We here present a largely non-destructive way to measure the dielectric constant for a particular board, using measurements of resonant frequencies. But first we present a simple method using capacitance, which is useful in some circumstances. All measurements were made with a build of Scotty’s Modular Spectrum Analyzer, also known as the MSA, whi