What are the frequency limitations on the various probes utilized with the Agilent 85070E Dielectric Probe Kit and why?
The various 85070E Dielectric Probe Kit probe types and frequency ranges are:
|Probe Type||Option Number||Frequency via Network Analyzer||Frequency via E4991A Impedance Analyzer*|
|High Temperature||020||200 MHz to 20 GHz||10 MHz to 3 GHz|
|Slim Form||030||500 MHz to 20 GHz||500 MHz to 3 GHz|
|High Performance||050||500 MHz to 50 GHz||500 MHz to 3 GHZ|
*The E4991A maximum frequency is limited to 3GHz.
The high frequency cap is due to the dimensions of the coaxial line and the requirement to not propagate higher order modes.
Materials measurements at low frequencies can become less accurate due to the fact that network analyzers measure reflection coefficient Gamma directly. The accuracy of the analyzer is expressed as a certain percentage of Gamma, both magnitude and phase. As shown below, the measured reflection coefficient Gamma corresponds to the complex dielectric constant of the measurement material:
Additional detailed information pertaining to the definition of permittivity is available in the application note, Basics of Measuring the Dielectric Properties of Materials (in Related Links below).
In other words, there is bidirectional mapping of the dielectric constant, epsilon sub r, and the reflection coefficient, Gamma. This illuminates how a polar display of Gamma is transformed into values of dielectric constant and loss tangent. The figure below presents the mapping of the complex dielectric constant epsilon sub r of the material into a polar reflection coefficient plot for two frequencies, 200 MHz and 2.45 GHz. The 200 MHz plot depicts a random Gamma vector and on the tip of the vector, there is a circle, representing the uncertainty (not to scale). Compare the display at 200 MHz and 2.45 GHz. At 200 MHz, a wide range of dielectric constant is compressed into a small area of the polar plot. This implies that a small percentage uncertainty for measuring reflection coefficient now becomes a large percentage error for dielectric constant or loss tangent. The dielectric constant error at 200 MHz is much larger than the error at higher frequencies. This explains the start frequency limitations on the probe specifications.
In addition to the Agilent family of network analyzers, the 85070E software supports E4991A impedance analyzer. Impedance analyzers operate on different principles as compared to a ratioed vector network analyzer. The E4991A RF Impedance Analyzer measures impedance directly via an RF-IV methodology. The RF-IV based impedance analyzers do not measure reflection coefficient Gamma. Thus, the E4991A is able to produce better results at low frequencies and the dielectric measurement is specified down to 10 MHz when using the high temperature dielectric probe. Impedance analyzers do not exhibit the same advantage at higher frequencies.