Custom and Production
Isolators and Circulators

DROP-IN TEST FIXTURES

At Sonoma Scientific, all of our drop-in style units are designed and tested using quality 50W microstrip fixtures. The actual physical characteristics of our fixtures vary based on frequency of their usage and the tab requirements of the devices being tested.

A common problem in making measurements in microstrip media is the need to separate out the effects of the transmission medium from the device characteristics. At microwave frequencies, systemic effects such as leakage, test port mismatch and frequency response will affect measured data.

In microstrip measurements, it is difficult to build impedance standards that are easily characterized. For example short circuits are inductive, open circuits radiate energy and it is difficult to build a high quality purely resistive load. Typical problems in determining the parameters of the device being tested and having a significant effect on measured data are:

• Impedance discontinuities occurring at the coax to microstrip launch. The reflection at this point is typically larger than that of the device interface.
• The signal is attenuated in both the coax and microstrip portions of the fixture.

At Sonoma Scientific, we prefer to use a calibration method known as "LRL" as opposed to using a standard 12 term calibration. The LRL (Line-Reflect-Line) provides an enhanced capability for error compensation when making a measurement in microstrip, and currently provides the highest accuracy in measurements available today.

The LRL calibration technique uses the characteristics of a length of transmission line as the calibration standard. A full LRL calibration requires two transmission line measurements, a high reflective measurement and an isolation measurement. Taking into account that there must be adequate separation between the coax/microstrip launches, (a minimum of two wavelengths are recommended), to avoid unwanted variations in error correction caused by the coupling of higher order modes. Since microstrip is a dispersive media, the test equipment Sonoma Scientific uses, applies dispersion compensation using specific microstrip parameters. These being:

• Characteristic Impedance (Reference)
• Effective Dielectric Constant Z_c
• Substrate Dielectric Constant
• Thickness of the Substrate
• Width of the Trace