HP 8753E manual How !CRL*/LRM* Calibration Works, TRL* Error Model

How !CRL*/LRM* Calibration Works

The TRL*/LRM* calibration used in the HP 8753E relies on the characteristic impedance of simple transmission lines rather than on a set of discrete impedance standards. Since transmission lines are relatively easy to fabricate (in a microstrip, for example), the impedance of these lines can be determined from the physical dimensions and substrate’s dielectric constant.

TRL* Error Model

[SAI

8 Error Terms

pm 2cd

Error Adapter *

Figure 6-52.

HP 8753E functional block diagram for a 2-port error-corrected

measurement system

For an HP 8753E TRL* 2-port calibration, a total of 10 measurements are made to quantify eight unknowns (not including the two isolation error terms). Assume the two transmission leakage terms, Ezr and EXR, are measured using the conventional technique. The eight TRL error terms are represented by the error adapters shown in Figure 6-52. Although this error model is slightly different from the traditional Full 2-port 12-term model, the conventional error terms may be derived from it. For example, the forward reflection tracking (Em) is represented by the product of ~10 and &O1. Also notice that the forward source match (Esr) and reverse load match (ELR) are both represented by ~11, while the reverse source match (EsR) and forward load match (ELF) are both represented by &22. In order to solve for these eight unknown TRL error terms, eight linearly independent equations are required.

The first step in the TRL* 2-port calibration process is the same as the transmission step for a Full 2-port calibration. For the thru step, the test ports are connected together directly (zero length thru) or with a short length of transmission line (non- zero length thru) and the transmission frequency response and port match are measured in both directions by measuring all four S-parameters.

For the reflect step, identical high reflection coefficient standards (typically open or short circuits) are connected to each test port and measured (%I and S22).

For the line step, a short length of transmission line (different in length from the thru) is inserted between port 1 and port 2 and again the frequency response and port match are measured in both directions by measuring all four S-parameters.

Application and Operation Concepts 6-93