Commonly used DEFINITIONS

Calibrations in a Load Pull System
The term “Calibration” or “Calibration File” is used throughout the L/P system with different meanings:

Network Analyzer (VNA) Calibration

Using a set of coaxial or waveguide standards and VNA internal software we determine the Calibration Coefficients, that allow us to make measurements at reference planes other than the VNA internal ports. The best, most accurate and efficient, calibration technique with regard to Load Pull and programmable tuner operation we found to be Thru-Reflect-Line (TRL).

A VNA calibration file contains all error terms for all calibration frequencies (up to several hundred frequency points)

Because of drift, VNA calibration should be repeated frequently; depending on the VNA model several times a week, or even every day.

Tuner Calibration

This is in reality a “tuner characterization” procedure, in which the tuner two-port S-parameters are measured using a, previously calibrated, VNA at a certain number of mechanical positions of the tuner probes (=tuner states) for one given frequency fo (and possibly its harmonics 2fo and 3fo). The tuner calibration files are saved on hard disk and contain a table with S-parameters as a function of motor (or tuner probe) positions, one file per frequency. Reliable results are obtained when the VNA is calibrated using TRL. VNA calibrations using SOLT method do not yield acceptable tuner data above 3-4GHz.

The tuners should be recalibrated only once every few months or if they have been dis- and re-connected frequently.

Setup Calibration

This is a characterization of all component two-ports included in the load pull setup; it is done using a VNA. All 4 S-parameters of each component are measured and saved in [.S2P] type ASCII files on hard disk.

Setup calibrations can be used for unlimited time, again subject to limited connect- disconnect operations.

Test Fixture Calibration

This procedure uses, in general, a custom-made TRL method to characterize the two halves of the test fixture. FOCUS’ own TRL algorithm, which is part of the basic software package, uses simple standards (Open (Short) – Thru – Line) and allows full test fixture characterization, not assuming symmetrical fixtures and allowing for quarter lambda transformers and bias networks to be included on the fixture. Test fixture calibrations are saved on hard disk as [.S2P] ASCII files and can be used indefinitely.

Wafer Probe Calibration

The wafer probes and the cables connecting them to the tuners are treated by FOCUS’ calibration software the same way as an ordinary test fixture (see above).

In-situ Tuner Calibration

This is a technique that allows us to characterize both fundamental and a single harmonic tuner at fo, 2fo and 3fo, without removing them from the setup . In order to do so we must calibrate the VNA at the input and output port reference plane of the setup and then “de-embed” to the tuner reference plane. This option is standard with the basic FOCUS software. However Isolators can not be part of the calibrating section of the setup, because they are not reciproque.

8-Term Calibration

A technique used in VNA calibrations for on-line travelling wave measurements through external couplers in load pull setups, whereby the tuners are connected between the couplers and the load/source and the DUT is connected between the couplers. Focus uses this method in its Hybrid (active/passive) Harmonic load pull system: HAILP.