A tuner or variable matching network or variable impedance transformer is used when the standard 50 ohm source or load impedance needs to be changed. We talk about load (source)-pull when we are examining the performance of an amplifier or a single device in the dependence of the load (source) impedance. So a tuner is required to perform these measurements.
A mechanical, passive slide-screw tuner is a uw/RF transmission-line structure with a deliberate disturbance or probe in it. By varying the depth of the probe ('Y') we largely vary the magnitude of the reflection coefficient as it is seen when 'looking' into the tuner. When the probe is moved along the transmission line ('X'), mainly the phase of the reflection coefficient is changed.
Typically a tuner is characterized, at the operating frequency, simply by collecting the 2-port S-parameters versus a large number of mechanical positions, 'X' and ‘Y’. In order to correct e.g. the power measurements for the losses of the output tuner, the full 2-port S-parameters of the tuner are needed in the calculation.
There are different methods to drive an automated tuner in terms of mechanical positions or electrical parameters and obtain the electrical data (e.g. loss) from the database:
- ‘High level control’: Through a complex measurement control program like IVCAD or ATS. <links>
- ‘Low level control’: Through MS-Windows link libraries (.dll) that you can link to a user program. This feature is available in almost every program environment like LabView, VEE, Visual-basic, Matlab, Phyton, etc.
- ‘Direct control’: Most tuners from Maury Microwave have a TCPIP interface and are completely LXI compatible. Also you can control them via the USB interface which is TMCUSB compliant. In short that means the tuners can be controlled with simple and intuitive (SCPI alike) commands, just as you would control a modern multimeter or a power supply! The tuners have all the intelligence on-board to handle the commands to determine the mechanical positions when you command it to go to a certain reflection coefficient (e.g. for command “TUNe 0.7 120”). Also the tuners have a web-interface through which you can follow the remote control and there is even a Smith chart in wich you can click to move the tuner to a certain impedance.
The tuner characterization to come to a database of tuner S-parameters versus various positions is always necessary. Compare this with the user-calibration of your Vector network Analyzer! In all methods, so called interpolated tuning is possible. This means that tuning is not only restricted to the characterization points in the database, but can be anywhere in between. Also (de-)embedding S-parameter files can be included between your DUT and the tuner and between the tuner and the termination/source. Also the termination/source (with a 1-port S-parameter file) can be included in the tuner calculations.