A mechanical load-pull tuner is complicated name for a simple device: it is an open transmission line that can be perturbed by the presence of metal “plungers” that can move in between the conductors of the transmission line to create a localized reflection. As these plungers can also move along the line, a variable impedance matching can be realized. Connecting these variable impedances to input and output of a non-linear gain block allows for the determination of optimal matching conditions in large signal operation, which is a must for power-efficient device design.
As an impedance tuner is a mechanical device, it is extremely slow when compared to the speed of the RF amplifier: positioning of the tuner to synthesize an impedance requires a delay of a few seconds while the time constants of the amplifier are at least thousands of times faster. To properly cover the Smith chart with test impedances for a single device therefore requires tens of minutes of measurement time. As load pull testbenches are extremely expensive to acquire and to operate, this measurement time is a serious roadblock for industrial characterisation labs.
For an instrument 2.0, this is clearly way too slow to be acceptable. To reduce the timeframe, the instrument 2.0 will use the knowledge that the device is fast, while the load is slow. The cycle of moving the load, then wait for stable configuration, then measure is therefore replaced by a continuous measurement during the movement of the load. This requires the design of a smooth, low vibration movement controller for the tuner and an in-line data processing capability that monitors the measurement accuracy during the measurement and adapts the trajectory of the tuner accordingly.
To tackle this engineering challenge, you will
- assess the influence of the current stepper motor vibrations on the stability of the impedance
- design a low-vibration drive to improve stability and control of the RF impedance while moving the chariot and the plunger
- design a mechanical trajectory that covers the impedance Smith chart optimally in a minimal time
- use advanced signal processing techniques to control the measurement errors that could be induced in the behaviour of the amplifier by the load variation, and limit those to obtain measurements comparable to the slow scanning technique