But the question remains, how do you actually determine the impedance of that device you are trying the match? You may also like: The tuner is calibrated at each frequency in the manner of an ordinary wideband tuner, resulting in well-behaved calibration patterns for amplitude and phase Fig.
However, as operating frequencies reach into the tens of gigahertz, the subtleties of impedance measurement and matching become more critical to overall design success and efficiency. The slightest imperfections or mismatch in cables References 3 and 4 , fixturing, and many other factors can result in results which are obviously incorrect or worse, look correct but are not.
For example, the engineers and scientists at the National Institute of Standards and Technology NIST have extremely clever advanced ways to independently check the results which commercial instruments provide, leading to traceable results and qualification of those instruments. Traditionally, load-pull tests were performed manually, as there was no alternative. Although most RF systems are designed for nominal 50-ohm input and output impedance and 75-ohm for cable TV , the reality is that many components do not present that preferred 50-ohm impedance due to the unavoidable realities of their design, device physics, packaging, and construction.
PDF Download. University of California at Santa Barbara. Modern PA designers require the most accurate possible transistor data for a wide range of RF and DC parameters as functions of DC bias, RF impedance at different frequencies , source power, and other variables.
The tuners, which are compact enough for positioning with wafer-probing equipment, provide a high reflection coefficient at the reference plane of a device under test DUT , whether in a test fixture or on wafer.
Therefore, the Smith chart and other tools must be used to create an optimal matching network. The corresponding frequency, axis positions, setup and tuner losses and actual impedance seen by the DUT are automatically switched and displayed on a Smith Chart.
Typical microwave transistors are stable when loaded with an impedance of 50 omega at one of their ports. In addition to issues of accuracy and validity of the result, these tests were time-consuming and often frustrating.
Why is knowing this important? Oscillations most often occur when a DUT is presented with a high reflection coefficient; oscillations will start at frequencies where the phase of the DUT and the tuner satisfy the oscillation condition.
Impedance matching between RF stages has always been a priority to ensure maximum transfer of RF power along with minimal RF-power reflections. The microcontroller autonomously determines the positions of the three probes required to present a given reflection coefficient at the DUT reference plane. The complex setup is expensive and tedious to calibrate and operate, especially when running on-wafer measurements.
A full harmonic load-pull measurement system for testing at fundamental, second-harmonic, and third-harmonic frequencies incorporates DC sources, RF signal sources, power meters, driver amplifiers, passive signal-coupling components, analyzers, and six impedance tuners Fig.