Why load pull a power device?

The performance of an active device is a function of many things:
Frequency
Bias point
Temperature
Source/load impedance at fundamental frequency
Load impedance at harmonic frequencies
Power level

When you measure active devices on a network analyzer, you are looking at the small-signal response in a fifty-ohm system, as a function of frequency and bias point, and perhaps temperature if you are fortunate enough to have a temperature controller. Using linear CAD software you can accurately predict the small-signal response if the device sees impedances other than fifty ohms. It's more difficult to predict performance under large-signal conditions. Perhaps you can obtain a large signal model of your power device, or use Steve Cripps method for predicting saturated power performance. But there are limitations to each of these methods; large-signal models are notoriously inaccurate. This is where load pull comes in--it can be employed to empirically gather all of data you need to design a power amplifier and predict its large-signal responses, including compression characteristics, efficiency, harmonics and intermodulation products.

To review large-signal data, most of the plots that are of interest will be on Smith charts. Typically you examine data one frequency at a time, by plotting contours of constant output power, gain, efficiency, etc. The contours look like potatoes, a minor irritation if you are on a low-carb diet. Of course, you can also plot Pin versus Pout on Cartesian coordinates, and now you can do it at the "sweet spot" where power is maximized.


Load pull bench block diagram

Below is a simplified system block diagram that shows most of the necessary components of a load pull system that can measure both CW and two-tone (intermodulation) signals on a transistor. The DUT resides in the middle, surrounded by microwave tuners, then bias tees (this assumes that the tuners provide a DC path from one port to the other, which is not always the case). One component that is absent from the block diagram is an RF probe station (probes plus coax cables) which is used to make contact to the device. Note that the loss of all components between the DUT and the tuners will affect the maximum reflection coefficient to which you can load-pull, so you want to carefully choose low-loss components in this case.




A pair of tuners are used. The input-side tuner allows the source match to be "pulled" to an impedance where the device has appreciable gain, then it is generally left at this fixed impedance for all measurements on a device at a fixed frequency. The output-side tuner is the one that gets a work-out!

The test-set extender interfaces the DUT to a suite of measurement equipment that makes up the rest of the test gear (this is where Auriga's expertise shines). A signal generator (or perhaps two for two-tone measurements), and quite often a power amplifier, a coupling network and power meter, comprise the input network. The output interface of the test set extender may include a high-power attenuator depending on the available power of the transistor, then perhaps a power meter and/or spectrum analyzer. A network analyzer is also used to measure input/output response. All of the test gear inside of the test set is hooked up with electrically-controlled RF switches which are extremely repeatable. For convenience, system-controlled power supplies and DC current meters are part of the setup; without these you would have to calculate efficiency by hand, and who wants to do that for 1000 measurements that you can make in one hour?

Of course, the entire bench is controlled by easy-to-use software running on a dedicated computer. Once the setup is calibrated and the device installed, the operator interface is entirely by computer. Grab a donut and let's go!


Calibration and verification procedures

Calibrating the load pull station is far more complicated than a normal S-parameter measurement, and can take several hours. A vector network analyzer must be dedicated to the setup, because the S-parameters of all of the components must be be recorded, over the frequency band of interest. This is not that big a deal for the cables, bias tees, probes and the stuff behind the test-set extender, but for the tuners, hundreds of tuner positions must be measured and stored. No question, the tuners are the heart of the system, and the accuracy of the measurement is most affected by the repeatability of the tuners.

Calibrating a load pull system starts with some basic setup information: what frequency band (start/stop/step size?) What area of the Smith chart? What input power levels?

Once a load pull system is configured and calibrated, its performance should be verified. This can be accomplished by measuring something that is linear. A "thru" turns out to be a very good verification standard and by measuring it we can verify that we can make accurate measurements of device gain. Absolute power is verified by putting a power meter directly onto the output of the input tuner to verify input power at the DUT, and directly on the output of the DUT to verify output power. This is not practical for a device that is on-wafer, but demonstrating such verifications at the tuner's coax (or waveguide) interface shows that the methodology is sound.

Richard: From internet, respect to the original authors.