Active mmwave Load Pull System

Today, mechanical tuners are suitable up to a maximum frequency of about 110GHz. In this product range, the automatic tuners by Maury-Microwave achieve an exceptionally high quality.

However, due to the ohmic losses of the HF line between probe and tuner, only the inner area of the Smith Chart is useable as load impedance with this type of tuner. For higher mmWave frequencies, for example in the WR-2 waveguide band for 325-500GHz, the losses caused by commercially available probe tips are substantial; this means that the use of mechanical tuners for these applications is not advisable.

From a technical perspective, the optimum method for conducting load pull characterizations in the high-frequency range is an active load pull process. We are able to offer a complete mmWave load pull measurement system, consisting of the following components:

Required basic devices:

  • Network analyzer with mmWave modules for a waveguide band (e. g. mmWave extender modules by Virginia Diodes). The maximum frequency is 1100GHz (WR-1 waveguide band); 2 complex ports are required
  • mmWave power meter (the most suitable instrument is the PM5 by Virginia Diodes)
  • Waveguide calibration kit plus calibration substrate, if the measurement is to be performed with semiconductor probes
  • DUTs with waveguide connections may be contacted directly; for semiconductor wavers, a probe station is required (e. g. by our supplier, Semiprobe)
  • Optional: Power supply for the DUT (e. g. SMUs with remote control by Keysight)

 Add-on devices and und SW for active mmWave load pull:

  • Add-on modules (2 per complex 2-port-system) with PXI control unit. These modules are suitable for all waveguide bands from WR-15 (50-75GHz) to WR-1 (750-1100GHz)
  • Control and measurement data acquisition software, "mmWave Studio", with SW modules for precise power calibration over a large dynamic range and for controlling the power supply. Similar to the add-on modules, this SW is suitable for all waveguide bands from WR-15 to WR-1

Figure 1 shows an example of a typical 2-port measuring station with a probe station, suitable for characterizing semiconductors with WR-10 extender modules in the 75-110GHz range. If measurements in a different waveguide frequency band are required, both extender modules have to be exchanged. The add-on system can be used without modification. Only the settings for calibration and measurement in mmWave Studio have to be adapted accordingly.

Figure 1: Active 2-port mmWave load pull measurement system, including network analyzer, mmWave extender modules and add-on modules. In this picture, the PXI frame for controlling the add-on modules is hidden underneath the network analyzer.

The following figure shows the user interface of the "mmWave Studio" software. The measuring station can be easily set up and calibrated with the following menu-driven procedure:

  • Step A: Connect the measuring instruments (network analyzer, power meter, power supply). The interfaces for the connection are generally GPIB and USB. The PXI frame is connected with an appropriate interface card by NI (National Instruments).
  • Step B: Calibrate S parameters and power on both waveguide ports.
  • Step C: Calibrate the probe tips by means of the calibration substrate.
  • Step D: Contact the DUT and select the appropriate parameters for the automated measurement (mmWave input power, load impedance range, supply voltage).
  • Step E: Start the measurement and store the measurement data.
  • Step F (optional): Evaluate the measurement data, e. g. by inserting load circuits into the Smith chart.
Figure 2: Controller Software GUI (example)

Figure 3 shows an example measurement of a HF transistor. A "power sweep" was measured for each selected load impedance (this corresponds to input vs. output power). In this case, the maximum available power of the extender module determines the achievable impedance range in the Smith chart. When purchasing the extender module, it is therefore imperative to make sure that the mmWave power is sufficient for your DUTs. We provide support for project planning tasks and for selecting the appropriate module.

The measurement data can be conveniently evaluated by marking the individual measurement curves with a simple mouse click. All measurement values which are of interest to the developer of an mmWave amplifier are shown: load impedance and mmWave power in the Smith-Chart, S parameters, power sweep measurement data, efficiency including the power supply. It is also possible to implement negative load impedances: in this case, the mmWave power that is fed back into the DUT is higher than the output power of the DUT. In order to protect the DUT, setting up negative impedances is prohibited by software in the current basic configuration of our systems. Depending on the use case, we can however adapt the load pull system to your specific requirements and also offer additional measuring options.

Figure 3: Measurement results for a transistor at 90GHz