Keysight Technologies (formerly Agilent Technologies) develops world-leading equipment for solving tough measurement challenges. The company’s Infiniium 90000 Q-Series oscilloscope is the first to reach the 60 GHz barrier, enabling engineers to make measurements on a new generation of fiber optic transponders and systems that provide higher levels of data communication speeds than previously possible.

High-performance oscilloscopes are capable of measuring signals at very high frequencies and are primarily used in high-speed serial communications, radio frequency, radar, aerospace, and other high-speed physics applications.
To ensure the oscilloscope could set a new standard for measurement accuracy, Keysight needed to develop a new electrical calibration source. The major challenge was cooling the package for the calibration head. To address this challenge, the engineers tried a cross-flow heat exchanger approach and turned to ANSYS® CFX® computational fluid dynamics (CFD) software to model and simulate this new design configuration.

The engineering team determined that the standard fan and exchanger configuration was not compatible with the connector placement on the front of the instrument that the head is used to calibrate. The team considered the alternative of a cross-flow heat exchanger design in which the airflow is perpendicular to the face of the heat sink. Since the wrapped flow configuration was previously unproven, simulation is critical in optimizing the design. It would take six to eight weeks to build the physical prototype parts, so a failed initial prototype design would lead to budget and schedule overruns.

Using CFX within the ANSYS Workbench™ environment, the Keysight team evaluated several design variations to analyze the cross-flow configuration. The primary concern was that the flow rate of the blower would be reduced due to the pressure drop associated with redirecting the airflow over the heat sink. The engineers created a proposed design and used CFD to analyze the pressure drop at various flow rates, reasoning that the pressure drop could be reduced by increasing the heat exchanger channel area. Subsequently, the team built a heat transfer model using the airflow predicted by the first simulation as a mass flow input. The heat transfer simulation showed that the case temperature rise was well within the 15 °C design specification. After constructing a crossflow prototype based on the latest design configuration, physical measurements show that the heat sink rises in temperature by 9.5 °C. This perfectly matches the results predicted by the CFX simulation of the prototype, and provides validation that built confidence in the simulation methodology.

Heat transfer results showing contours of temperature. Red shows lower temperature and blue higher temperature.
The simulation results helped Keysight engineers convince management that the cross-flow design would deliver the desired results. Keysight oscilloscope customers demand worldclass measurement accuracy to enable emerging technologies. In producing the 90000 Q family of oscilloscopes, the Keysight design team needed to develop a superior calibration source to ensure the 90000 Q could deliver the industry’s highest real-time bandwidth, lowest noise, and best jitter performance. In designing the N2806A calibration source, the Keysight R&D team utilized ANSYS CFX computational fluid dynamics software to simulate the environment and optimize the design. As a result, the team successfully produced a highly robust solution that exceeded the requirements for thermal stability, signal integrity, and usability. Success was achieved on the first prototype, and this helped Keysight become the first oscilloscope manufacturer to break the 60-GHz real-time bandwidth barrier and enable a new class of ultra-accurate, high-bandwidth measurements.

This article was written by Matt Richter, Expert R&D Engineer, and Brad Doerr, R&D Project Manager, at Keysight Technologies. For more information visit www.ansys.com/Products/Simulation+Technology/Fluid+Dynamics .