July 31, 2015
Design and production engineers have become more reliant on intermodulation distortion (IMD) measurements due to the highly sophisticated modulation techniques used in today’s RF and microwave amplifiers. Traditionally, IMD products are measured at various points in the spectrum with spectrum analyzers, which have also been used to measure amplifier harmonics and spurs. The added complexity of today’s high-speed designs and subsequent advanced measurement requirements have led to vector network analyzers (VNAs) now being a superior alternative. In today’s post we will discuss how enhancements have been made to VNAs to more accurately, quickly, and cost efficiently conduct IMD measurements.
Identifying third order – and higher – intermodulation distortion products (IPn) is as important as quantifying harmonic and second order distortion products because IP3 products are located in-band and cannot be easily filtered. The Third Order Intercept Point (TOI or OIP3) of power amplifiers is an important metric to quickly identify the intermodulation characteristics of the active device. During the design process, engineers often use measured IPn data to characterize devices at the chip level. In production, vendors often need to specify and measure the OIP3 performance of their amplifiers. Using a VNA to measure these parameters is more time- and cost-efficient, as it can replace multiple instruments.
Simplified IMD Measurements
To improve test time and cost, solutions such as IMDView™ from Anritsu (figure 1) have been developed to help design and production engineers validate their amplifiers. IMDView provides a graphical user interface (GUI) for setting up the IMD measurement to simplify measurement complexity and control the VNA hardware for the wide range of measurements.
Figure 1
Numerous tools to help configure the IMD measurement are designed into these software products. In addition to setting up individual trace responses, the software provides extensive trace and channel management for easy display optimization. The software also tracks the frequency bands of interest, as well as the power levels required of the two tones. During power calibration, the software automatically switches the paths during the procedure to provide calibrated tones at the DUT input.
Improvements in trace and channel management provide easy optimization of complex measurements. Engineers can set the VNA in an Active Channel Only mode when multiple channels are configured. This will update the display of only the active channel to provide the best measurement speed for the designated channel. If multiple response traces are configured, a menu can provide the ability to activate only the desired traces, as needed. Engineers can use the software to modify input parameters and observe IMD results immediately without switching screen displays.
Hardware Helps, Too
Software is only one way VNAs have been enhanced to simplify IMD measurements. Dual source and combiner hardware can be integrated into a VNA and provide the ability to switch from S-parameter measurements to IMD measurements. The internal combiner and switch option transfers the path of the second source to the Port 1 side of the VNA. During S-parameter measurements, the switch is set for the normal path to perform standard tests, such as S-parameter, harmonic and gain compression, of the active device. When a two-tone IMD measurement is required, the IMD software routes the switch to combine the two internal sources for a two-tone input to the DUT. With the combiner option, single-connection IMD measurements can be made with the VNA without the need to re-cable the measurement setup. The result for engineers is shorter test time and improved overall accuracy because concatenation errors of multiple calibrations and system setups are reduced.
VNA Architecture
Of course, these enhancements are only good if the base VNAs in which they are added can deliver the necessary performance. Fortunately, a new generation of VNAs has been designed with Non-linear Transmission Line (NLTL) architecture that is advantageous in IMD applications. For example, the receiver provides exceptional linearity while maintaining very low noise floors even at millimeter-wave (mmWave) frequencies, as well as strong IP performance, all of which are critical for acquiring accurate IMD analysis.
VNAs with this architecture can provide receiver IP3 performance in the +35 dBm range while maintaining strong performance even at narrow tone deltas. In fact, in combining the NLTL-based receiver with unique source control capabilities, such as common offset mode, there is minimal difference in IP3 performance when measuring under wide or narrow tone deltas, as shown in figure 2. This is particularly advantageous for design engineers who need to stimulate the DUT under a wide range of modulation conditions for the greatest possible confidence.
Figure 2
A VNA that offers independent source and receiver control gives engineers the ability to set the receiver at programmed frequencies and measure pre-determined points in the spectrum. Source and receiver power calibrations provide the opportunity to calibrate for absolute power. The result is that there is no need for power meters and spectrum analyzers to conduct a full suite of amplifier measurements.
A key parameter in performing IMD measurements is measurement speed because each IMD trace may require multiple sweeps to gather all information necessary for response plotting. Again, the unique NLTL architecture provides benefits.
Conclusion
IMD measurements have become critical to validate the design and performance of high-speed amplifier designs. Software and hardware advances in VNAs make it more cost- and time-efficient when conducting these tests, helping engineers lower test costs and improve productivity. You can learn more about conducting IMD measurements by downloading this free application note.
