December 16, 2015
In a previous VNA Reflections post we discussed how a VNA with enough instantaneous wideband performance could be used to analyze PA performance under real-world modulated conditions. The advantage of using a VNA for PA measurements is the accuracy gained when compared to other single receiver instruments, such as spectrum analyzers (SPAs) and vector signal analyzers (VSAs). VNAs that incorporate Nonlinear Transmission Line (NLTL, a.k.a. shockline) samplers may not have the bandwidth or linearity limitations associated with traditional architectures and can give engineers greater confidence in their 5G wideband modulated designs.
VNAs can provide excellent measurement accuracy due to an architecture centered on error correction and calibrations. Without error correction, instruments performing high frequency measurements can have mismatch measurement uncertainties in the 1-2 dB or higher range. VNAs using error correction reduces system mismatch errors to tenths of a dB by correcting for DUT output and system mismatch, directivity, and other systemic errors. In addition, an important requirement of a VNA is to provide ratioed S parameters and other measurement needs. A VNA performs ratioed measurements through the use of multiple receivers and this allows increased stability by taking into account the drift of any driver amplifiers that may be used. Diagram 1 illustrates the basic configuration of a VNA with typical four receivers for calibration, error correction and ratioed measurements.
Diagram 1: Basic VNA configuration
High Speed IF Digitizing
The VNA source in traditional systems provides a CW signal and, consequently, typical VNA receivers are optimized for CW measurements. The widest IF bandwidth capabilities in many VNA receivers are often limited to 10 MHz or less due to expectations of CW reference signals. For wider IF bandwidths, high-speed ADCs are required. For example, the Anritsu VectorStar® has a High Speed IF Digitizing option that allows IF bandwidths of 200 MHz. Diagram 2 illustrates how the IF digitizer option can be used to capture a digitally modulated signal for VNA analysis.
Diagram 2: Block diagram of VNA with high-speed digitizer option
Note that because of the inherent VNA error correction of a solution such as VectorStar, the captured signal is error corrected for minimal mismatch uncertainty. This is critical when measuring a PA since the return loss of the PA may be quite poor and can contribute to high levels of uncertainty if uncorrected.
In comparison, the architecture of a single receiver instrument (diagram 3) without error correction can result in uncertainties in the 1-2 dB range rather than the tenths of a dB range of a VNA and perhaps multiple dB more if driver amplifier drift is not handled appropriately.
Diagram 3: Typical block diagram of a VSA/SPA
Calibration
A VNA provides a choice of calibrations to accommodate a range of possible DUT scenarios such as connector type or devices on-wafer, multiple frequencies, multiple ports, and others. The VNA can perform calibrations from a simple normalization with no port match correction to full error correction of the DUT mismatch. Single receiver instruments are often challenged to perform a simple normalized reference.
The following chart provides a comparison between a simple normalization and a VNA error corrected measurement, both under a modulated condition. One can appreciate the differences gained in measurement accuracy when performing the measurement using an error corrected VNA. An improvement of a few dB in measurement accuracy can make a substantial difference in verifying PA performance for measurements such as ACPR and, consequently, network performance. The ability to perform accurate VNA measurements of PAs under real-world conditions is something that has been long sought and will offer an opportunity for advancements in RF measurements.
Conclusion
In our next post, we will discuss further additional benefits of characterizing PAs under modulated conditions and how that might further improve device characterization. To learn more about the NLTL VNA architecture and how it can be helpful in high-speed designs such as 5G, download our free white paper.
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