September 29, 2015
With all the talk surrounding 5G one would think that it is right around the corner and in some respects it is. Engineers have seen this before – i.e. 2G, 3G, and 4G. So, many of us understand that while it will take years for the technology to reach the consumer, R&D work is well underway to achieve the widely accepted goal of a 2020 rollout similar to what is depicted in figure 1.
Though 5G standards are still being finalized, general purpose instruments such as Vector Network Analyzers (VNAs) are already being used to measure early stage 5G designs. That is because VNAs do not have to be constructed around waveform standards, as they don’t demodulate the signal. Rather, VNAs detect complex envelope variations.
Engineers working on 5G designs use VNAs to conduct active device measurements on power amplifiers, characterize devices during modeling, and perform signal integrity measurements on backplane connection designs. These may sound commonplace, as they are routinely done today for LTE designs, however the significantly higher modulation rates and increased linearity demands associated with 5G change the rules of the game. VNAs must have a series of advanced specifications in order to conduct accurate modulated measurements.
Typical Measurement Sequence
Engineers typically use a VNA to measure linear S Parameters and conduct gain compression measurements to verify designs. In the second scenario, the VNA needs to be calibrated for absolute power, so that an accurate power sweep of a CW signal can be made to precisely identify the linear and non-linear regions of the device under test (DUT). In a typical measurement sequence, the DUT will be analyzed for intermodulation using a two-tone CW signal.
A typical IMD measurement using a VNA can provide very accurate analysis of the intermodulation properties of the DUT and is commonly used to raise the performance to a high level. While this measurement has been performed with a spectrum analyzer, modern VNAs offer the ability to accurately measure the IMD performance without switching to a different test instrument. A VectorStar® model takes this one step further by providing internal combiners and switches to offer automatic single connection measurements from S Parameters, gain compression, and IMD, and with higher levels of correction to improve accuracy.
The next challenge is to analyze the DUT performance in a real-world condition and determine if it will conform to the ACPR specifications when stimulated with a digitally modulated signal. Until now, typical VNAs did not provide enough instantaneous bandwidth to characterize DUTs under real-world conditions. For example, the analyzer needs an IFBW that is wide enough to capture the bit rate to detect envelope variations of the modulated signal. The IFBW of most VNAs limits instantaneous bandwidth to <10 MHz. This is not enough for comprehensive and accurate measurements under modulated conditions.
Fortunately, VNAs such as VectorStar are evolving and can be configured to meet the measurement requirements associated with 5G designs (figure 2). These analyzers can be designed with 200 MHz of instantaneous bandwidth to explore device characterization under modulated conditions. To achieve this performance, the conventional VNA architecture includes a wideband receiver rather than the narrowband mixers commonly found.
This new generation of VNAs incorporates Nonlinear Transmission Line (NLTL) samplers among other improvements including those to the IF systems. NLTL-based samplers are configured to provide scalable operation characteristics and a number of performance benefits. NLTL-based VNAs don’t have the bandwidth limitations associated with traditional architectures and can give engineers greater confidence in their 5G designs.
The wider bandwidth allows the test and reference receivers to capture modulated waveforms, and the complex analysis capabilities of the instrument can be used to identify error terms and provide calibration capabilities. It provides enough modulation bandwidth to accurately monitor the upper and lower channels for ACPR and often other spectral regions of interest. As a result, VectorStar allows the user to move from CW S-Parameter measurements, to gain compression measurements, to IMD measurements, and then confirm ACPR performance under a modulated condition without switching instrumentation.
Measurements using VNAs, VSAs, and Spectrum Analyzers
VNAs with this type of architecture and performance are a superior choice than other general purpose instruments, such as a Vector Signal Analyzer (VSA) or spectrum analyzer. Unlike these instruments, the multiple-receiver design of the VNA makes it better suited for calibration and error correction while offering 14 bit dynamic range all the way to millimeter-wave range frequencies. Plus, VSAs and spectrum analyzers are prone to significant errors due to uncorrected raw receiver mismatch, and their linearity and accuracy deteriorate as carrier frequency rises above 26.5 GHz.
In our next post, we will outline how to conduct wideband characterization of ACPR performance of PAs for millimeter-wave frequencies using VNAs that have these advanced performance and features. 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.