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Faster Testing with High-Performance Spectrum Analysis in a VNA

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Faster Testing with High-Performance Spectrum Analysis in a VNA

Whether engineers are designing, developing or testing a new component, they will often use a network analyzer and a spectrum analyzer to fully understand the device under test (DUT). A vector network analyzer (VNA) measures or calculates informative responses such as Sparameters, gain compression and noise figure. A spectrum analyzer (SA) provides additional insights about the presence of harmonically related signals, non-harmonic spurious signals and more. During a typical troubleshooting session, the need to frequently connect, disconnect and reconnect the DUT to the VNA or SA will soon become inconvenient and time-consuming. One answer is to incorporate both capabilities in a single instrument.

Of course, the notion of performing basic spectrum analysis with a VNA is not new. This capability exists, to some extent, in a few previous-generation analyzers. Today, faster digitizers, digital signal processors (DSP) and central processing units (CPU) are making it possible to implement an SA capability that is fast enough to accelerate crucial and often tedious measurements, such as spur searches.

Accelerating Measurement Speed

In the past several years, VNAs have become significantly faster in all essential measurements: match, gain/loss, noise figure, two-tone IMD, leakage, conversion gain, compression, delay and more. Today’s most advanced models cut test time per device by 10× in defense electronics, 6× in satellite converters and more than 3× in wireless infrastructure devices.

The technologies underlying these performance improvements were the necessary foundation for a high performance spectrum analyzer capability. Keysight’s goal was to find a new and better way to provide an SA capability in a VNA without altering the core architecture of the instrument. This led to Option 090 for the Keysight PNA Series VNAs. This capability addresses the spur search problem in four ways. First, it provides fast searches over broad frequency ranges, up to 67 GHz, and test times are 10 to 500 times faster than a stand-alone SA when looking for medium-level spurs. Next, multi-channel spectrum analysis is synchronized with internal swept signal generators, providing additional insight into cause and effect. Third, it provides in-fixture spectrum measurements that benefit from the added accuracy of VNA calibration and de-embedding techniques. It is also compatible with the singleconnection, multiple-measurement (SCMM) capability of the PNA-X microwave network analyzers. With these solutions, engineers have new ways to reach deeper insights into device performance.

Leveraging the Existing Architecture 

The proven block diagram of a microwave VNA is one of the reasons it delivers excellent measurement results. This is why Keysight’s design team wanted to leave the receiver block diagram untouched while adding meaningful SA capability. As implemented in the PNA Series, the receiver has a bandwidth of 38 MHz and uses a wideband anti-aliasing filter. The 100 MSa/s analog-to-digital converter (ADC) has a theoretical maximum bandwidth of 50 MHz. The key to the successful addition of SA capability was the addition of a new data-processing flow that pulls digitized data from the output of the receiver ADC. Similar to any advanced signal analyzer, the process creates a finite-sized time record of data, applies a windowing function and then computes a fast Fourier transform (FFT) to produce the frequency spectrum of the incoming signal. As shown, a few additional steps ensure an accurate and useful spectrum display: video averaging to reduce trace noise, calibration to ensure amplitude accuracy, image rejection to remove unwanted artifacts and peak detection to enhance frequency accuracy.

Retaining the expected level of VNA performance precluded the addition of the hardware preselection filter or “preselector” that is used in stand alone spectrum analyzers to block unwanted harmonics or mixing products (i.e., images). Within the data flow, the solution was the development of a powerful software-based preselector that is part of the SA capability. On the test bench, engaging this capability virtually eliminates images. In addition, the analyzer can be configured to make harmonic measurements on one or more channels using the receiver attenuator and a narrow resolution bandwidth (RBW) to reduce the displayed noise floor.

Achieving Unrivalled Excellence 

Providing spectrum analyzer measurements inside a VNA is not a new idea. However, the technology available today — ADC, DSP, CPU — makes it possible to implement those measurements at a speed that makes them truly useful to engineers who are designing and testing advanced components. When characterizing active devices, the right mix of speed and performance provides a competitive edge. In R&D, this provides a level of measurement integrity that helps developers transform deeper understanding into better designs. On the production line, it ensures the throughput and repeatability needed to transform great designs into competitive products. With the SA capability in the Keysight PNA series, the ability to make multiple simultaneous network and spectrum measurements lets engineers gain deeper insights into their DUT and, ultimately, achieve unrivalled excellence in measurements and designs.

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