Shifting Signal Analysis into High Gear with Multiple Simultaneous Measurements
By Abhishek Shukla, Agilent Technologies
As wireless designs evolve, dense integration increases the likelihood of unwanted interaction between the baseband, RF, IF and digital sections of a device. In testing and troubleshooting, the long-standing paradigm has been to make one measurement at a time using a spectrum or signal analyzer. This process of looking in one place at a time can impede progress—and any type of delay is problematic in product categories that depend on rapid introductions of new or modified designs.
Today, the new alternative is to make multiple measurements simultaneously using vector signal analysis software with one or more measurement front ends. This solution has been implemented in the Agilent 89600 VSA software, release 15. The multi-measurement capability enables simultaneous measurements—from one or more signals or devices—in the time, frequency and modulation domains. It can do all of this using digitized waveforms from a variety of supported instruments, and the software can coordinate measurements from multiple instruments at the same time.
This article describes three problem scenarios, introduces the multi-measurement concept and presents three example use cases, two simultaneous and one sequential.
Sketching a Few Typical Problems
Three relatively common situations highlight the shortcomings of the one-at-a-time approach:
support for multiple formats, the demands of highly complex measurements, and the need for more testing of more prototypes.
Developers of current-generation devices need to test multiple standards at the same time: W-CDMA, LTE, Bluetooth®, 802.11 variants, and more. Within devices that use these standards, the analog portion of the signal path is shrinking. As individual engineers become responsible for more or larger functional blocks, the troubleshooting process becomes more difficult without a system-level view of signals throughout the block diagram.
Whether the engineer works in R&D, system integration or design validation, certain measurements have become so difficult that they require multiple instruments, complex setups and long test times. One example is simultaneous testing of transmit spectrum masks while searching for low-level spurious signals. Until now, this combined task might have required either separate measurements with one signal analyzer or parallel measurements with two analyzers set up with very different configurations. Unfortunately, the use of more equipment is contrary to one of today’s most common mandates: “Do more with less.” As a result, these difficult measurements are sometimes omitted, leaving doubt about a device’s performance or compatibility.
Finally, prototype testing is changing with the emergence of multi-carrier, multi-format smartphones, tablets and so on. Naturally, this requires more tests per device to characterize the various radios, signals and formats. Less obvious is the need for more prototype devices; because more complexity may cause greater deviation in performance, testing of more devices helps ensure success relative to a company’s quality goals.
Across all three of these problem scenarios, the multi-measurement technique can save considerable time and provide new insights when compared to the one-at-a-time approach.
Understanding the Faster Alternative
To quickly reveal signal interactions and simplify troubleshooting, the multi-measurement technique utilizes multiple single-channel measurements. Currently, these can take three forms: shared acquisition and independent acquisition, which are simultaneous; and a “fast-switched” approach, which is sequential.
Shared acquisition uses a single measurement frontend such as a signal analyzer or oscilloscope. Measurements are truly simultaneous because they are performed on a single block of digitized waveform data that was acquired by the scope or analyzer.
Independent acquisition uses multiple frontends to acquire digitized waveforms that are the basis of one or more simultaneous measurements per frontend. Because this approach can provide virtually unlimited bandwidth, it is useful when measurements are needed at different parts of the frequency spectrum or with different bandwidths. For example, this method can produce LTE measurements centered at 700 MHz and Bluetooth measurements centered at 2.4 GHz.
Fast-switched measurements use one frontend, which is configured to sequence through a user-defined collection of measurements. As an example, this could be an identical set of signal-quality measurements performed at various carrier frequencies with different demodulation formats.
In all cases, the set-up process is the same. The first step is to define the set of measurements. Each can have its own settings, such as center frequency, frequency span, resolution bandwidth, triggering, and so on. Such a collection might include a full-span frequency spectrum and CCDF plot, a zoomed GSM spectrum and its constellation, a zoomed W-CDMA spectrum and its constellation, and a zoomed LTE downlink signal.
All measurements reside in RAM and the whole collection is initiated simultaneously. Once executed, all results can be displayed side-by-side for detailed analysis (Figure 1).
Examining Three Example Use Cases
Multi-measurement capability enables a variety of possible test scenarios that go beyond the traditional one-at-a-time paradigm. Three examples will help illustrate what is possible:
Case #1: Shared Acquisition
This method can be used to rapidly test and analyze multi-format devices. In this case, the measurement configuration includes the VSA software and a signal analyzer. All signals produced by the device are within the 160 MHz IF bandwidth of the analyzer: GSM, W-CDMA and LTE (Figure 2).
Within the software, the user created four distinct measurements and the associated display traces. Each measurement had a different center frequency, span, and so on. All four measurements were generated simultaneously from a single set of digitized data provided by the analyzer.
All four measurements were completed in roughly the same time it would take to make a single measurement in the one-at-a-time approach. In addition, the side-by-side presentation of results reveals interactions that may not be visible when comparing asynchronous measurements.
Case #2: Independent Acquisition
In this case, the goal is to capture, measure and analyze widely spaced signals. The general measurement setup includes the VSA software and two or more instruments—signal analyzer, scope, logic analyzer, etc. Here, the use of two signal analyzers makes it possible to examine what’s happening at different frequencies that carry LTE and wireless LAN (WLAN) signals.
As shown in Figure 3, the instruments are configured to trigger at the same time, enabling coordinated acquisition and meaningful measurements. Acquiring signals in this manner helps provide new insights; the simultaneous measurements in Figure 3 reveal increases in LTE error vector magnitude (EVM) coinciding with WLAN transmissions.
This case can be extended to parallel sets of measurements taken simultaneously at key points within the device block diagram (Figure 4). For example, this could be used to check EVM at each test point—baseband, IF and RF. Software math capabilities can also be used to compute transfer functions across the block diagram.
Case #3: Fast-Switched Measurements
In this example, the software works in concert with one instrument to rapidly sequence through a diverse collection of measurements at widely dispersed frequencies. The enabler is the software’s macro function, which can be used to step through collections of preconfigured tests that check GSM, W-CDMA, Bluetooth and WLAN performance (Figure 5).
The overall concept is similar to the shared acquisition example in Case #1; however, in this case true simultaneity is not required. Still, the net benefit is similar; a diverse set of tests can be performed at several frequencies with a single analyzer, and can be completed more quickly than with the one-at-a-time approach.
With wireless devices and systems now handling multiple carriers and formats at the same time, testing becomes more efficient with solutions that can do likewise. Multi-measurement capabilities, such as those implemented in the 89600 VSA software, go beyond traditional methods. Shared acquisition provides true simultaneity by making several measurements at once from the same digitized data. Independent acquisition provides, in effect, unlimited bandwidth because a different instrument is tuned to each signal of interest. The fast-switched approach also provides essentially unlimited bandwidth for a series of measurements that needn’t be simultaneous.
In all cases, the common benefit is deeper insight into the complex signal interactions within today’s densely integrated wireless designs. For more information, please visit www.agilent.com/find/89600_VSA_MM.
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