Testing complex modulated wireless signals

Wireless communications systems such as LTE demand signal analyzers that are fast, accurate, and affordable

BY HIROSHI GOTO
Anritsu, Morgan Hill, CA
http://www.anritsu.com

Next-generation wireless communications systems are becoming increasingly sophisticated with higher speeds, wider bandwidths, and multiple modulation methods in which the signal changes dramatically with time. To ensure sufficient bandwidth for new and emerging services and applications, new technologies are operating at higher frequencies. For example, LTE operates at 3 GHz.

A common test instrument used to measure these signals is a signal analyzer. To acquire accurate measurements without impact to transient changes on these complex signals measuring instruments such as signal analyzers require excellent accuracy and wideband analysis performance at frequency bands that exceed 3 GHz.

Today’s spectrum analyzer need to be both fast and accurate to keep up with the latest wireless communication systems.

Another important element is reducing – or, at minimum, controlling – the cost of test. That’s why today’s signal analyzers must also be able to conduct a variety of measurements that historically required multiple instruments. An easy upgrade path is also desirable, so the instrument can efficiently address future requirements as standards evolve.

For all these reasons, engineers designing wireless components, devices, and systems need to look for a signal analyzer that has advanced specifications and performance capabilities to ensure their designs. First and foremost, today’s signal analyzers should have basic frequency coverage up to 6 GHz so that it has frequency coverage that exceeds the frequency of the technology it is testing. This allows the analyzer to conduct the necessary measurements with a high degree of accuracy.

From a spectrum analysis perspective, a wide dynamic range can help ensure total level accuracy. For complex modulating signals such as LTE uses, signal analyzers should produce display average noise level (DANL) of approximately −155 dBm/Hz, third-order intercept (TOI) of ≥22 dBm, adjacent channel leakage ratio (ACLR) of ≤−78 dBc at 5-MHz offset, and total level accuracy of ± 0.5 dB. It is imperative to maintain this type of performance over the entire frequency range, so that measurements reflect the true values across the entire dynamic range.

New design benefits

Signal analyzers have had to undergo a change in design to reach this type of performance. A high-performance RF front end, 16-bit ADC, advanced DSP, and a fast CPU are all now required in signal analyzers so they can meet today’s testing requirements.

The evolution of the signal analyzer has helped the instrument meet the Category B spurious test standard established by the 3GPP, which requires an instrument to have a wide dynamic range to ensure true measurement of the spectrum. A signal analyzer with the aforementioned performance will meet the industry specification and allows tests to be conducted without using correction devices, such as filters and amplifiers. The true values of devices and base stations are measured easily, and spurious tests can be performed with less test equipment as well.

Built-in vector signal analysis (VSA) is also increasingly valuable, both from a measurement standpoint and cost of test. If the instrument has a bandwidth of 31.25 MHz or more and a built-in function that can conduct FFT analysis over a 125-MHz bandwidth, complex signals can be easily tested with high accuracy.

Also, signals can be acquired seamlessly using a digitizing function that accurately captures signal waveforms with no signal dropout. This allows for analysis from various views such as frequency, power, and time for easy and quick troubleshooting.

Memory is also important. A large-capacity memory lets waveforms be captured over long periods of time. The maximum capture time depends on the frequency span (see table ).

A signal analyzer with a large memory of 128 Msamples or more can capture 200 frames of an LTE signal and store them as a file. The file can then be replayed by LTE measurement software to perform analysis, such as EVM measurements. The ability to support low residual EVM of

In R&D applications, a large memory allows users to save data so that comparisons can be done for each device under test (DUT). It also supports comparison of retrofitting improvement effects. At the production level, this kind of memory supports rechecking of performance data for troubleshooting post-shipping faults.

Application-specific software can be used to analyze DUTs to ensure compliance with the standard. In LTE applications, an effective software tool can create waveform patterns for 3GPP LTE (FDD) uplink and downlink TRx waveforms. Rx measurements, such as dynamic range tests, can be made by having the software create a waveform pattern that is loaded in the signal analyzer and output to the DUT.

Measurement speed

Software is also integral to another key attribute – speed. Accuracy is only part of the equation in today’s wireless test world; conducting tests quickly is also a prerequisite. The faster the tests can be conducted, the quicker devices and components can get to market, sometimes making the difference between being a leader and an also-ran.

Advanced software can work with the high-speed CPU of a signal analyzer to achieve fast measurement speeds over a 125-MHz span. The combination of software and hardware can raise efficiency in R&D while cutting production-line tact times.

Fortunately, signal analyzers have evolved in measurement capability, performance, and overall design to accommodate next-generation wireless signals. When selecting an analyzer to measure high-bandwidth signals with complex modulation schemes, the instrument must have a high-end frequency that comfortably exceeds the frequency of the technology it is measuring. Large-capacity memory, wide dynamic range, and fast measurement speed are also required.

Advanced design techniques have allowed signal analyzers to deliver this kind of performance, which lets designers create products that are in specification while improving time to market. ■