Modulation Analysis for Transient RF Signals

The Tektronix Real-Time Spectrum Analyzer (RTSA) provides a variety of triggering, capture, and analysis capabilities that greatly simplify the task of thoroughly characterizing RF devices that employ analog or digital modulation techniques. Real-time tools like the frequency mask trigger, seamless capture, and time-correlated multi-domain analysis can be used to efficiently troubleshoot and solve complex modulation analysis problems. These unique benefits of RTSA technology are especially valuable for engineers working with modern communication signals that are often transient in nature. This technical brief examines several measurement challenges involved with modulation analysis for RF communications signals and the comprehensive set of tools that the Tektronix RTSA provides to address these problems. This includes a discussion of the unique triggering, capture, and analysis capabilities of the RTSA and a survey of examples drawn from a variety of common industry problems.

Measurement Challenges
Modulation technology has continued to grow more complex with time. The wireless communications industry, once dominated by simple continuous wave amplitude and frequency modulated signals, has evolved to much more complicated modulation schemes that involve sophisticated techniques to spread information in the time, frequency, and code domains. Intermittent packets of digital multi-carrier vector modulations have become commonplace. This trend toward more complex modulations has created a demand for a better approach to analyze these signals. Whether searching for phase hits on a high capacity data link with a residual bit error floor of 10-12, or trying to capture an elusive packet collision in a WLAN combo device, it is imperative to have equipment that can reliably detect and trigger on transient RF signals.

Transient RF signals have long posed a difficult measurement problem. Trying to trigger the acquisition of an RF signal using traditional time domain voltage and power levels, though useful for some cases, can be fraught with problems in low Signal to Noise (SNR) environments or in the presence of other strong signals. Many of today’s most difficult RF diagnostic problems arise from intermittent signals in the presence of other larger signals. Packet collisions in WLAN combo devices, microprocessor interference, local oscillator phase-hits and surveillance of low probability of intercept signals are but a few of the many real-world examples of the utility of being able to capture information based on spectral shape.

Modern modulations also demand new ways to analyze signal structure. Vector modulation, spread spectrum and multi-carrier techniques require more insightful approaches to visualizing signals and their impairments than traditional analysis tools provide. The Real-Time Spectrum Analyzer

Tektronix has designed the RTSA to address the measurement challenges associated with dynamic RF signals such as transmissions using variable modulation types and bursted communications packets. The fundamental concept of real-time spectrum analysis is the ability to trigger on an RF signal, seamlessly capture it into memory, and analyze it in multiple domains. This makes it possible to reliably detect and characterize RF signals that change over time. This technical brief contains the following examples:

– Troubleshooting complex 3G cellular wireless devices and systems.
– Locating interference in combo devices that implement multiple standards such as 802.11a/b/g, 3G, and Bluetooth.
– Integrating and debugging advanced software defined radio systems.
– Characterizing unwanted modulation in RF communication systems.
– Detecting microphonics and phase-hits in local oscillators that impact overall system performance.

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