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Spectrum Analysers and Real Time Analysis

IMEX has a wide range of expertise that can help you make your purchase.

Introduction

Much has been said about the transition to a wireless world, with mobile networks usually cited as the defining example. As we embrace new technology, wireless telephony is only the first large step among many.

We are seeing wireless technologies displacing older wired and even infared architectures just about anywhere people need to communicate with one another or with electronic systems. Many of us have 802.x networking in our home. Radio frequency identification (RFID) systems monitor container shipments at seaports. Wireless point-of-sale terminals speed up car rental check-in at airports. Third generation (3G) mobile phones deliver a wide range of bandwidth-intensive services with enhanced network convergence with the evolution of WLAN and Bluetooth on a chip solutions. In short, RF signals are doing more work for more users than ever before... and this is a trend that is accelerating.

Inevitably, this has had an impact on the characteristics of wireless signals, requiring them to support more applications and services. Today's RF signals carrying digital content are far more complex than their predecessors, with many of them featuring time varying modulation schemes, appearing only in short, non-repeating bursts.

The basic amplitude and frequency modulated signal formats of the past simply can't deliver the required information rich data stream that may carry digitised voice, text, pictures and even video.

It all adds up to a challenging environment for the designer who wants to get a new product to market on time. Today's complex formats contain details that can only be understood by reliably capturing brief, momentary events and analysing changes over time.

Real Time Acquisition

Real time signal activity matters to the RF designer. Consider, for example, the characterisation of an RFID signal, a common use with the RSA2000A and RSA3300 Series Tektronix spectrum analysers. These signals are sporadic in nature, appearing occasionally, but transmitting for a relatively long period of time and containing a large amount of modulated data. The RF engineer is challenged to verify the 'traditional' RF performance while still also performing analysis on a very short section of a long signal.

For many RF engineers, the task above may be addressed with a swept spectrum analyser available to hand on the bench. It is a critical tool that provides great results when given a stable repeating signal to work with, as in the case of many basic RF measurements. The swept spectrum analyser builds up a detailed image over time, often over an internal of several seconds.

However, when monitoring a less predictable signal, the analyser, can miss important events that occur in one frequency band while the instrument is looking at another. Transitory frequency changes defy capture by a swept spectrum analyser

Real-time phenomena such as phase lock loop (PLL) settling time, RFID transmissions, W-CDMA compressed mode behaviour and GSM frequency hopping imply a need for real time acquisition. There must be symmetry between the problem and its solution. Imagine a methodology based on looking at all of the frequency bands at once. This implies an entirely different, massively parallel acquisition architecture. The traditional concept of a slow and deliberate sweep across the frequencies of interest no longer applies.

This alternative architecture can continuously capture the frequency band of interest over a long period of time, as opposed to sweeping across the input signal and building an image from serially acquired frequency steps. The rapidly recurring parallel acquisitions maximise the likelihood of capturing the dynamic changes from, for example, a particular RFID transmission.

Moving Forward to Analysis Methods

A real-time approach is good but it is only the beginning of the solution. The true strength of real-time acquisition lies in its ability to support analysis methods that inform the design process and speed both evaluation and compliance procedures.

Each real-time acquisition is like a single frame in a film. Its many simultaneously acquired frequencies tell the whole story about the signal that has been captured - steady-state power, peaks and transients. Like the movie, hundreds of these 'frames' in sequence can reveal dynamic action. This is the new dimension, the improved view of the signal that designers need to help them understand today's complex modulated RF signals.

Examples of these views include spectrograms and codograms. Both analysis formats bring out otherwise unseen trends and details. A spectrogram depicts the behaviour of the signal in terms of power and frequency, and also concatenates a series of acquisitions to produce a time axis. Looking at the spectrogram, the engineer can easily discern frequency shifts or the details of PLL settling time.

A second tool, the codogram, displays code power versus time power in graphical form. If real-time analysis is integrated into the measurement platform then it enables engineers to assess the big picture of performance and in context. The approach offers many advantages for high pressure design projects, emerging products and NPI.

So far, the high cost of components has meant that real-time spectrum analysis techniques have been limited to the highest of high-end applications. As RF technology moves into more and more areas, particularly consumer applications, more engineers require improvements in their view of RF signal analysis. Accessible real-time spectrum measurement solutions are emerging to support the trend towards RF wireless connectivity. Designers developing the next generation of RF based communication and control technologies will benefit from real-time acquisition and the innovative signal viewing perspectives that it brings.

Contact IMEX now to discuss your needs and the options available to you.

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