13 Nov 2010

Realtime Spectrum Analyzers - what are they?

Darren Tipton of Rohde and Schwarz UK investigates what realtime spectrum analyzers are and how they can be used to find signals other analysers cannot see.

Spectrum analysers are devices that allow us to look at signals in the frequency domain rather than in the time domain. They are frequency selective devices that can be used to analyse characteristics of wanted signals, such as channel power, bandwidth, level and also unwanted characteristics of signals such as unwanted side band power, spurious or other interference. In many cases a spectrum analyzer can be used to gain more information about a signal such as its phase. Such instruments are normally called vector signal analysers due to this added capability and they are able to perform for example modulation quality or frequency vs time measurements on signals by capturing the signal and post processing it.

In this short article we will discuss the differences between swept and real time spectrum analysers as well as what real time means and what advantages or disadvantages it has over traditional methods.

Traditional Analyzers

A traditional spectrum analyzer sweeps across the spectrum using a heterodyne mixer principle, looking at and capturing small parts of the spectrum at a time and building this into a complete picture over time. Analysing the spectrum from 100MHz to 6GHz, for example, will take some time to accomplish because of the need to sweep over each frequency point to be investigated. For this reason it can be said that a normal swept spectrum analyser, even if the sweep time to cover the spectrum is extremely fast, is blind for the some of the sweep time. If an event occurs in one part of the spectrum when a different part of the spectrum is being examined, the event will be missed.

Real-time spectrum analysers have become more interesting in recent years to capture events that have never been possible to see before. Introduced to help identify signals that appear for very short periods of time within a given bandwidth, such as pulsed radar signals or hopping signals, the ultimate aim of real-time spectrum analysis is to capture signals with 100% probability of detection in a way that traditional swept spectrum analyzers are not able to do.

The Real-Time Spectrum Analyser

Compared to a swept spectrum analyser, a real-time analyser works in a different way. The analyser is tuned to a given centre frequency and the signal is sampled in the time domain. The sole purpose of the real-time analyser is to capture every event in a defined bandwidth with no gaps.

In the world of oscilloscopes, real time oscilloscopes sample signals fast enough to satisfy Nyquist sampling theorem. These are classed as real time systems. However, in the world of spectrum analysis, real-time does not just mean satisfying Nyquist sampling theorem, but also ensuring that no information is lost in the time taken to process the data. This is one of the main problems with traditional spectrum analysers. Where a signal needs to be processed, there is a gap where no capture is taking place, therefor events that occur in the spectrum are lost.

FSVR Real Time Spectrum Analyzer

FSVR Real Time Spectrum Analyzer from Rohde and Schwarz

For a traditional spectrum analyser to perform measurements on modulated signals such as modulation quality (EVM), it must capture (sample) the whole bandwidth of interest at once without sweeping in order to perform the processing. This in itself is a form of real time spectrum analysis, as the signal is sampled in realtime and stored in a capture buffer for offline processing. We call this, "Offline Realtime" and for this reason, all spectrum analysers can be classed as real time instruments when we consider what realtime means in terms of oscilloscope users. What is special about realtime spectrum analysers is that they performs it's processing in an "Online Realtime" mode, meaning no information is missed while data is being processed.

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About the author

Darren Tipton is a Product Manager at Rohde & Schwarz UK. He received a BEng (Hons) Degree from The University of Liverpool in 2000 and started his career in RF, test development and later concentrating on field applications for mobile and base station test on GSM and WCDMA. Since 2006 he has been working for Rohde and Schwarz UK and is responsible for Spectrum Analysers, Signal Generators and Power Meters in the UK.

Rohde & Schwarz is an independent group of companies specializing in electronics. It is a leading supplier of solutions in the fields of test and measurement, broadcasting, radio monitoring and radio-location, as well as secure communications. Established more than 75 years ago, Rohde & Schwarz has a global presence and a dedicated service network in over 70 countries. It has approx. 7400 employees and achieved a net revenue of 1.2 billion (US$ 1.7 billion) in fiscal year 2008/2009 (July 2008 to June 2009). Company headquarters are in Munich, Germany.

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