Characteristics Analysis of Oscilloscopes with Time Domain Measurement and Spectrum Analyzer Functions

Many laboratory-quality oscilloscopes have the function of spectrum analyzers in addition to time domain measurements. As telecommunications applications become more common in today's design environment, modern oscilloscopes have added features such as dedicated spectrum analysis.

Many factors contribute to improvements in the accuracy and resolution of measuring instruments. For example, an 8-bit oscilloscope with floating-point FFT operation can observe signals as low as -100dBm (approximately 2μV). However, some oscilloscopes will also show large harmonics at this level, while spectrum analyzers will not.

A good spectrum analyzer

There are three metrics used to judge the performance of an oscilloscope-based spectrum analyzer: resolution bandwidth (RBW), noise floor, and dynamic range. These three indicators can be used to compare dedicated spectrum analyzers and oscilloscopes.

* Resolution bandwidth

RBW determines the ability to distinguish adjacent signals. The smaller the RBW index, the better the ability to distinguish adjacent frequencies. The smaller the RBW, the better.

*Noise floor

The noise floor of an instrument is its own inherent noise, which determines the minimum level of the input signal that can be observed. If the input signal is to be distinguished from the noise floor as a valid input, it must have a higher amplitude than the noise floor. The lower the noise floor, the better.

* Dynamic Range

This is the maximum ratio of the input signal amplitude to the noise floor amplitude, and the larger the dynamic range, the better.

These three parameters, along with several other instrument characteristics, must be considered. In addition, the relatively important characteristics related to the application cannot be ignored. With the recent development of data acquisition technology, some oscilloscope platforms have emerged that are comparable to top spectrum analyzers.

When selecting an oscilloscope with spectrum analyzer features, many other features must also be considered. This includes actual dynamic range, sensitivity, phase accuracy, filter windows, and more. Other, less practical features are also important. For example: In a time-pressured design environment, sweep time and ease of use both impact the use of an oscilloscope as a spectrum analyzer. When a product launch deadline has been set, a tool that takes minutes (rather than seconds) to display results will definitely not do.

Low resolution bandwidth greatly increases scan time. As the resolution bandwidth approaches its lowest value, the scan time increases to 29 seconds. Time is precious, so ease of use is important. Analysis with an oscilloscope should follow the pattern of conventional spectrum analyzer analysis. Most engineers have used standard spectrum analyzers to analyze factors such as frequency (including span, center frequency, and RBW), reference level offset, and reference level. Following these basic proven procedures allows the user to follow familiar steps. Familiar results.

hidden advantages

Some advanced oscilloscopes offer a feature not found in dedicated spectrum analyzers: the ability to threshold the acquired data that represents changes in the spectrum. Setting thresholds is useful for analyzing complex waveforms that may have different frequency components in many segments.

By storing a sample of the background EMI waveform generated by passive circuits and using spectral analysis to create a reference spectrum of the background radiation, it is possible to separate locally generated EMI and ambient radiation. The results are stored and subtracted (using mathematical functions within the oscilloscope) from the reacquired data and spectrum changes, and what remains is the spectrum of the local EMI radiation.