VIDEO: Identifying and Measuring Spurious Signals

A Tektronix RSA306 PC-Based Spectrum Analyzer is used to identify and measure spurious signals from the Tektronix Demo Board.

Greetings and welcome to our 78th Test and Measurement Video. Our subject for today is how to identify and measure spurious signals. The instrument for accomplishing this task is the Tektronix RSA306 PC-Based Spectrum Analyzer.

A brief aside: The spectrum analyzer, traditionally a bench-type instrument, resembles the oscilloscope. Today’s models of both these measuring tools use liquid crystal flat screen technology to graphically display electrical waveforms. The difference is in how these displays are constructed. The oscilloscope graphs signals in the time domain, where the Y-axis is scaled to show amplitude typically in volts and the X-axis represents time in seconds. Most all digital storage oscilloscopes also have Fast Fourier Transform capability. This means they can display signals in the frequency domain, where amplitude is still shown on the Y-axis but now logarithmically in power rather than volts. Frequency instead of time is represented on the X-axis.

The spectrum analyzer is optimized to display signals in the frequency domain, which may seem to be a decrease in functionality, but actually its specifications and capabilities model for model exceed those of the oscilloscope in many respects. This makes possible a display that unlike the time domain lends itself to spectral analysis. A great amount of information is shown in a way that once you get used to it is uniquely user friendly.

The RSA306 connects via a BNC cable to the proprietary Tektronix Demo Board that provides an array of signals. It also connects to the user-supplied PC via Ethernet for interactive two-way communication. A specialized Y-configured USB cable powers the Demo Board from two of the PC USB slots.

One aspect of this setup together with the SignalVu-PC programing that is done in the PC is to perform electromagnetic precompliance interference diagnostics. This means that in-house engineers can make preliminary assessments with regard to spurious emissions in new products before submitting them for formal evaluation, a cost-effective strategy.

In the Demo board, we use the same settings as in Video 77, except that by means of row and column buttons, we shift the signal type from QPSK1 back to CW.

To see how spurious emissions can be detected and displayed in the RSA306 instrument, in the SignalVu-PC interface, click Preset.

Then click the Display button in RF Measurement and select Spurious from the available displays box.

Click Add to move the application to the selected displays list. Click Remove to clear the icon from the list. Click OK.

Click the Settings button. In the Scale tab, select the Log scale to reset the display to show the frequency axis in a logarithmic scale.

Under the Ranges and Limits tab, click Load to select the appropriate limit line and measurement condition.

Select FCC Part 15, Rad, Class A (10 m).CSV for this example. A larger new resized window of range and limit can be displayed by clicking Expand.

You can edit the start and stop range, filter shape, RBW, VBW, detector, threshold and excursion and limit line.

After loading the spurious table, the display should look like this:

The green bar is the boundary within which emission levels comply with published FCC standards.

Thanks for watching. New videos are added periodically, so check back frequently.