Oscilloscope cursors complement other measurement tools

Article By : Arthur Pini

It may seem somewhat archaic to retain an oscilloscope’s original measurement tools of screen graticules and cursors, but these tools are hardly outdated.

Most oscilloscopes these days offer from 25 to 50 automated basic measurements with optional measurements for specialized analysis running close to 100. With all this measurement horsepower available it may seem somewhat archaic to retain the oscilloscope’s original measurement tools of screen graticules and cursors. But these original measurement tools are hardly outdated.

Oscilloscope measurements come in three flavors. The first, and most primitive, is using the screen graticule: a measurement technique that goes back to the first oscilloscopes. Cursors, sometimes called markers, came in with the last few generations of analog scopes. They allow the user to manually identify points on a scope trace that the instrument will use in making measurements.

The final measurement method is the use of automatic measurement parameters, often referred to simply as measurements. This technique uses the scope’s computing power to automate the analysis and measurement of common waveform characteristics such as period, amplitude, and mean level without using manual markers. But this automation does not replace the earlier two methods. The use of scope graticules and ‘box’ counting, for instance, survives because it is useful for quick estimates of amplitude and horizontal units of time or frequency.

The use of cursors or markers, while not as powerful as automated measurements, provides great flexibility under a variety of measurement situations that cannot easily be handled by automated measurement parameters. Cursors work because they are user-controlled and -interpreted; it’s hard to beat the human brain for understanding a measurement situation and responding quickly and correctly.

Consider the situation shown in Figure 1. Here a square wave signal has a small crosstalk coupled signal riding on it (upper grid). Measurement parameters easily measure the square wave frequency a 1 MHz, as shown in the measurement parameter P2 below the grid. The crosstalk signal is too small to be detected using automated measurements without a wholesale resetting of the measurement thresholds and measurement gates. However, cursors can be easily engaged to read the crosstalk signal’s frequency as 50 MHz.

dual cursor measurementFigure 1 Using dual cursors to measure the frequency of a crosstalk signal requires that cursors be placed over the waveform being measured, while the cursor horizontal readout appears in the lower right corner of the display, outlined in orange.

The cursor horizontal readouts appear in the lower right corner of the figure under the timebase and trigger dialog boxes marked by an orange outline. The readout shows the time, relative to the trigger point, of each of the two cursors. It also contains the time difference and the reciprocal of the time difference (frequency). A zoom trace showing the horizontally expanded view of the waveform is in the lower grid. The cursors track across all related displays, like zoomed versions of the waveform. The accuracy of cursor measurements depends on the accuracy of their placement on the waveform. Using the zoomed view permits a more accurate placement of the cursors.

The cursors’ vertical or amplitude readouts appear in the respective vertical channel dialog boxes. There are two active waveform traces: channel 1 (C1) and the zoomed view of it (Z1). The cursor amplitude readouts appear in those dialog boxes in the lower left of the figure, outlined in light green.

In some instruments, like the one used here, there are two cursors, which allows readout of the vertical or horizontal difference value. Again, the amplitude at each cursor position relative to the channel’s offset appears along with the difference of the values, marked as ΔV, and the equivalent time difference in the horizontal readout field, designated ΔX.

This particular oscilloscope, a Teledyne LeCroy WaveMaster 820Zi, offers several different types of cursors which operate on any of the scope’s available display traces, be they time domain, frequency domain, or histograms (Most oscilloscope suppliers offer cursor measurements similar to those used in these examples). The example shows dual horizontal cursors with two vertical lines marking the horizontal values at their locations. Up and down directed arrows are cursor icons that ride on the waveform being measured to mark the amplitude at the cursor location and differentiate between the cursors. Having the cursor icons riding on the waveform improves cursor placement and thereby accuracy.

The other horizontal cursor type is a single or absolute horizontal cursor using only a single vertical line with amplitude value of the waveform marked by a small cross icon.

This scope offers both single and dual vertical or amplitude cursors as well. These show up as either a single or dual horizontal line, respectively, and measure the amplitude of the cursor’s vertical position relative to the scope channel’s vertical offset.

Figure 2 is another example of a measurement that is easy for cursors but difficult for automatic measurement parameters, measuring the duration of a signal burst. This is difficult for automated parameters because the vertical decision thresholds are near zero volts and are in the noise. Both vertical and horizontal cursors are used in the example to determine the peak to peak amplitude and the burst duration. The horizontal cursors read the duration as 2.49µs and the vertical cursor readout shows the peak to peak amplitude of 365 mV.

dual cursor rf burstFigure 2 Measuring the duration and amplitude of an RF burst using cursors shows that the pulse duration is 2.49µs and the peak to peak amplitude is 365 mV.

The use of cursors is not restricted to time waveforms only. Considering that most current oscilloscopes have Fast Fourier Transforms (FFT) and other advanced mathematical capability, cursors can work on almost all waveform types, even those from the frequency domain or from statistical domain traces like histograms. Figure 3 offers an example of a common frequency domain measurement.

frequency domainFigure 3 Measuring the roll-off of a low pass filter in the frequency domain using dual cursors shows a -72dB roll-off over the octave frequency range from 60 to 120 MHz.

This example measures the roll-off of a 50 MHz low pass filter in the frequency domain. The vertical scale of the trace is in decibels (dB) and the horizontal scale is in Hertz, units of frequency. The cursors are placed an octave apart in frequency. The cursor horizontal readout has the left-hand cursor at 60 MHz and the right-hand one at 120 MHz, one octave apart. The vertical cursor reading, in the F3 dialog box shows an amplitude difference of -72.4dB. The filter roll-off is -72.4dB per octave. This is another measurement that is easier to make using cursors than with measurement parameters.

Making cursor measurements on an X-Y display yields some interesting capabilities especially for interpreting vector signals. Consider the sixteen-state quadrature amplitude modulated (QAM16) signal shown in Figure 4. The X and Y inputs are the in-phase (I) and quadrature (Q) components of the 16QAM-like signal. When rendered on the X-Y display these waveforms produce a state transition diagram that shows the waveform’s vector nature.

vector measurementFigure 4 Using cursors to measure vector signals on an X-Y display has the cursor readouts (magnified), which include vector magnitude and angle.

This signal has 16 vector values. There are 12 vector states spaced at equal angular increments and four additional vector states with reduced amplitude on the 45°, 135°, -45°, and -135° arms of the diagram. Each of these vectors represents a 4-bit digital state.

The X-Y cursors on this oscilloscope move on the X-Y display and can be placed on any of the vector end states. In this example the cursor, represented by the cross icon, is placed at the end of one of the vectors. The X-Y cursor readout field under the X-Y has been magnified for easier reading and shows the vector’s slope, angle, and radius. In this case it shows the vector radius (magnitude) as 354.2 mV and the angle as 44.9° (slope is equal to 0.996). Note also that the cursor tracks on the X-T and Y-T plots so the location of the I and Q components that make up the vector on the X-Y display can be located on the I and Q waveforms.

Switching to a dual horizontal cursor delivers another surprise as shown in Figure 5. As was seen in the basic cursor examples, dual cursors result in two cursor markers. In this example each is placed at the end of two adjacent vectors. The vector cursor readout shows the vector difference between the longer vector and the shorter one. The magnitude of this difference is 167 mV at an angle of -90°. Again, the cursors on the X-Y display track on the X-T and Y-T traces allowing easy identification of anomalies on either display.

vector measurementFigure 5 Using dual horizontal cursors yields the ability to read the difference between two vectors, the error vector magnitude, for the vectors marked by the cursor icons.

In essence, these X-Y cursors offer a limited vector signal analyzer functionality without a separate instrument or software package. Those are available, if you have need and budget.

Cursors are very useful measurement tools complementing the automated measurements. The examples show that cursors are most effective in situations where some interpretation of the acquired data is required in order to place the cursors in correct positions on the trace. This man-machine teamwork is an interesting part of the cursor measurement process.

Arthur Pini is a technical support specialist and electrical engineer with over 50 years experience in electronics test and measurement.

Leave a comment