Basics of manual and automatic oscilloscope measurements

Other than displaying electrical signals in real-time and stored in memory, an outstanding capability of our modern digital storage oscilloscope is to measure those signals automatically or manually and to display the results. Metrics can appear above and below the onscreen waveforms or separately in table format. The accuracy surpasses, by orders of magnitude, that achieved by technicians counting divisions and ticks.

Manual measurements are performed on acquired data using cursors, on-screen vertical or horizontal markers that the user superimposes on displayed waveforms. In the MDO3000 Series oscilloscope, these functions are accessed by pressing the Cursors button, located on the front panel to the left above the Wave Inspector. Pressing the button a second time turns Cursors off and holding the button briefly in the On position activates the on-screen Cursor menu.

To demonstrate Cursors, the internal AFG with sine wave displayed has been connected via BNC cable to an analog input. When cursors are On, the enclosed values appear in the cursor readout at the upper right of the display. The top lines are positive and negative values bracketed by the active cursors. The bottom lines are delta values representing the difference between each pair of cursors.

The second cursor menu selection lets the user choose the source, channel or reference, of the signal that is to be measured. The third selection toggles between horizontal and vertical bars, determining which pair can be moved by the Multipurpose Knobs. The fourth selection links and unlinks the bars so Multipurpose Knob a moves them together or separately. Either way, Multipurpose Knob b moves only the lower bar. The last menu selection determines the units for the values shown in the menu readout. The choices are Seconds, Hz, ratio expressed as percent, and phase in degrees. The final two units are relative to cursor positions, 100% and 360° respectively.

Cursors are also applicable when the oscilloscope is in the XY mode, as when displaying Lissajous patterns. Automatic Measurements can be taken on signals in either the time or the frequency domain. For both, the procedure is basically the same, but because the menus contain different selections, the user is led in different directions and the displays are dissimilar.

To take an automatic measurement in the time domain, it is best to begin by defaulting the instrument. If you do not wish to do that because you need to retain some prior settings, ensure that the oscilloscope is in the time domain by activating one of the analog channels.

For manual measurements, described above, press Cursors. For automatic measurements, press Measure. The horizontal measurement menu appears across the bottom of the screen. Press the first selection, Add Measurement. (As in automatic measurements, the internal AFG sine wave or equivalent signal should be connected to Channel One.)

Pressing Add Measurement activates the corresponding vertical Add Measurement menu. The top menu selection, Source, should be set of course to the analog channel input to which the AFG is connected.

The second menu selection, Measurement Type, refers to an interesting on-screen list of measurements, any one of which may be selected by turning Multipurpose Knob b. For this example we have chosen Frequency. To the left is displayed the relevant equation, a diagram clarifying the way in which a frequency measurement is made and comments noting that frequency is the reciprocal of period and that the measurement is taken over the first cycle.

To activate Frequency, press the soft key associated with OK Add Measurement. Then, press the analog Channel One button. Once again the AFG sine wave is displayed. Now the automatic measurements for Frequency appear in the measurement bar. The nominal frequency of the sine wave coming out of the AFG is 100.00 kHz. The measured value, mean, minimum, maximum and standard deviation fluctuate a slight amount from this ideal.

Multiple automatic measurements of different signals and parameters can be taken, and they are stacked in the measurement bar. Returning to the Measure menu, note that these lines can be removed individually from the Measure bar, or all measurements can be removed.

If a triangle icon with an exclamation point appears in place of the measurement, vertical clipping is happening. This can be mitigated by adjusting vertical scale and position.

If there is a Low Resolution message, increase the acquisition record length so the instrument has more point for the purpose of calculating the measurement.

Reference Levels are horizontal lines, set by the user, that intersect waveforms at specified amplitude levels so that time intervals can be measured. They are needed to ascertain rise and fall times, and they can be set as percentages or units.

To access Reference Levels, press Measure and Add Measurement. Then press More as needed to scroll down to Reference Levels. The vertical menu that appears allows the user to set High Reference, Mid Reference and Low Reference levels. (Some measurements require only one reference level. Others require two or three. The frequency diagram, for example, shows a single mid-reference level in place for the frequency measurement.)

To access Automatic Measurements in the frequency domain, first press RF. (AFG needs to be shifted from the analog channel input to the RF input using an RF adapter.) Then press Measure. With AFG off, the oscilloscope is displaying a zero-amplitude signal. The irregular trace you see near the bottom of the screen is the noise floor of the instrument. It is not a manufacturing defect, but rather a consequence of the fact that all resistive loads (even a resister sitting on a shelf, not in a circuit, with leads not shunted) has a small but measurable voltage across it due to thermal energy.

When we switch on the AFG, a long, slender spike rises up from the noise floor, almost to the top of the screen. This is the default 100.00 kHz sine wave signal from the AFG. Despite its amplitude, this fundamental is difficult to see because it almost exactly coincides with the grid line at the left edge of the screen.

To make this signal more visible, press the Freq/Span button, directly below RF. The center frequency is the default 1.50 GHz, and that is the problem. Using the number pad, reset the center frequency to the AFG output, which is 100.00 kHz. That brings the fundamental to the center of the screen, where it is now prominent.

Because the AFG output is a sine wave, where all the power is in the fundamental, we will expect to see no harmonics. To see a signal with harmonics, push AFC>Waveform>Square. Still no harmonics. Most likely the span is problematic. Press Freq/Span and reset the Span to 12 MHz. Now we see harmonics, beginning at 10 dBm below the fundamental and dropping off as spectral distance increases.

Now we are ready to do some Automatic Measurements in the frequency domain. Press Measure, then Select Measurement. The menu selections are: Channel power, which is the total power within the bandwidth, defined by channel width.

Adjacent channel power ratio — which is the power in the main channel and the ratio of channel power to main power — for the upper and lower halves of each adjacent channel.

Occupied bandwidth, which is the bandwidth that contains the specified percent of power within the analysis bandwidth.

To summarize, the Measure mode is useful for checking the bandwidth of a signal where applicable and for quantifying the spectral distribution of power outside the fundamental.