Measurement gates, which limit the source of measurements, can be in either of two forms. The first are fixed gates, set by the operator at fixed locations within the acquisition. The second type of gating is waveform gating, here the state of another waveform determines when measurements will be made. The gating criteria are independently adjustable for each parameter on WaveRunner, WavePro and WaveMaster oscilloscopes.
Acceptance criteria allow the user to accept only those measurement values falling within a user specified range.
This tutorial will examine all of these methods of selectively choosing which parts of an acquisition are measured.
A LeCroy Xi or Zi series oscilloscope.
1 passive Probe
1 coaxial cable
Displays shown in the tutorial are based on the following initial setup on a WaveRunner 6 Zi scope:
- Recall the default setup: File pull down > Recall Setup> Recall Default.
- Verify that the input coupling on channel 1 is DC 1 MΩ: Touch or click the channel 1 annotation box>touch or click on the coupling field >select DC 1MΩ.
- Set the channel 2 input coupling to DC 50Ω: Touch or click the channel 2 annotation to bring up the C2 dialog box>touch or click on the coupling field >select DC 50Ω.
- Connect the passive probe to the channel 1 input of the oscilloscope and connect the probe tip to the Cal output test point. The associated probe ground lead should be connected to the adjacent ground lug.
- Connect the coaxial cable from the channel 2 input on the oscilloscope to the AUX output connector on the scope’s lower front panel.
- Use the Utilities pull down menu to select the FastEdge signal to be output from the Aux output connector. Utilities > Utilities Setup> Aux Out (tab)> Use Aux Output For>FastEdge.
- In the Aux Output tab of the Utilities Setup set the frequency of the calibration output to 1 MHz using Use Calibration Output For>Frequency > 1 MHz.
- Auto setup the scope by pressing the Scope Setup button on the front panel and selecting AutoSetup from the fly out menu.
- The scope display should appear a shown in Figure 1.
- Press the Normal button in the front panel trigger control group.
This completes the initial setup.
Turn off channel 1 by touching or clicking on the C1 trace annotation box. The C1 dialog box will appear. Uncheck the Trace On checkbox.
Set up measurement parameter P1 to measure the frequency of channel C2 (Measure pull down, Measure Setup>Check Show Table checkbox> P1 tab> check the On checkbox>Source C2>Measure Frequency). Go to the P2 tab, change the source from C1 to C2. You should have P1 and P2 both displaying the frequency of C2. Go to the Measure Tab, check the checkboxes for Statistics ON and Histicons. Press the Clear Sweeps button. The screen should appear as shown in Figure 2.
Press the Single button in the Trigger control group on the front panel of the oscilloscope. Press Clear Sweeps on the front panel. Both P1 and P2 table entries should be cleared. Press the Single button on the front panel again. Both parameters should show that 49 measurement values have been acquired. This is an example of LeCroy’s “All Instance Measurement” where the parameter is measuring the frequency of each of the displayed cycles.
Go to the P2 tab. Touch the right hand tab labeled Gate. Double touch or click on the Start field. The Start pop up should appear. Enter the value 3 then press OK. The Start value on the Gate tab should now show 3.0 div. Double touch or click on the Stop field. When the pop up appears set the value to 3.35 div. The display will show the measurement gate with a dot-dash pattern vertical line at 3 divisions from the left hand edge and a dashed vertical line at 3.35 divisions as shown in Figure 3.
The parameters make measurements of those portions of the waveform between the measure gate locations. Now parameter P2 is only measuring one cycle (the one between the measure gates). This is shown in the P2 parameter statistics where only one value is recorded. Using the measure gates it is possible to select any part of the displayed waveform for measurement.
The measurement gates can also be set by touching them with your finger or a mouse until a move left/right arrow appears. Dragging with your finger or mouse will move the gates.
Try moving the gates and see what effect it has on the results tabulated for parameter P2. When you are finished, press the Default button on the Gate tabs of the Measure setup dialog. This will reset the gate locations to Start at zero and Stop at ten divisions which includes the whole waveform. Note that parameter P2 now report that there are 49 measurements included in the parameter statistics.
Turn on channel 1 by pressing the “1” button in the Vertical control group on the front panel. Touch or click on the P2 measure table entry on the screen. The P2 tab of the Measure setup dialog will appear. Touch or click on the right hand Accept tab. Touch or click on the Values Based on Waveform State checkbox. The other entry fields in the tab will be active. If not already set to these values enter them manually:
The parameter P2 will use the state of the waveform on channel 1 to determine when to make measurements. It will only measure when C1 is in its high state. Note that the number of measurements included in the P2 statistics is now 10 or one measurement for each high state of C1 as shown in Figure 4.
Change the State is field from High to Low. Note that the number of measurements is now 20, corresponding to number of cycles on C1 that correspond with the low state of channel C1. Note that since the frequency parameter requires about 1 ½ cycles per measurement the sum of the waveform gated measurements is less than the 49 original measurements.
You might use waveform gating if you were probing a bus and only wanted to measure when a certain chip on the bus was enabled by its chip select line.
Touch of click on the Values Based on Waveform State checkbox to turn of waveform gating.
Turn off channel 1
Accepting Values in a Range
Change the trigger mode from stopped to Normal. The scope parameter statistics should be updating. Note the min and max values for parameter P1, they should be about 4.99959 to 5.00029 MHz, a range of about 700 Hz.
Go to the P2 tab of the Measurement setup. On the right hand Accept tab touch or click on the Values in Range check box to turn on value acceptance. Set the value in the between field to 4.99990 MHz and the And field to 5.00000 MHz. Press Clear Sweeps on the Front panel and let the parameter statistics accumulate. Note that the min and max range of the P2 statistics are now limited to the range between 4.99990 and 5.00000 MHz. Note also that the histicon has been truncated and no longer shows a Gaussian distribution as shown in Figure 5. Compare it to the histicon for parameter P1.
The parameter P2 is now only accepting values in between the selected range. You might use this feature if you were measuring a parameter that varied between two states and you were only interested in one of the states.
You have seen three different methods of limiting measurements to specific regions of a waveform, when certain other events were occurring, or when multiple values had to be separated.
This completes the tutorial.