Special Trigger Modes
Software Assisted Trigger is used to find the trigger-level crossing point closest to the hardware trigger point. It then adjusts the time offset of the waveform so that it is aligned with the specified trigger level and slope.
Using Measurement triggers you can leverage parameter measurements as waveform trigger conditions.
Measurement triggers can be either the only trigger or the final trigger in a chain of trigger events including hardware triggers.
The Cascaded trigger allows you to define successive trigger Stages (referred to as Stage A, B, C, and D) as arm, trigger/arm, trigger/arm, trigger criteria, respectively. This means you can qualify a trigger by up to three pre-conditions.
WaveRunner 6 Zi oscilloscope
Displays shown in the tutorial are based on the following initial setup on a WaveRunner 6 Zi scope:
- Connect a coaxial cable from channel 1 to the Aux connector on the front panel.
- Recall the default setup: File pull down > Recall Setup> Recall Default.
- Set the input coupling on Channel 1 to be 50 Ohms: Touch or click the channel 1 annotation box>touch or click on the coupling field >select DC 50 Ω.
- Set up the Aux output to be the Fast Edge signal. Utilities pull down > Utilities Setup >Aux Output Tab>touch or click on Fast Edge. The Fast Edge signal is a 5 MHz, 450 mVp-p, square wave.
- Set up the Cal Signal to be a 1 MHz square wave. Utilities pull down>Utilities Setup >Aux Output Tab>Use Calibration Output For –Frequency 1 MHz
- Connect the passive probe to the channel 2 input of the oscilloscope. Connect the probe tip to the CAL test point on the front panel of the scope.
- Turn off channel 2.
- Auto Setup the scope: Press Scope Setup then select Auto Setup from the fly-out menu.
- Using the C1 dialog box, set the channel 1 vertical scale to 100 mV/division and the vertical offset to 0 V.
- Set the trigger level to 0 V.
- Set the trigger mode to normal.
- This completes the initial setup. The scope display should be similar to Figure 1.
Decrease the horizontal scale to 100ps/division using the front panel horizontal scale knob. Touch or click the trigger annotation box or use the front panel Trigger Setup button to open the trigger dialog box. Touch or click on the Software Assisted Trigger tab. The screen should look like Figure 2.
Note that in Figure 2 the trigger location shows the waveform being slightly below the trigger level at the horizontal trigger indicator. This occurs because the trace and the hardware trigger paths are different and there are small timing differences. Software Assisted Trigger measures this difference and deskews the trigger and data so the trace crosses the vertical trigger threshold at the trigger time.
Touch or click the Software Assisted Trigger Enable check box. The result should be similar to Figure 3.
The Software assisted trigger was used in its default Auto mode. It is also possible to selectively gate the desired trigger point in the normal mode.
Turn off Software Assisted Trigger by unchecking the Enable box.
Change the timebase setting to100 ns/division. You should see five cycles of the square wave on the screen. On the Software Assisted Trigger Tab change the Horizontal Gate Start to 190 ns and the Stop to 210 ns. Change the Mode to Normal. Enable Software Assisted Trigger. The trigger point shifts two divisions right and the edge located between 190 and 210 ns from the original trigger location is now the trigger point as shown in Figure 4.
The hysteresis control is used to minimize the effects of noise on the trigger point. The Hysteresis selection imposes a limit above and below the Level, which precludes measurements of noise or other perturbations within this band. The width of the band is specified in milli-divisions.
Some Guidelines for use of these Controls
Hysteresis must be larger than the maximum noise spike you want to ignore.The largest value of hysteresis usable is less than the distance from the level to the closest extreme value of the waveform. Unless you know the largest noise and closest extreme level that will ever occur on any cycle, leave some margin on both sides of the level. The default value is a good starting point unless your signal is very noisy.
Measurement Trigger allows the user to trigger the scope based on the result of a measurement. Any of the oscilloscope’s measurement parameters can be used. Measurement conditions including ‘less than’, ‘greater than’, ‘inside a range’, ‘outside a range’, or ‘don’t care’ can be applied to trigger on specific measurement values. Figure 5 shows the measurement trigger set up in the Trigger dialog box.
Touch or click the trigger annotation box or press the front panel Trigger Setup button to display the trigger dialog box.
Turn off the software assisted trigger. Make sure the trigger mode is set to normal and the scope is acquiring data.
Use the Measure pull down menu to access the measurement setup.
Check the Show table checkbox.
Press the clear All Definition buttons on the right side of the measure dialog box.
Check the Statistics on check box.
Set up parameter P1 to measure the risetime of Channel 1. Turn P1 on using the On checkbox on the left side of the P1 tab. The screen should appear as shown in Figure 6. Using the measurement statistics we can see that the risetime of the FastEdge signal varies from approximately 278 to 382 ps. Let’s trigger the scope on values greater than 350 ps.
Go to the Trigger dialog box. Select Measurement as the trigger type as shown previously in Figure 5.
Set up the following on the trigger dialog box:
Condition : Greater than
Limit: 350 ps
The oscilloscope is now triggering on a risetime greater than 350 ps. Note that the oscilloscope is acquiring data and then using a software based trigger to position the trigger point at the first measurement meeting the trigger condition. Because this is done after acquisition the trace may be truncated as shown in Figure 7.
You can try the measurement trigger using other parameters.
Return the trigger type to edge trigger.
Cascaded triggers allow the scope to be triggered based on multiple events by arming the trigger on a single first (“A”) event or a succession of multiple events (up to 3) and then triggering on a specified condition. Holdoff and reset capabilities are provided between each set of events.
Let’s set up a simple 3 stage cascaded trigger.
Turn on channel 2. Set the vertical scale to 200 mv/division. This is a 1 MHz square wave from the Cal test point on the front panel which we had setup initially.
Use the trigger dialog box to select the Multistage trigger type. Press the Cascaded trigger button. On the cascaded trigger tab you will see 4 trigger stages labeled A through D. The default is stage A set to edge trigger on channel 1 (C1).
Set up the following conditions as shown in the following figures:
The result of this trigger setup is shown in Figure 12.
Referring to Figure 8, the scope trigger was armed by a positive edge on channel 2 exceeding 0.5V followed after the 200 ns holdoff by a positive edge on channel 1 exceeding 0V. After the dual arming condition is met, the scope triggers on a risetime in excess of 325ps. In Figure 12 a second risetime measurement is setup using measurement gates to measure only the edges at the trigger time. While the general risetime measurement has a range of values from 280 ps to 395 ps, the gated measurement shows a range of values all exceeding 325 ps.
This concludes this tutorial.