Electronic Design

Precise Trigger Aids In Recording Scope Signals On Plotter

Analog oscilloscopes remain the lowest-cost means for viewing waveforms. Capturing single-shot or repetitive signals using a film camera, digital camera, or camcorder1 can be costly, however. Another drawback is that the resulting 2D format renders subsequent signal processing and storage difficult. Recording with a digital storage oscilloscope (DSO) or a DSO board with a dedicated PC, on the other hand, offers many advantages. Unfortunately, these aren't always readily available.

Repetitive signals displayed on an analog oscilloscope may be recorded using the circuit in Figure 1 with the previously described 1-GHz sampling circuit2. This technique is useful for all oscilloscope settings up to 50 ns/div. While monitoring several signals in sequence on a single-trace oscilloscope, multiple records may be made on an X-Y plotter in their proper temporal positions. Also, the circuit provides a highlighted panoramic view and a low-jitter output pulse for triggering a sampling oscilloscope2 or DSO, for displays and plots with much faster sweep speeds.

The buffered CRO time-base ramp T triggers the MAX997 comparator at the plotter-voltage X, which is chosen manually on the 250-Ω, 10-turn potentiometer. Advancing this delay-control advances the plotter pen to plot the waveform. Concurrently, the intensified portion of the CRO beam advances along the displayed signal VIN (Fig. 2, trace A).

This highlighted edge acts as a cursor to monitor the progress. The beam is modulated by the MAX997 output, which is connected to the CRO's Z-input (Fig. 2, trace B).

Other similar channels may be added as needed to trigger on the same ramp T at other phases on the waveform. This comparator's negative transition also triggers the MAX961 strobe comparator. If this circuit is installed in place of the fast ramp section, its outputs drive the T/H sampler in reference 2.

Feedback via the 1-nF, 1.5-k network to its latch enable (LE) terminal locks this comparator's state for 650 ns (Fig. 2, trace C). An active probe1 with a 50-Ω output is used to eliminate the effects of sampler kick-out. This probe connects the input signal to the 50-Ω sampler input VIN, to acquire samples Y for the plotter. Even the graticule can be downloaded along with the plotted traces, if pen marks are made as the cursor crosses the extreme horizontal or vertical lines. Additional lines may be added with a ruler.

Times and voltages at the cursor point may be read at X and Y using a digital voltmeter. These uncalibrated, filtered outputs may also be read by any DSO board, or even by a PC's stereo sound card. Alternatively, incrementing a digital word representing X can drive the MAX997 via a DAC. As a result, the corresponding values of Y may be read by an ADC.

Level triggering on the trace A signal will often produce an unstable display since, like many signals, it contains several possible trigger points. The display may be reliably stabilized by using external triggering on a programmable counter output, set to countdown to the signal pattern's fundamental or subharmonic frequency.

Another option relies on the oscilloscope's variable trigger hold-off to adjust the sweep repetition rate. It does this until it triggers on a single feature of the signal at its pattern fundamental or subharmonic frequency. A different method is used for oscilloscopes lacking this control. In addition to indirectly providing variable trigger hold-off, this technique involves adjusting the variable time/div control. Although this places the time/div setting in an uncalibrated state, the horizontal-magnifier can still restore the desired time per division with the aid of a calibration signal.

It's often necessary to display fine features in a signal concurrently using a faster sweep. Again, level triggering could occur at several points, leading to an unstable display. The highlighted signal illustrates how this circuit can provide a single delayed trigger for triggering a sampling oscilloscope (Fig. 2, trace B, again). This unrefined method will exhibit considerable jitter due to slight ramp and signal fluctuations, however (particularly when the two sweep speeds are widely disparate).

The proper solution is to retrigger on the signal after the delay, which is accomplished using a second MAX961. Most dual-trace oscilloscopes can trigger their delayed sweep in this manner3. But since the bandwidth of a sampling oscilloscope is higher, this circuit provides better resolution. Rather than triggering the second oscilloscope itself, the delayed trigger briefly operates the latch enable terminal of the second MAX961.

For the reintroduced VIN signal to produce a trigger pulse as shown here, the comparator must be enabled by the delayed trigger (Figure 2, trace D). The 500-Ω control sets the enable window between 100 ns and 600 ns. Using the sign of the voltage level set on the 1-k potentiometer, it can be determined whether the triggering occurs on a positive or negative edge.

This resynchronized, delay-qualified trigger will reliably trigger any other oscilloscope with only a 4.5-ns propagation delay through the MAX961. The trigger system routinely displays fine features, like this short central pulse on a 1-GHz oscilloscope with less than 20-ps jitter (Figure 2, trace A, again). It consistently rejects all other possible trigger edges outside the enable window. Together with its display oscilloscope, this circuit furnishes a precise delay-qualified trigger peripheral for a sampling oscilloscope2 or any other instrument lacking this feature.

References:

  1. Hickman, "Towards A 500-MHz Scope Add-On," Electronics World, March, 2000, p. 202.
  2. H. Houtman, "1-GHZ Sampling Oscilloscope Front End Is Easily Modified," Electronic Design, September 18, 2000, p. 175.
  3. J. Ganssle, "Delayed Sweep: A Critical Tool," EDN, April 27, 1995, p. 50.
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