Create short pulses with a function generator

Use the burst mode to circumvent the limitations of a function generator's duty-cycle adjustment.

Cheryl Diller, Agilent Technologies Loveland, CO -- Test & Measurement World, 2/1/2001

When you need to create simple pulses and don’t have a pulse generator, you can often substitute a function generator. By using a function generator’s triggered burst function, you can create pulses with pulse widths that are relatively short (or relatively long) with respect to their periods.

Function generators create square waves, and you can vary their duty cycle—within limits typically between 15% and 85%. Outside of those limits, engineers think of the waveforms as pulses with periods and pulse widths rather than as a square waves with duty cycles and frequencies.

Figure 1. A comparator compares a sine wave to a voltage level. When the sine wave reaches that level, the comparator’s output goes high.

Many function generators create square waves by applying a bipolar sine wave to a comparator’s input. When this input signal exceeds the comparator’s threshold voltage, the instrument’s output is V_high . Otherwise, the comparator’s output is V_low. When the sine wave’s DC level reaches the comparator’s threshold voltage and that voltage is set to 0 V, the function generator produces a square wave, a signal with a 50% duty cycle (Output 1 in Figure 1).

But the comparator has practical limits. As the comparator’s threshold level rises (DC2 in Figure 1), the output pulse narrows to Output 2. As the threshold reaches the sine wave’s peak, there’s no longer enough voltage difference between the peak and the threshold to produce clean, reliable transitions from the comparator’s output.

Narrow pulses

You can get a function generator to produce pulses beyond the duty-cycle limits of the comparator. Assume you need to generate a pulse that’s high for 1 ms, returns low, and repeats every 10 ms—effectively a signal with a 0.1% duty cycle. You can produce this pulse by using a function generator’s triggered burst mode.

In its internally triggered burst mode, the function generator generates n cycles of a waveform every t seconds. (Some function generators refer to the repetition rate in frequency rather than in time, so you may need to convert units in your waveform calculations to match those of your instrument.) To produce a 50% duty cycle square wave that’s high for 1 µs, set the function generator’s output frequency to 500 kHz.

Set up a square wave with a 50% duty cycle. Assume that the time the signal is high will be the duration of your positive pulse. You also must set the voltage levels for high and low.

Use the function generator’s amplitude and offset voltage settings:

Amplitude = V_high – V_low
Offset = (V_high + V_low)/2
For a TTL signal where V_high = +5 V and V_low = 0 V, set the amplitude to 5 V and the offset level to +2.5 V.

Figure 2. A burst of one cycle can produce a narrow pulsed, repetitive signal.

Figure 3. Use burst delay to produce narrow low pulses.

Now, set your function generator to produce one cycle of the square wave every 10 ms. If your function generator refers to the burst rate in terms of frequency, then use a frequency of 100 Hz. Figure 2 shows the resulting waveform.

Wide pulses

You can use the same concept to produce the complementary waveform, one that’s low for 1 µs and high the rest of the time. Because you can’t get inside the function generator and swap the signals on the comparator’s input terminals, you’ll need another method.

For high duty-cycle waveforms (pulses almost as long as their duration or narrow low-going pulses), you can use the burst mode to extend the high level of the pulse waveform. To do this, you must set the function generator’s burst phase so the waveform begins and ends at the high level. This process effectively delays the phase of the instrument’s sine wave so it starts at nearly 1808.

For a 50% duty-cycle waveform that starts low and ends high, adjust the function generator’s burst-phase adjustment to just under 1808. When the burst starts, the sine wave in Figure 3 will be just above the high-to-low threshold. Soon after the burst starts, the sine wave will drop below the threshold and the comparator’s output will fall—remaining low for one-half cycle of the sine wave.

When you set the triggered burst to one cycle, the cycle ends before the sine wave crosses back below the threshold. The instrument’s output remains high until just after the next burst trigger occurs. Setting the square wave’s frequency to 500 kHz produces a low-going pulse that is 1 µs wide.

The next time you need a short pulse and you don’t have a pulse generator handy, use a function generator. By using a function generator’s burst mode, you can produce pulses that are shorter than a function generator will otherwise let you create. T&MW

Cheryl Diller is the product manager for sources in Agilent Technologies’ Measurement Products Generation Unit in Loveland, CO, where she has worked for six years. She has a BSEE from the University of Michigan, Ann Arbor.