Today's general-purpose function generators are highly versatile signal sources. Most often they are based on some sort of digital signal-generation scheme, such as direct digital synthesis. They offer an extremely stable frequency, low distortion, many types of waveforms, and often have built-in sweep and modulation. Many also include an arbitrary waveform generation capability as well. However, one area where their performance is often lackluster is in the generation of low duty-cycle pulses. While they may be able to generate a variable duty-cycle square wave, typically this can only be adjusted over a 20% to 80% range -- often in 1% increments.

How then does one go about extracting low duty-cycle pulses from one of these generators? The trick is to use the burst mode capability available in many of today's function generators. The general principle is to first set up a square wave with a period corresponding to the short pulse width that is required. Next, set up the generator to repetitively send a single cycle or burst of that square wave at a much lower frequency corresponding to the desired interval between pulses.

The following example -- generating a 1-ms pulse that occurs once every 200ms, corresponding to a duty cycle of 0.5% -- will provide a clearer understanding of what this process entails.

The first thing to be done is to calculate what the frequency will be of a square wave having a 1-ms positive pulse width, i.e. a 2-ms period. Taking the inverse results in a frequency of 500 kHz to which the function generator's output will be set.

However, since the objective is to burst a single pulse of this waveform once every 200 ms, the inverse will give a 5-kHz repetition rate. This is called the burst rate (see Fig. 1.)

Fig. 1: A graphical representation of burst rate, generating a 1-ms pulse with a 0.5% duty cycle.

To build a low duty-cycle pulse stream, the HP 33120A function generator, a general- purpose function generator with built-in burst capability, will be used as an example.

Fig. 2: The HP 33120A Function / Arbitrary Waveform Generator

Step 1. Set up the pulse: First, select the square wave function and set the output amplitude for the pulse at 1 Vpp. Next, set the appropriate frequency for the pulse width that is needed. As calculated previously, this is 500 kHz for a 2-ms period. (Positive for 1 ms, negative for 1 ms.)

Step 2. Set up Burst Mode: The next step is to select the BURST mode on the HP 33120A. Press SHIFT, BURST (Note that the BURST indicator lights up), and set the BURST COUNT to 1. This is accomplished by pressing the key sequence SHIFT, RECALL MENU, which leads directly to the BURST COUNT command in the display. To verify that the count is set to 1, move down a level in the menu structure by pressing the down arrow, "Ú". Press ENTER to save this value in memory. Next, to make sure there isn't any phase shift in the 500-kHz signal, set the BURST PHASE to zero degrees. Once again, press SHIFT, RECALL MENU, then the right arrow, ">", until BURST PHASE appears in the display. Use the down arrow, "Ú" to verify that the phase is zero degrees.

Step 3. Set the Burst Rate: Now, set the BURST RATE to the desired frequency of 5 kHz. Again, use the key sequence SHIFT, RECALL MENU to jump into the menu. Press the right arrow, ">", until BURST RATE appears in the display, and use the down arrow, "Ú", to enter the value of 1 kHz. Again, hit ENTER to save this value.

Step 4. Set the Burst Source to Internal: One final thing: With the HP 33120A, BURST can be triggered either externally or internally. In this case, the internally triggered burst mode is chosen. For a final time, press SHIFT, RECALL MENU, then the right arrow, ">", until BURST SOURCE appears in the display. Use the down arrow, "Ú", to verify that the BURST SOURCE is INT (Internal.)

Fig. 3: Scope display showing a 1-ms, 0. 5% duty-cycle pulse

Check the output on an oscilloscope (see Fig. 3 for how it should look), showing an expanded scale on the bottom trace and showing the pulse itself. Note that the readouts indicate that the frequency is 5 kHz, the pulse width is 1 ms, and the duty cycle is 0.5%, just as we desired.

This technique is not just limited to square waves. Other waveforms could also be used, including sine, triangle, even arbitrary waveforms. The phase and offset can be adjusted for whatever type of pulse is desired.

Fig. 4: Using burst to output an Arbitrary Waveform with a low duty cycle

Depending on the frequency capabilities of the function generator used there will be limitations on just how narrow a pulse can be generated and at how high a repetition rate. However, using this technique will allow you to generate low duty-cycle pulses from a general-purpose function generator without having to resort to a much more expensive pulse generator.

Biographical sketch:
Mark D. Bailey is the voltmeter product manager at Hewlett-Packard Company's Electronic Measurements Division. Bailey joined HP in 1983, when he worked in service support and then in on-line field sales support. He has now been involved in customer research, product definition, and several product launches, including the HP 34401A Digital Multimeter, the HP 33120A Function Generator, and, most recently, the HP LogicDart Advanced Logic Probe.

Prior to joining Hewlett-Packard, Bailey worked for nine years as an R&D design engineer for Tektronix, Inc., where he was project leader on several different instruments; he holds a B.S. in Electrical Engineering, which he earned at Brigham Young University. He also owns MDB Sound, a business specializing in sound reinforcement.

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