Pulse Fundamentals with a VXIbus Waveform Generator Abstract VXIbus test system developers are familiar with the functionality of analog "synthesized" pulse generators for test and measurement applications. Digital waveform generators can do everything that their analog counterparts do, with the added capability of shaping and randomizing the occurrence of a pulse. Some examples are presented showing specifically how to program rectangular, trapezoidal, sparse, multiple, non-linear rise time, and shaped pulses with the Racal Instruments Model 3151 Waveform Generator. Introduction Analog pulse generators are being obsoleted by digital waveform generators which offer all the features of traditional pulse generators plus added advanced features of a waveform generator. Traditional pulse and waveform generators can produce double pulses, programmable delays, programmable rise times, etc. But only digital waveform generators can output random pulse trains, add noise or overshoot to a pulse, or create pulses with odd or "defective" shapes. In the past digital waveform generators have not been considered to replace pulse generators because of comparatively low sample rates, and inadequate built-in pulse capability. But with VXIbus instruments, output frequencies of today's waveform generators equal or exceed those of pulse generators. Waveform generators are beginning to appear with a variety of built-in pulses such as rectangular, trapezoidal, exponential, Gaussian, triangular, and even sinc. Arbitrary waveform generators are assuming the role of waveform generators. This implies a more general usage for generation of waveforms like pulses, triggers, noise, or arbitrary signals. Digital waveform generators are finally ready to replace archaic pulse generators. The following examples present a few of the possibilities available for using waveform generators to produce useful, real-world pulses. The flexibility of the waveform generator's digital memory allows pulse creation limited only by the imagination of the test developer. Rectangular Pulses Digital waveform generators make good rectangular pulse generators because of fast rise and fall times. In this mode, anti-aliasing filters are disconnected to produce rectangular waveforms with high-frequency harmonics. Rectangular pulses are used to turn things on and off quickly, trigger other instruments, repetitively stimulate circuits to produce transients, and simulate digital data. Arbitrary waveforms may be used to simulate open-loop PWM waveforms. Amplitude and offset controls enable the simulation of different digital levels such as RS-232, TTL and ECL. Figure 1 shows an example RS-232 serial data transmission. The Model 3151 is used to output the ASCII string "3151" at 9600 baud, 8 data bits, no parity, and 1 stop bit. Table 1 shows the code used to generate this string. Figure 1: "3151" Serial Data String Example Note that TRACE definition data (in Table 1) is enclosed in brackets to denote that this data must be sent as a true binary string in Word Serial Binary format. This is easily done in a BASIC or C' program. Table 1: "3151" RS-232 String Example Trapezoidal Pulses The ability to vary the delay, rise, high, and fall times of a pulse, not available on all synthesized pulse units, enables a waveform generator to test digital logic, comparator thresholds and hysteresis, or to simulate different logic families. An example test stimulus might be: a 10ms rise time, a 4 ms high time, a 15 ms fall time, and a 20kHz repetition rate. Table 2 gives the SCPI commands to generate this waveform while Figure 2 shows the waveform. Table 2: Stimulus Pulse Example's SCPI Code Figure 2: Stimulus Pulse Example Sparse Pulses Synthesized pulse generators have the ability to set long delays between pulses. This is a good way to set a precise delay between the triggering of two events. Waveform generators offer the same capability; in two different ways. The first uses waveform memory to store the pulse and the time interval. The second uses the waveform generator's internal trigger timer to initiate the pulses at a fixed repetition rate. In either case, the pulse may be a single pulse or a burst of pulses. Table 3 shows programming examples of sparse pulses on a waveform generator. Table 3: Sparse Pulse Examples An example repetitive event might require a narrow stimulus pulse and a 1MHz repetition rate. Figure 3 shows an example of this as programmed by the first entry in Table 3: Figure 3: Sparse Pulse Example Multiple Pulses Common stimuli for communications systems are repetitive bursts of two or more pulses. Waveform generators can produce these signals by triggering a counted burst of pulses either with an internal trigger source or by sequencing the pulses with a fixed delay. For example, a 10 pulse stimulus repeating at a 10kHz rate is required to test a detector. Table 4 shows the commands to program a 3151 to do this using internal trigger generation. Some communications applications require double pulses repeating at a 1MHz rate. This exceeds the repetition rate of the internal trigger found in some waveform generators. The Racal Instruments 3151 overcomes this by providing the capability to generate a sequence of an arbitrary waveform and an arbitrary delay. SCPI code is shown in Table 4 and Figure 4 shows the example graphically. Table 4: Example Commands for Programming Multi-Pulse Stimuli Figure 4: Double Pulse Example Pulses with Non-Linear Rise Times The best synthesized pulse generators can generate pulses having rise and fall times with linear slopes. Only waveform generators can generate pulses with non-linear rise and fall times. A common example of this is the exponential rise and fall times which simulate the charging or discharging of a capacitor. The leading edge of an exponentially rising pulse transitions smoothly from flat to steep without ringing or overshoot. SCPI syntax and Figure 5 are shown for a 100kHz train of pulses with exponential rise times: RES reset state FUNC:SHAP EXP exponential shape FREQ 100E3 100 kHz EXP:EXP 80 time const: 80 VOLT 5 10Vpk-pk OUTP ON output on Figure 5: Exponentially Rising Pulse Train Example Although this non-linear pulse train was generated with built-in waveform commands, other non-linear pulses can be created with arbitrary waveforms. Arbitrary non-linear pulses can be created and imported from math or spreadsheet software. They can also be created using Racal Instrument's WaveCAD software. Shaped Pulses Pulse Shaping is required when simulating pulses for data transmission or modulation. Gaussian pulses may be used to simulate variable amounts of spreading from different lengths and types of fiber optic cables. These smooth pulses may be spread over wider and wider amounts of the pulse period by increasing the time constant. For example, a 12.5kBaud pulse train from a fiber optic cable is to be simulated to test a repeater. SCPI code for the model 3151 is as follows: RES reset state FUNC:SHAP GAUS Gaussian pulse FREQ 125E2 12500kHz VOLT 5 10Vpk-pk GAUS:EXP 80 width parameter OUTP ON turn on output The waveform is shown in Figure 6: Figure 6: Gaussian Pulse Train Pulse shaping is important for high speed digital communications system research and development. Because communications channels are bandlimited, it is often desirable to use pulses of limited bandwidth instead of square digital pulses which have high frequency harmonics. Improperly shaped pulses tend to spread into each other and create intersymbol interference which can degrade communication links. The sinc function is an example of a pulse shape which has a bandlimited frequency spectrum with a sharp cutoff frequency. Waveform generators can produce pulse trains of sinc pulses to characterize communications channels with limited bandwidths. The Model 3151 provides a built-in sinc pulse function. An example sinc pulse train is shown below along with SCPI code to produce a sinc pulse train with a 100 Hz repetition rate: RES Reset parameters FUNC:SHAP SINC Sinc pulse FREQ 100 100Hz rep. rate VOLT 6 12 Vpk-pk OFFSET 4 4 Volt offset SINC:NCYC 5 10 zero crossings OUTP ON Turn output on Three cycles of the 100 Hz sinc pulse are shown below: Figure 7: Sinc Pulse Train Other types of bandlimited pulses are used to optimize digital communications systems. A more optimal pulse shape known as the root-raised cosine is similar to the sinc function except the bandwidth is not cut off as sharply and the pulse decays more quickly. Root-raised cosines and other modulation pulses may be generated mathematically (i.e. with WaveCAD equation entry or a PC based math program) and may then be transferred to the waveform generator as an arbitrary waveform. Summary The examples presented here provide compelling evidence that modern digital waveform generators surpass synthesized pulse generators in many performance categories. VXIbus test system developers should be aware of this for two reasons. First, if both "arbitrary" waveform generator and pulse generator functionality are required in a system, it is more cost-effective and space-efficient to replace the two units with one waveform generator. Second, if only pulse generation is required, the waveform generator makes a far more flexible choice then does a synthesized pulse generator. The waveform generator duplicates the functionality of the pulse generator but allows the test developer to use his or her imagination without being limited by "squared-off" pulses. TOC 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13