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Arbitrary Waveforms
Arbitrary waveforms can meet needs not met by the instrument's standard waveforms. For example, you might need
a unique stimulus, or you might want to simulate signal imperfections such as overshoot, ringing, glitching, or
noise. Arbitrary waveforms can be very complex, making them suitable for simulating signals in modern
communications systems.
You can create arbitrary waveforms from a minimum of 8 points up to 1,000,000 points. The instrument stores
these numeric data points, known as "samples," in memory and then converts them into voltages as the waveform is
generated. The frequency at which points are read is the "sample rate," and the waveform frequency equals the
sample rate divided by the number of points in the waveform. For example, suppose a waveform has 40 points and
the sample rate is 10 MHz. The frequency would be (10 MHz)/40 = 250 kHz and its period would be 4 μs.
Waveform Filters
The instrument includes two filters to smooth transitions between points as arbitrary waveforms are generated.
– Normal filter: A wide, flat frequency response, but its step response exhibits overshoot and ringing
– Step filter: A nearly ideal step response, but with more roll-off in its frequency response than the Normal filter
– Off: Output changes abruptly between points, with a transition time of approximately 10 ns.
Each filter's cutoff frequency is a fixed fraction of the waveform's sample rate. The Normal filter's response is -3 dB at
27% of the sample rate and the Step filter's response is -3 dB at 13% of the sample rate. For example, for an
arbitrary waveform at 100 MSa/s, the Normal filter's -3 dB frequency bandwidth is 27 MHz.
Turning the filter off may change the sample rate to a lower rate if the sample rate was greater than 250 MSa/s
before the filter was turned off.
Quasi-Gaussian Noise
The Noise waveform is optimized for both quantitative and qualitative statistical properties. It does not repeat for
more than 50 years of continuous operation. Unlike a true Gaussian distribution, there is zero probability of getting
a voltage beyond the instrument's Vpp setting. The crest factor (peak voltage divided by RMS voltage) is
approximately 4.6.
You can vary the Noise bandwidth from 1 mHz to the instrument's maximum bandwidth. The energy in the noise
signal is concentrated in a band from DC to the selected bandwidth, so the signal has greater spectral density in the
band of interest when the bandwidth setting is lower. In audio work, for example, you might set the bandwidth to 30
kHz, to make the audio band signal strength 30 dB higher than if the bandwidth were set to 30 MHz.
PRBS
A Pseudo-Random Bit Sequence (PRBS) has two levels (high and low), and it switches between them in a manner
that is difficult to predict without knowing the sequence generation algorithm. A PRBS is generated by a linear-
feedback shift register (LFSR), shown below.
Keysight EDU33210 Series User's Guide
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