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Agilent 33500 Series
Waveform Generators
PRBS Pattern Generation
Technical Overview
Testing digital devices typically requires sending a series
of alternating high and low voltage values (bits) to the
device under test (DUT). If these bits are easily predict-
able, the test coverage will be limited to a small set of
possible scenarios. For this reason, it is often helpful
to send a near-random stream of bits, such as the one
shown in Figure 1, to the DUT.
To generate this type of bit stream, the Agilent 33500
Series waveform generators include a pseudo-random
binary sequence (PRBS) waveform generator. A PRBS
waveform is generated by a linear feedback shift register
(LFSR) like the one shown in Figure 2.
Figure 1. A near-random stream of bits
The branches after register cells 5 and 9 are called taps,
and the configuration of the LFSR and its taps determines
the PRBS waveform.
Figure 2. A linear feedback shift register
How to generate a PRBS waveform from the
front panel
From the front panel, press [Waveforms] > More > PRBS
> [Parameters]. You can then specify the polynomial
function used to generate the sequence (7, 9, 11, 15, 20 or
23 register cells), the bit rate at which the sequence plays
(up to 50 Mbps), and an edge transition time that is used
for both rising and falling edges.
Figure 3. PRBS menu
FUNCtion:PRBS:DATA The number after the PN
How to generate a PRBS waveform using SCPI {PN7|PN9|PN11|PN15|PN20|PN23} specifies the LSFR length.
An LFSR of length L results
You also can generate PRBS waveforms using the Standard in a bit sequence of length
Commands for Programmable Instruments (SCPI) program- 2L-1.
ming language. The equivalent SCPI commands are:
FUNCtion:PRBS:BRATe The can range
from 1 Mbit/s to 50 Mbit/
sec
FUNCtion:PRBS:TRANsition[:BOTH] The edge transition time for
leading and falling edges,
from 8.4 ns to 1 s.
Summary
Use the PRBS waveform to improve the robustness of your
digital device tests. By sending a near-random bit stream to
your device, you can test your device under a wide variety
of conditions.
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