Digital Delay Generators Expand VXI Testing in Time Domain

It is now possible to build into VXI/VME systems the fine resolution, low jitter and variable output levels that have been available only in benchtop digital delay generators (DDGs). Because of major design enhancements, VXI/VME delay generators have significant advantages over benchtop instruments. These include agile, error-free, real-time programming; the ability to combine multiple timing channels into delay, gate, delay and gate configurations; triggerability via the bus; and the ability to stack several modules to form complex serial sequences.

What Is a Digital Delay Generator?

When triggered, DDGs provide low-jitter, highly resolved, stable timing pulses that cannot be provided by other means. The alternatives–such as one-shots, standard pulse generators, delay lines and lumped delay modules–increase jitter and deteriorate pulse-edge speeds.

DDGs produce digitally adjustable time intervals and fast-edged pulses which mark 1) the end of each interval and 2) precise gate widths (Figure 1). The edge speeds do not deteriorate with increased time. DDGs can be triggered at random by some external event or be self-triggered by an internal oscillator. They can use an external clock as reference.

An example of DDG performance–six channels of delay in one single-width module–is shown in Figure 2. Channel delay and width can be adjusted up to 168 ms. Jitter is less than 50 ps even at delays of milliseconds. If you provide your own system clock as an external reference, the basic jitter is reduced to 25 ps.

Attributes Useful for ATE

Some properties broaden the suitability of DDGs for testing applications:

Faster programming times than with GPIB or RS-232.

Real-time, on-the-fly programming that is error-free.

Flexible combining of modules and individual timing channels.

Adjustable pulse top and baseline levels.

Multiple channels in one single-width module at a low cost per channel.

Faster Programming

Speedy programming of delay, width or amplitude is critical for real-time test systems. Parameters are changed on-the-fly as measurements or calculations are made. Programming times of microprocessor-based GPIB delay generators is typically 30 to 40 ms per channel. VXI DDGs can be programmed in less than a microsecond.

Error-Free Programming Anytime

The ability to program new timing or amplitudes, error-free, as soon as measurements or calculations warrant is important. Conventional programming schemes must take into account illegal windows during which programming would cause errors in the data. For example, data is transferred from the bus into a delay generator before or after a timing sequence. If new data is inserted during this transfer time, the delay generator may or may not recognize this new data and erroneous information may be conveyed (Figure 3). The VXI/VME delay generators provide error-free programming by storing and then transferring the new data when it can be done safely.

Combined Timing Functions

The delay channels can be configured as delay only, delay and gate, gate or combinations of the above. Several modules can be grouped in series to generate complex timing sequences.

Adjustable Baseline/Pulse-Top Levels

Because it has adjustable output levels, a DDG can provide signals for a broad range of devices and logic families. The levels can be adjusted to check level sensitivities, noise margins and discriminator levels.

Low Cost/Channel

There are significant cost savings to bundling many channels in one small module. A small space-per-channel requirement is a clear advantage.

Conclusion

VXI/VME digital generators give ATE systems integrators significant cost, size and performance advantages, especially when compared to stand-alone units. They add precise, low-jitter and accurate control of delays and widths to this fast backplane, modular architecture.

About the Author

John Yee is Product Manager at BNC. He joined the company 20 years ago as a senior engineer. He has a B.S. degree in electrical engineering from Stanford University and an M.S. degree in electrical engineering from the University of California/Berkeley. Berkeley Nucleonics Corp. (BNC), 3060 Kerner Blvd. #2, San Rafael, CA 94901, (415) 453-9955.

Instrumentation

Copyright 1995 Nelson Publishing Inc.

September 1995

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