When making timing (propagation delay) measurements, ensure that the positive input and negative input of the first test point are in the same direction on the second test point.
Also, remember that measurement-system bandwidth includes scope, probe, and source. Calibrate and characterize instruments, probes, and output devices as a "system." To improve measurement accuracy, system bandwidth should be three to five times the signal to be measured. Knowing your application helps in selecting your probe type. Consider these factors:
Signal type being measured—voltage, current, logic, other
Signal frequency content—dc, Hz, kHz, MHz, GHz
Signal source impedance—resistive, capacitive, inductive
Physical connection considerations —DUT and instrument
Instrument input—50 Ù, 1 MÙ, other
Instrument bandwidth or rise time
Measurement tools have limitations, so there is a need to reduce the amount of known error when making crucial design and engineering measurements. The errors are either electrical (amplitude errors, phase-angle shifts, propagation delays, changing source impedance) or mechanical (physical geometries, attachment degradation, attachment capability). Figure 4 provides an indication of the amount of error involved when the measurement tool isn't fast enough.
Timing measurements are always a concern. As differential and single-ended signals are merged, making these timing measurements will become more and more crucial. For example, when using a differential probe on a processor or bus structure using Rambus, deskew resolution requires timing alignment of the differential signal to the corresponding bus signals. These nanosecond and subnanosecond signals will require additional bandpass from the measurement tool in order to avoid adding errors to the measurement in question.
With any new technology comes new probe-to-DUT interface requirements. The ability to probe the IC will provide the most accurate and reliable signal measurements. Many of today's surface-mount ICs, however, will need to be accessed by probing close proximity pins that require unique probe-tip attachments. Compliance at the probe tips is necessary so probes can be positioned at any angle, and so sufficient force can be applied to assure excellent contact.
Today's computer speeds are limited by the bus structures used by the microprocessor, memory-management chipsets, and peripheral device chipsets. The present architecture used in memory management limits the bus speeds to less than 200 MHz.
Differential-technology applications have improved the speed of processors, displays, and memory buses. Rambus technology, with edge speeds from 600 ps down to 200 ps (600 MHz to 1.6 GHz), represents one of the technology directions for microprocessor memory, displays, and transmission architectures. Rambus technology employs a differential clock as opposed to relying on the present ground-based processor clocking systems. Considering that the clock signal is at the heart of the processor, and that the Rambus clock is differential, it's a fairly safe bet that differential probing technology will be necessary to meet measurement requirements.
IEEE 1394, also known as Firewire, is a rapidly developing plug-and-play technology in the computer peripherals field. It's a pure differential signal. True differential probing will greatly assist this technology's development. The increased dependence on high data-rate signals in video transmission, color printers, and DVD continues to create increased requirements on data-rate transmissions. Signal rise- and fall- speeds reach 260-ps rates with equivalent bandwidths of 1.5 GHz.
Local-area networks with a greater dependence on data transmission have placed increased demands on standard Ethernet connections. Gigabit Ethernet has been developed to alleviate the increased data-transmission demands.
For all of these fast technologies, the low differential-IC's voltage swings (0.8 V to 1.2 V, in some cases) for logic levels increase the susceptibility to noise and signal degradation. Thus, high signal-to-noise ratio is of significant importance. Differential-measurement solutions can provide this level of integrity and performance where a signal-ended system will start to stumble.
Today's signals are measured through all kinds of techniques: single-ended passive probes, single-ended active probes, differential-amplifier systems, battery-powered instruments, and differential probes. The trend is "faster is better." One needs only to look at high-performance technology products and watch how new technology trickles down into consumer products to see this.
As the speeds of devices increase and move into the next generation, the nature of probing will become more complex. Differential probing techniques will continue to evolve and improve. Measured signals will require more direct-connection techniques to provide the reliable and accurate measurement speeds needed to design, verify, and manufacture future products. Today's differential solutions are expanding to meet these and future needs.
Despite all of these voltage-measurement techniques, other measurement styles and approaches may be needed to produce the accurate and reliable measurements required by tomorrow's products. The differential current measurement approach may help improve the speed and accuracy of these measurements, though it's a little harder to use. Higher-frequency current probing solutions are now available for this technique. Electro-optical, another arena that is changing direction quickly, also may lead the way in meeting future measurement needs.
We may not know what the exact structure of future technology will be, but we do know that tomorrow's designs will be faster and smaller. So the measurement tools will have to be faster, more accurate, and easier to use, too.
Recommended Reading:
"ABC's of Probes," Tektronix Inc., Literature number 60W-6053-7, July 1998.
Feign, Eric, "High-frequency probes drive 50-ohm measurements," RF Design, October 1998.
"Measurement Solutions for Disk Drive Design," Tektronix Inc., Literature number 47W-8257-2, April 1992.
Parham, Johnny, "High-Speed Probing," Tektronix, Inc., Literature number 55W-12107-0, June 1998.
Sekel, Steve, "Differential Oscilloscope Measurements—A Primer on Differential Measurements, Types of Amplifiers, Applications, and Avoiding Common Errors," Tektronix Inc., Literature number 51-W-10540-1, July 1997.