Fast Scopes and Probes Save Signal Integrity

Oct. 1, 2002
Accelerating clock speeds and ever-faster digital circuits pose extreme challenges for even the best oscilloscopes. To measure clock speeds in excess of 200 MHz and edge rates less than 100 ps, real-time oscilloscopes must have extremely high sampling ...

Accelerating clock speeds and ever-faster digital circuits pose extreme challenges for even the best oscilloscopes. To measure clock speeds in excess of 200 MHz and edge rates less than 100 ps, real-time oscilloscopes must have extremely high sampling rates, broad bandwidths, and must be complemented by single-ended and differential active probes with comparable performance. Fast scopes have been available, but high-performance probes have been scarce. Fortunately, the new 6-GHz model 54855A and 4-GHz model 54854A Infinium oscilloscopes and a novel probe architecture known as InfiniiMax from Agilent Technologies (Colorado Springs, CO) effectively address these challenges, delivering sampling rates to 20 GSamples/s simultaneously on four independent channels.

The probes feature a new architecture to ensure full bandwidth even when accessories are used at the probe tip to make the physical connection to the device under test (DUT). Probe amplifiers can be configured for single-ended or differential operation through different probe heads, so a single probe amplifier can be used to make either type of measurement. The two new oscilloscopes (Fig. 1) retain all of the features of other Infinium models, including an 8.4-in. (21.34-cm) thin-film-transistor (TFT) color liquid-crystal-display (LCD) screen, 10/100 local-area-network (LAN) Ethernet interface, 10-Gb hard drive, and remote operability from the Internet. However, the operating system has been upgraded to Windows XP, a compact-disc-read/write (CD-RW) drive has been added and there is dual-monitor support for running third-party applications on the oscilloscope. Standard memory is 262 kpoint/channel, which can be increased to 1 Mpoint/channel with no loss of sampling rate, or up to 32 Mpoint/channel at sample rates of 2 GSamples/s and slower. Memory of 1 Mpoint creates a time-capture window of 50 µs per channel at 20 GSamples/s, which is more than adequate for most applications. Acquisition memory of 32 Mpoints at 2 GSamples/s and slower sample rates allows the designer to capture long time windows at high resolution, such as identifying glitches due to a power-supply start-up from reset.

The ability to maintain a full 20 GSamples/s on all four channels simultaneously when performing either single-ended or differential measurements is unique to the new scopes. All other four-channel scopes require two channels to deliver real-time, full-rate sampling at 6 GHz without aliasing, essentially turning them into two-channel instruments. The independent channels provide an important advantage, since in many measurement situations at least three signals must be viewed in relationship to each other to accurately determine performance margins.

In addition to the scopes and probes, Agilent has introduced an optional jitter-analysis package that will be useful for designers of high-speed clock circuits. It is integrated into the scope-application suite, and includes a setup wizard that guides the user through the jitter-measurement process, including tips on the various types of measurements and when to use them. Measurements include cycle-to-cycle, n-cycle jitter, and period jitter, as well as time-interval error, setup and hold time, measurement histograms, trending, and jitter spectrum.

While the bandwidth and sampling rates of the 54855A and 54854A are impressive, the use of a conventional probe architecture would have made these specifications almost meaningless. This is because the performance of traditional active probes drops precipitously at high frequencies as wire accessories are added to the probe tips to facilitate physical connection to the circuit. For example, a 2-in. (5.08-cm) long wire attached to the end of a probe with specified 6-GHz bandwidth will reduce the probe's effective bandwidth to only 1.5 GHz, rendering any additional scope bandwidth useless. In addition, as the maximum operating frequency of the probe increases, the size of the probe decreases, which can make it extremely difficult to use when "browsing" by hand from point to point in the circuit.

The new InfiniiMax architecture eliminates these drawbacks. In a conventional active probe, a length of transmission line in the measurement path (such as the aforementioned wire accessories) becomes a tank circuit at high frequencies that can resonate causing unwanted oscillation, variations in impedance, and reduced bandwidth. In contrast, the transmission line in the InfiniiMax probe circuit path is well-controlled, properly terminated, and compensated by the probe amplifier.

As a result, the InfiniiMax probe system (Fig. 2) offers the highest performance available for any use model— browsing, solder-in, socket or probe holder. To maintain usability of these small probes, Agilent has designed a sleeve that fits over the probe head, making it easier to hold for long periods of time in a "browsing" fashion. The company offers a considerable number of probe choices to meet customer needs ranging from low cost to the highest performance. Probe amplifiers with 3.5-, 5.0-, and 7.0-GHz bandwidths are available. Combined with "connectivity kits," these probes are capable of either single-ended or differential measurements, or both if two types of kits are selected. The kits contain a small browser (and sleeve), a socketed probe head, a solder-in probe head, and fully characterized performance plots for all of its various probe heads. These performance plots include swept-frequency response, common-mode rejection versus frequency, impedance versus frequency, time-domain probe loading, and time-domain probe tracking.

The InfiniiMax probe architecture circumvents issues that have plagued active probes for years. Since conventional probe designs have not been able to keep pace with bandwidth requirements, they have limited performance of even the fastest scopes to the maximum speed of the probe itself. In addition, these probes cannot achieve their specified performance when any wire accessory is attached to them, a situation that worsens rapidly as wire length is increased. Finally, as measurement frequencies have increased, probe designs have become smaller and smaller, which has made the probe difficult to hold. This is compounded by the task of inserting probes into the fine-pitch geometries of integrated circuits (ICs). All of these problems are successfully addressed by the new InfiniiMax architecture. P&A: 54854A (4 GHz) $49,995.00, 54855A (6 GHz) $58,995.00, InfiniiMax probes $3850.00 to $8550.00, Jitter Analysis Software $3995.00. Agilent Technologies, (395 Page Mill Rd., Palo Alto, CA 94303, (800) 452-4844,

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