Electronic Design
As Scope Bandwidths Rise, Probes Must Follow Suit

As Scope Bandwidths Rise, Probes Must Follow Suit

Some of the sharpest design engineers in the world ply their trade at test-equipment companies. Designing an oscilloscope with a bandwidth of 45 GHz, as in the case of LeCroy’s WaveMaster 8 Zi-A, is no mean feat. That instrument and others such as Agilent’s Infiniium 90000-X series scopes bring a wealth of measurement capabilities to the testbench. They can reveal minute details of signals with lightning-quick rise and fall times.

But these scopes would not be as capable as they are if they were not equipped with equally capable probing systems. The last thing test engineers want or need is a probe that’s going to influence their measurements or fail to deliver the full bandwidth that’s available to them on the scope. Fortunately, today’s high-end probes are constructed to sidestep these issues.

Logic Probes Boost Bandwidth
On the high-performance side of the logic-probe equation, test vendors are finding that customers needed more bandwidth and accuracy in their probes along with greater signal visibility. One example of a probe that accomplishes this goal is Tektronix’s P6780 differential-input logic probe (Fig. 1).

Designed for use with Tek’s MSO70000 series scopes (see “Scopes Deliver High Signal Fidelity To 20 GHz"), the P6780 is the company’s first logic probe with a 2.5-GHz bandwidth that exceeds that of many traditional scope probes. With it, users can connect the digital channels on the MSO70000 series scopes to the digital buses and signals in the target system. The probe breaks out 16 data channels and one clock/data channel over two flying-lead sets.

With a probe (Fig. 2) such as the P6780, users are looking to acquire a number of different signals from a device under test (DUT) or board and observe them in parallel. It’s a style of probing that’s traditionally employed with logic analyzers, but here Tektronix has adapted it to use with a mixed-signal oscilloscope.

“It’s an interesting mechanical engineering challenge to have these 16 data points, all running at 2.5 GHz, in a package that’s 1 to 1.5 inches wide and 1.8 inches thick,” says Chris Loberg, senior marketing manager at Tektronix.

The most prevalent application for such a probe is logic qualification. Tektronix has found that the biggest need for the probe was among customers doing Double Data Rate 3 (DDR3) designs and trying to make closing market windows for their end products. In doing so, they must examine many different data points as they debug the timing relationship between the DRAM and memory controller.

In designing the probe, Tektronix’s engineers faced mechanical constraints related to interference or crosstalk between the probe’s channels. It was critical to ensure that each of those 16 signals is accurately captured at the board and brought back to the scope while preserving the frequency content. Frequency content from adjacent channels is paired and brought into the probe in two blocks of eight. These blocks merge in a ribbon cable to the instrument connector.

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The biggest concerns in the mechanical design of the probe were electromagnetic interference (EMI) issues with adjacent channels and on the board under test. Debugging a DDR3 design is a challenging task with a high-speed bus and processor both radiating some amount of EMI. Capturing logic signals in such an environment is challenging enough without corrupting interference. Thus, very careful consideration was given to shielding and grounding.

In a probe with so many channels, there is also the issue of timing skew between channels. The mechanical length of the cables is carefully considered to minimize skew. “What we want is to bring the signals up to the front end of the scope and not have the user concerned with skew from channel 1 in the digital bus set up to channel 12,” says Loberg.

An interesting side note to the design of a multi-channel differential probe is how a manufacturer like Tektronix decides how many channels to build in. What is the “sweet spot” for the number of channels? “I see the sweet spot in the range of eight to 10 channels,” says Loberg. But Tektronix chose to build the probe with 16 channels, as it is a good “building-block” number.

One limitation on how many channels a probe can have is the amount of information that must be pushed to the scope’s display. “We find that beyond 16 channels, a logic analyzer is better for parallel system use. But 16 channels is the industry’s standard for mixed-signal scopes and digital signal capture,” says Loberg.

Probe In Action
In using a probe such as the P6780 to debug a DDR3 design, one task might be to apply a setup and hold trigger on an adjacent processor to evaluate its performance. For attachment, a solder-in tip could be placed onto a basic controller chip with, say, an I2C bus feeding the memory system.

“You can monitor that with a couple of those extra probes and capture those transactions. The beauty is that you can use an event table to get a protocol view of that bus’ performance,” says Loberg. Doing so provides a simple logic-qualified view of an adjacent bus. Is my processor control misbehaving, and if so, is that causing the fundamental DDR3 bus to go awry?

Annother scenario may be that the I2C bus is feeding a power-supply controller. When that controller switches, could it be causing a glitch in the DDR3 that throws off its timing? The combination of the P6780 probe and an MSO70000 mixed-signal scope enables users to attach a logic probe on the controller chip in the power supply. Then it’s a relatively simple matter to determine whether the controller’s switching is coincidental in time with reads or writes to the DDR3.

Mechanical attachment of 16 probe tips can be a challenge, but Tektronix provides a broad range of technologies for getting signals out of chips and board traces. Probe tips can be as simple as a square pin connector attaching to an interposer-type board or fixture. Other options include a manual solder-down attachment. Each probe ships with a kit of more than half a dozen different connection options.

Agilent’s N2887A and N2888A soft-touch probe heads allow high-density probing of 36 single-ended signals or 18 differential signals, respectively. These adapters connect Agilent soft-touch connectorless probes to the input connectors of the Agilent InfiniiMax I and II series probe amplifiers.

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The probe heads enable engineers to probe high-density signals with up to 4 GHz of bandwidth. The probes also permit engineers to make the multichannel measurements commonly required in DDR memory testing and in other high-speed applications where space is tight.

For wafer-probing applications, Agilent’s N2884A Infiniimax probe (Fig. 3) is a 12-GHz differential fine-wire probe tip that enables R&D and test engineers to debug and test high-speed active ICs using an oscilloscope. Claimed as the only differential scope probe for wafers on the market, the N2884A uses Agilent’s low-cost, zero-insertion-force (ZIF) probe head technology, which provides a flat frequency response over its entire 12-GHz bandwidth.

The tip eliminates the distortion and loading that affect probes with in-band resonance. The probe measures a voltage versus an adjacent local ground or other node on the DUT. This technique delivers better common-mode noise rejection.

At The High End
Agilent’s biggest contribution to the scope-probe market of late is its InfiniiMax III probing system, which it calls the world’s highest-speed probing system. The system consists of four probe amplifiers (16-GHz to 30-GHz bandwidth), four probe heads, three probe head tips, three probe adapters, one performance verification and deskew fixture, and three probe-bandwidth upgrade options.

A wide range of probe heads allows connection using a browser, ZIF tip, 2.92-mm or 3.5-mm SMA cable (Fig. 4), or solder-in tips. The probing system complements Agilent’s Infiniium 90000-X series oscilloscopes with bandwidths of up to 32 GHz.

The InfiniiMax III browser uses a novel “crisscross” blade grounding system for lower-inductance grounding, a poly-iron wrap of coax tips to reduce standing waves, and very low parasitic replaceable resistor tips to achieve 30-GHz performance.

Passive Aggression
On the topic of passive probing, Tektronix recently polled customers to learn more about their probing challenges and how the company might improve its probing options and measurement accuracy. According to Randy White, technical marketing manager at Tek, two results jumped out.

From a performance standpoint, the number-one survey answer was to improve probe accuracy by imposing less loading on circuits under test. Tek’s answer to this issue is its TPP series of passive probes, which sport a bandwidth of up to 1 GHz with loading of just 3.9 pF (Fig. 5).

Most of today’s scopes ship with passive probes, and most of those probes come with a bandwidth of 500 MHz. Tektronix doubled the bandwidth in the TPP probes through a new architecture that includes a built-in ASIC. At the same time, the company has halved the loading found in the typical 500-MHz probe, which usually comes in with an input capacitance of around 8 to 9 pF.

These probes will potentially save money for Tek’s customers. Say you’ve purchased a 1-GHz scope. This probably means you intend to measure signals of up to 1 GHz. But if your scope came with 500-MHz probes, you’re now looking at purchasing an optional active probe to make up the difference. The TPP passive probes obviate the need to make that optional purchase.

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There’s also a good possibility that the probes will also save users’ time. Passive probes are typically compensated through manual adjustment of a screw on the side of the compensation box. This process must be undertaken each time the probe is plugged into a different scope. Failure to do so can result in waveforms and pulse shapes that are distorted (Fig. 6a).

The ASIC within the TPP probes not only communicates to the scope to tell it about itself, but an active compensation network also runs an ac-compensation process automatically to compensate for both the low- and high-frequency bands. This is a boon to users, particularly for the high-frequency compensation portion of this equation.

Typically, a service technician must perform high-frequency compensation by opening up the probe and making adjustments with the aid of special signal generators (Fig. 6b). The TPP series probes automatically perform both the low- and high-frequency compensation process in about five seconds.

The second notable result from Tektronix’s user survey was that users requested longer ground leads for their probes. Now, if you want more performance on passive or active probes, you usually need short ground leads for a lower-inductance ground path. But because of the active compensation network in the TPP series probes, there’s no problem with using a standard 3-inch ground lead, which gives users a pretty good reach on typical boards.

The movement toward automatic calibration is also in effect at Agilent. The N2893A is a 100-MHz ac-dc current probe that has degaussing and auto-zero capability built in. The probe works with software in the scope to auto-zero itself with the push of a button on the probe pod. “Zeroing used to be done with a wheel on the side of the probe that users would spin to get zero function,” says Tim Figge, Agilent's probes and accessories manager. 

Simulation In A Scope
Another trend in oscilloscopes that relates to probes and fixtures is de-embedding, which Agilent implements with its InfiniiSim technology. “InfiniiSim allows us to de-embed fixtures and perform better probe correction,” says Mike McTigue, Agilent’s lead probe architect. The technology also enables you to move the observation point.

“If you have a model of the device you’re measuring, you can measure on one point and then on the scope screen move to a point you can’t actually probe and get a measurement there,” says McTigue. This is full de-embedding, in which you account for and correct for all of the input and output impedances in the circuit.

Agilent has added a family of ball-grid array (BGA) interposer probes that uses the InfiniiSim technology in the scopes to take measurements of otherwise inaccessible points. The interposers are very thin printed-circuit boards (PCBs) that are soldered between the user’s BGA and the landing pattern for that BGA on the PCB. The interposer physically fans out the signals from the balls to pads or header pins.

“This goes hand in hand with simulation,” says McTigue. “The interposer adds parasitics in front of the probe. If you have a good model of that interposer, we can back out and cancel out the effects of the interposer. So what you see on the screen is what you would expect to see at the ball of the BGA.”

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The Importance Of Design
At LeCroy, the design of probe amplifiers and probes is an ongoing topic of importance. Each probe that LeCroy ships is individually tested and calibrated with coefficients stored within it. The probes’ frequency responses are calibrated to be perfectly flat. Thus, plugging a 25-GHz probe into a 25-GHz scope results in a bandwidth of 25 GHz.

With scopes moving higher in bandwidth, users want to at least de-embed the probe, making it transparent to the measurement process. They want to make sure the probe’s frequency response and scope have no effect on the measurement results. LeCroy’s approach is to design the scope and probe as a system. In doing so, the company feels it is satisfying most user needs around probe de-embedding.

“We design to a system specification as opposed to probe only or scope only,” says Larry Jacobs, senior design engineer in probe development at LeCroy. “When you connect the probe to the front end of the scope, that probe becomes part of the instrument. That’s how we approach it.”

Consider a scenario in which you have your scope set to 50 mV/div and you’re preparing to measure a signal with an amplitude of around 400 mV. When you attach the probe to your scope, the scope now sees the probe’s attenuation. Let’s assume it’s a 3· attenuation value. Plugging in the probe forces you to divide your 50 mV/div by the attenuation value. You’ll probably also have to set the scope’s gain higher and deal with more noise on your signal.

The bottom line is that lower probe-attenuation values are a key to enabling scopes to operate over a wider range. “This applies to both the frequency- and time-domain response of the instrument as well as noise,” says Jacobs. “We store detailed calibration data inside the probes so we can maintain it perfectly.”

Combating Loading
A key to making very high-frequency probes function with minimal circuit loading is to get a resistive element as close to the point of contact as possible. This is most difficult to achieve with browser leads, which are (relatively) long wires that will often resonate at higher frequencies.

LeCroy’s approach to this problem is embodied in its WaveLink Dxx05-PT (Fig. 7), a 22-GHz positioner tip accessory that uses carbon-fiber composite tips to optimize browser signal fidelity and minimize loading. The conductive carbon fibers place a distributed resistance at the point of probe contact to minimize parasitic impedance. The fibers improve signal fidelity by eliminating the skin effect that’s typically present at high frequencies with purely conductive tips (Fig. 8).

“Because you have these thousands of carbon fibers in the tip, it’s very flat in frequency,” says Jacobs. “Further, the skin effect doesn’t apply because the fibers are so small. They’re actually carbon nanotubes within the epoxy element.” The tips are made by extruding epoxy mixed with the carbon filaments. The end result is a composite material that’s stronger than the sum of the parts. In addition, it has resistive characteristics that are tailored for the probe.

Making The Difference
When it comes to probes for mid-range and lower-bandwidth scopes, it’s not so much about bandwidth but rather factors such as form factor and accessories, and, in general, making it easier for users to connect their scope to the DUT. LeCroy’s latest launch in this regard is its ZD series differential probes, which cover 200 MHz to 1.5 GHz.

The ZD series replaces an older LeCroy probe with a larger form factor and with limitations in terms of how it connects to DUTs. The new series is more streamlined, making it easier to probe in tight spots. It also comes with a broader variety of connection mechanisms, including solder-in tips and pivoting leads.

The 200-MHz probe is notable in that it offers a higher voltage swing, making it particularly suitable for use in the automotive industry. “It’s useful for diagnosing CAN and Flexray buses in automotive applications,” says Dan Monopoli, product marketing manager at LeCroy. Meanwhile, the higher-bandwidth ZD series probe, with its 1.5-GHz bandwidth, suits designers working with USB 2.0.

TAGS: LeCroy
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