Furthermore, HDMI signals have a high dc-bias level that’s problematic for going directly into a scope. With the SMA tips, users can set a termination voltage to match this dc bias level. As a result, they won’t load down the probe’s amplifier, and they can still acquire signals.
Embedded probe tips also provide users with improved voltage standing-wave ratio (VSWR) and skew matching. “We can control the probe much better than individual channels of the scope,” says Heltborg. The result is enhanced performance.
GOING THE CUSTOM ROUTE
Another avenue to consider for probe tips is the custom route, which can yield perfectly tailored results for a given application. Custom probes can be the answer for probing extremely fine-pitch IC packages. One vendor of such probes is AlphaTest Corp., whose probes can be constructed as small as 0.25 mm, center-to-center spacing. Probe makers like AlphaTest can deliver custom test fixtures as well as probes with custom contact forces and lengths.
“At least half our customers come to us with concerns about mechanical connections,” says Dan Rogers, AlphaTest’s vice president of marketing. The company’s probes are constructed simply, yet durably, of steel. It builds custom test fixtures into which the pins are inserted. The fixture holes serve as guides for the pins, which are accurate enough to hit a pad that is 5 mils in width.
Because AlphaTest built its own lathes for machining the probe parts, adjustments can be made to coil thicknesses, curves, and pitches. The angle of the coil within the tube is also adjustable, yielding different amounts of contact force and travel.
The vagaries of making a good connection with the circuit under test constitutes just one aspect of getting the most out of active probes. Most of today’s high-end active probes are differential probes that examine serial data streams. These data streams are referenced to each other and not to ground. Such probes come with built-in differential amplifiers that subtract the two signals, resulting in one differential measurement displayed on one scope channel.
For the design of its WaveLink 13- to 20-GHz differential probes, LeCroy Corp. went the non-traditional route. The probes use a differential traveling-wave amplifier architecture that’s commonly seen in ultra-high-frequency broadband amplifiers. This architecture brings a number of benefits, including maximum gain/stage and minimal attenuation.
“One key benefit is that there is no feedback, which always increases noise even as it stabilizes the amp,” says Ken Johnson, LeCroy’s senior product manager for scopes and probes. “As probe bandwidth rises, the expectations for noise performance grow more stringent. We have to make dramatic gains in noise spectrum density to make any headway.”
Too much noise in an eye diagram can cause excess thickness at the top and bottom. “That thickness is related to test equipment, not the user’s signal,” says Johnson. “In such instances, they can no longer discriminate between noise in the signal and noise from the probe and scope.”
The traveling-wave architecture involves a synthetic transmission line through the signal path that yields extremely low probe noise of 25 nV/√Hz. “We strove to make the amplifier more linear with as flat a frequency response as possible out to 20 GHz,” says Johnson. The amplifier comprises a line of active elements (FETs). Each transistor has an associated parasitic capacitance to which inductance is added to create a continuous impedance from stage to stage (Fig. 5).
A critical specification for active probes is ac loading. For the WaveLink probe, LeCroy specifies ac loading as just 175 Ω minimum. Johnson says ac loading is more a function of tip design. “It’s a matter of having a resistor that’s very small and attached as close to the point of contact as possible,” he says. “We have patents on how to do custom compensating resistors. It’s a simple and elegant way to control parasitic capacitance, rather than using excess capacitance to swamp the parasitics.”
Another benefit of this architecture is its ability to minimize the gain needed from each individual stage. “Each stage’s gain is additive to the previous stage, so each one isn’t being pushed too hard. The synthetic transmission line is a trick we play to get the gain stages to add,” says Johnson.