WHITE PAPER
Probing Is Everything
Enhancing probing accuracy through optimized probe
head connection
Probe Connection is Key
Probe connection plays a crucial role in achieving high signal integrity (SI) measurements. Most SI
engineers are aware that probing is a critical element in SI measurements. The probe is a vital aspect of
enabling a system’s full performance. However, one often-overlooked pitfall is the effect of the probe’s
connection to the target device.
The combination of an oscilloscope, a probe, and probe accessories forms a measurement chain. The
weakest link in the chain determines the overall performance of the measurement system. Even a
powerful high-performance oscilloscope and probe system that meets your overall measurement
requirements may not be good enough. A relatively weak link, such as a poor probe tip accessory, can
bring the overall performance down to fractions of your system bandwidth performance, which effectively
neuters your investment while providing incorrect results.
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InfiniiMax Probing System
The InfiniiMax probing solution delivers industry-leading performance, featuring the highest bandwidth
and a comprehensive selection of probe tips. It offers two input impedance topologies—RC and RCRC—
each suitable for different applications due to their distinct impedance (loading) characteristics across
frequencies.
The RC impedance profile, which exhibits a gradual roll-off due to capacitive loading, maintains lower
loading at mid-band frequencies. However, designing RC probe tips for frequencies above 25 GHz poses
significant challenges due to the intricacies of component design. In contrast, the RCRC topology
dominated by the probes’ resistance, then capacitance, then resistance again, then capacitance again
from low to high frequencies. RCRC probes typically achieve broadband attenuation and preserve wave
shape effectively, thanks to their high bandwidth and low high-frequency loading.
InfiniiMax Ultra RC 25 GHz
InfiniiMax 4 RCRC 52 GHz
Figure 1. InfiniiMax Ultra RC and InfiniiMax 4 RCRC Input Impedance chart
The Keysight InfiniiMax Ultra, with up to 25 GHz bandwidth, is the premier RC probing system ideal for
applications sensitive to loading variations, such as DDR or MIPI. The InfiniiMax 4 RCRC probe extends
up to 52 GHz, providing a superior measurement tool for high-speed digital applications, including
802.3CK and PCIe5/6.
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What about the probe tip?
The InfiniiMax probing amplifier features Keysight's cutting-edge designs and utilizes their proprietary InP
IC process technology. This setup delivers the highest performance for measuring both differential and
single-ended signals on compact ICs and densely populated PCBs. As electronic components become
smaller and faster, accurately probing these signals grows increasingly challenging. The Keysight
InfiniiMax Probing System offers the broadest range of probe heads and all necessary accessories,
ensuring effective and versatile measurements. Employing a modular system, the InfiniiMax provides an
extensive selection of probe heads to support a variety of usage scenarios. Improved tip connections
enhance the overall performance of the system.
Figure 2. The physical and electrical model of Keysight InfiniiMax differential probing system
Figure 1 shows the physical and electrical model of the Keysight InfiniiMax differential probing system.
From the left is a probe head with damping resistors and an RC network connected to the differential
InfiniiMax probe amp that terminates into the 50 Ω path of the oscilloscope.
In the InfiniiMax probe system model, the 10–15-cm-long coax cable in the probe head is essentially a
“controlled” 50 Ω transmission line allowing for optimal probe’s system performance. As a result, you have
the flexibility to get the probe head into very tight spaces without significant bandwidth or performance
loss. The section to the far left is the actual connection with damping resistors. This “uncontrolled” section
of the model with parasitic capacitance and inductance can affect the probe system’s bandwidth and
performance greatly, depending on the configuration of the connection.
What you need to be careful about is the uncontrolled parasitic components formed by the probe head
tip configurations.
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Consumable probe tips
Keysight’s InfiniiMax ZIF solderable probe head with the N5426A ZIF tip provides 18 GHz of bandwidth
when used with the MX0025A 25 GHz probe amp. Keysight offers two versions of InfiniiMax RC ZIF tips
— one with a 2 mm tip length(N5426B) and the other with a long wired ZIF tip(N5451A) that users can
adjust for longer reach.
Figure 3. InfiniiMax ZIP consumable probing tips
Convenience vs. measurement accuracy
As the input lead wire gets longer and the input wires spread out to create a wider loop area, you might
expect lower and lower bandwidth in a measurement. The example in Figure 4 increases the input wire
length by 9 mm and the pitch span a little bit, yet the probe or measurement system bandwidth falls to
3.3 GHz which the original system bandwidth is 18 GHz.
Longer wires may provide the convenience of probing physically separated test points easily, but there is
a trade-off in doing so.
Figure 4. Increased lead wire length and wider loop area created by two input leads reduce bandwidth. Keep it short
and within a small loop area
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Look at the frequency response characteristics of the ZIF probe head / tip. As the input wire gets longer
and the input wires spread to create a wider loop area, not only does the bandwidth of the probe get
lower, the frequency response becomes less flat. It possesses a greater variation in frequency response
as the tip span changes (Figure 5).
Figure 5. Longer input lead wire and wide loop area can cause non-flat frequency response
We learned that varying lead length and span affect the bandwidth and flatness of the frequency
response. Here is another case in which varying lead length and span can affect the probe loading.
Figure 6. Longer input lead wire and wide loop area can cause higher loading of the probe as the probe tip
capacitance gets higher
Figure 6 plots the probe’s input impedance (vertical) versus frequency (horizontal) of the ZIF head / tips.
Notice that as the lead gets shorter and the span gets smaller, the probe’s input impedance gets higher,
resulting in lower probe loading. In other words, as the lead gets longer and the span gets wider, the
probe loads more signal from the target.
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Figure 7. The step response in the time domain measured with a 2 mm ZIF (blue) and 11 mm long lead ZIF (purple)
Figure 7 shows a screen capture of the step response in the time domain measured with a 2 mm ZIF
(blue) and 11 mm long lead ZIF (purple). Notice that the 2 mm ZIF tracks down the source (in green) quite
closely, meaning that the probe’s loading effect is negligible. However, the purple trace measured with a
long lead ZIF deviates considerably from the source, loading down the signal.
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Probe Positioning and Undesired Signal
Coupling
Another issue of concern involves high-frequency electromagnetic (EM) signals coupling into the
measurement system. While most parts of an oscilloscope probe are shielded to block unwanted EM
signals from entering the signal path, there is often a short unshielded section at the probe tip, between
the input resistors and the amplifier. Additionally, some probe tips may not be fully shielded on all sides.
These shielding gaps, which arise due to practical usage and manufacturing constraints, are not ideal. If
the probe captures high-frequency components of the signal that are not attenuated by the input
impedance, it might display an unwanted initial spike or step during the signal transition. This can alter the
signal's appearance, introduce measurement irregularities, and affect frequency responses, which in turn
reduces measurement accuracy, shortens the 10-90% rise time, and decreasing digital eye openings.
Furthermore, if the probe tip is capacitively loaded by a Device Under Test (DUT), it could lead to a lower
measured bandwidth. Since these issues originate not from the DUT but from the measurement process
itself, troubleshooting can be challenging.
To reduce these issues, ensure the probe tip is positioned perpendicular to the Device Under Test (DUT).
Many probe tips come equipped with fixtures or positioners to facilitate this setup. If an angled position is
necessary, it's preferable for the probe to lean away from the incident signal. Additionally, different loading
models may be available to optimize the alignment of the probe tip with the DUT in use.
The following Figures 8, 9 and 10 show the effect of various positions of a 52GHz probe tip on the
resulting bandwidth and step response. The difference in -3dB bandwidth between the optimal tip position
and having the tip with the signal trace side leaned 45 degrees toward the incident waveform is
approximately 3 GHz and the step response suffers greatly. In the extreme case with the signal trace of
the probe tip flat against the signal trace of the DUT on the incident side, there is also a significant
“suckout” evident. Based on the geometry of the probe tip, how undesired energy couples into it, and the
electrical design of the tip, the effects of position may be even more significant for other probing solutions.
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Figure 8. Probe tip orientations tested. The probe tip was nominally positioned in these 5 positions. The input signal
was connected on the left side of the fixture. The right side of the fixture was attached to a 50 ohm oscilloscope
channel.
Figure 9. Step Response for different probe tip orientations. To make the measurements, a Keysight MX0031A
52GHZ probe with a MX0041A 52GHZ tip was connected to a UXR0702A 70 GHz oscilloscope. The probe tip was
connected to a MX0030-60001 PV/Demo Fixture in a manner similar to that described in the performance verification
section of the user guide for MX0031A. The source is a Keysight N2126A 1.85mm calibration module.
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Figure 10. Frequency response for different probe tip positions.
Flex Printed Circuit (FPC) probe head
The flex printed circuit (FPC) probe head renders the probe tip lightweight, flexible, and small, yet highly
functional. It not only enhances signal stability but also increases the flexibility of the probing process,
making it more user-friendly and efficient. The MX0100A stands out as the smallest and highest-
performing solder-in RC probe head available, specifically optimized for modern high-speed devices. This
micro probe head is designed for use with Keysight InfiniiMax RC probe amplifiers, enabling access to
devices with small geometries. The MX0100A features solderable lead wires that provide a robust
connection to the Device Under Test (DUT), with 4-mil diameter leads that are adjustable to target
devices spaced from 0 mm to 7 mm apart. Similarly, the MX0041/41A are two FPC probe heads for the
InfiniiMax RCRC system, offering up to 52 GHz probing capabilities.
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Figure 11. Example for InfiniiMax RC MX0100A FPC Soldering (position perpendicular to DUT)
Figure 12. Example for InfiniiMax RRCC MX0041A FPC Soldering (position perpendicular to DUT)
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Conclusion
Understanding what's happening in a circuit is crucial, and that's where measurement equipment comes
in. It's important for engineers to choose the right probe, consider its impact on the circuit, and connect it
properly to the device under test (DUT). This careful setup prevents time wasted on tracking down errors
caused by the measurement tools themselves. Using software and following device-specific guidelines
from the user manual can also enhance understanding and speed up research and design processes.
The configuration of a probe tip, especially for high-speed probes, significantly influences its bandwidth,
frequency response, and loading characteristics. While longer input wires can make it easier to reach
separated test points or fit into tight spaces, they can also reduce performance. Therefore, it's best to
keep input wires short and the loop area of the tips small.
To minimize electromagnetic interference, position the probe tip perpendicular to the DUT. Many probe
tips have fixtures or positioners to help with this. If an angle is necessary, angle the probe away from the
signal. Various loading models are also available to ensure the probe tip is correctly aligned with the
DUT.
The Infiniium UXR-Series real-time oscilloscope creates a powerful measurement platform for signal
integrity, especially when combined with the MX0025A InfiniiMax Ultra 25 GHz RC probe and/or
MX0032A InfiniiMax 4 RCRC probe.
The Infiniium UXR-Series offers bandwidth from 5 GHz to 110 GHz, as well as the highest ENOB, lowest
noise, and 10-bit vertical resolution on all channels all the time.
The Keysight MX0025A InfiniiMax Ultra 25 GHz RC probe has a suite of available accessories, which
includes the MX0100A InfiniiMax II microprobe head kit, MX0106A 23 GHz InfiniiMax differential
soldered-in head and high-performance probe heads with consumable ZIP tips. These probe heads and
accessories simplify connectivity while ensuring signal integrity.
The InfiniiMax 4, Keysight's latest probe system, offers an impressive bandwidth capacity of up to 52
GHz. Achieving this level of performance required the development of an innovative modular probe head
system, featuring a flexible printed circuit (FPC) probe head. This unique system comprises the MX0040A
probe head connector and two FPC probe heads of varying lengths (MX0041A and MX0042A), catering
to diverse testing needs. The design facilitates rapid attachment of the amplifier to different probe head
connectors, and the 3 different position cradles (MX0046A) aid in precisely aligning the probe and DUT.
The newly developed FPC not only enhances signal stability but also adds flexibility to the probing
process, making it more user-friendly and efficient.
Keysight enables innovators to push the boundaries of engineering by quickly solving
design, emulation, and test challenges to create the best product experiences. Start you r
innovation journey at www.keysight.com.
This information is subject to change without notice. © Keysight Technologies, 2018 – 2024,
Published in USA, June 18, 2024, 7120-1246.EN
