Oscilloscope Probes - High-Bandwidth Probes: Potential Improvement and Front-End Selection

As a component of the measurement system, the importance of oscilloscope probes is self-evident. Even today, when consistency testing has become the main application area of ​​real-time oscilloscopes and a combination of fixtures and cables is often used to pick up signals for circuit break testing, buses such as DDR and MIPI still have to rely on probes to access the circuit under test for testing.

Figure 1 Typical connection diagram for DDR and MIPI-DPHY test

There are many articles and discussions about the requirements for probes when testing these two typical buses, including the probe's bandwidth, impedance and capacitance, attenuation ratio, and differences in the probe's circuit structure.

With the LPDDR4 standard advancing to 4.2Gbps and DDR5 starting at 4.8Gbps, have you found that the typical 13 GHz bandwidth oscilloscope and probe systems purchased in previous years for 3.2Gbps rates, such as DSAV134A and 1169A/B, are no longer able to keep up?

At this point you will realize how wise it is to have purchased an oscilloscope with software upgrade bandwidth earlier. You can complete the upgrade in your own laboratory by purchasing a bandwidth upgrade license without returning to the factory! However, according to the flat response system bandwidth formula:

BWSystem = Min(BWScope, BWProbe)

Therefore, even if the oscilloscope is upgraded to a higher bandwidth, if the probe bandwidth is still 13 GHz, the entire system bandwidth is still 13 GHz.

How to liberate the productivity of 1169B and make it play a greater role?

Tip 1: tap potential, increase efficiency and let your assets appreciate in value!

Keysight Infiniimax differential probes are DSP-corrected with flat amplitude and phase response for extremely high accuracy. The bandwidth to be corrected is typically selected to be approximately 3 dB above the uncorrected bandwidth.

Increasing the bandwidth significantly beyond this value will generally result in an increase in the noise floor, and further increases may result in severe noise distortion.

The N5381A/B solder-in probe head combined with the Infiniimax 1169A/B probe amplifier can successfully extend the bandwidth to more than 3 dB because the peaking of the N5381A/B exceeds the normal 12 GHz bandwidth and the peaking of the probe head helps compensate for the roll-off of the probe amplifier bandwidth.

Tip 2: Details determine success or failure, how to make good use of the probe?

In addition to fully tapping the potential of the existing probe bandwidth, from an operational perspective, what other precautions can be taken to effectively improve the bandwidth and signal fidelity of the signal connection link? Many times we attach great importance to the selection of oscilloscopes, but often perfunctorily deal with the subsequent use and test details, and the negligence of these small details often leads to our efforts being twice as much as required! Details determine success or failure!

01 From a design perspective, do a good job of testability design (Design For Test) in advance, including reserving test points and ground test points as close as possible to the end of the signal to be tested

Taking MIPI bus testing as an example, due to the existence of common-mode bias voltage, a differential probe is used to perform single-ended testing on the positive and negative ends of the differential data and clock signals respectively, and then differential and common-mode operations are performed to obtain effective differential signals and common-mode bias respectively.

The signal detection method determined by this circuit characteristic leads to very strict requirements on effective reference grounding during signal detection.

Therefore, for example, for D-PHY, it is recommended that during design, in addition to reserving test points for the positive and negative ends of the differential signal, a ground test point as close as possible to the same ground plane should be reserved.

For other general SerDes signals, test points should be reserved at the end of the tested line or the receiving end as much as possible to reduce the reflection that may be caused by the test point being located in the middle of the line. In addition, under high-speed conditions, the stub effect of the via or pad at the test point in the middle of the line and the probe load have a great impact on the line.

02 Choose the right welding probe head

There are many probe front ends for the Infiniimax series probe amplifiers. What are the differences between these front ends?

For example, when testing DDR bus, many engineers prefer the ZIF probe front end N5425A/B. There are many ZIF Tips to choose from, such as N5426A, N5451A, etc. What are the differences?

Figure 2 N5426A (top) and N5451A (bottom)

Table 1 N5426A and N5451A different wire lengths and angle bandwidths

It can be seen that the longer the wire, the lower the bandwidth, the greater the loading effect, and the flatness of its frequency response characteristics will also decrease. The larger the angle between the two legs of the long wire, the lower the bandwidth will also be.

In addition to the ZIF probe, there are also commonly used direct welding probes E2677A/B, N2836A and N5381A/B and N5441A. The difference is that the damping resistors of the former two are outside the PCB at the front end of the probe, while the damping resistors of the latter two are on the welding PCB.

Figure 3 E2677A/B (left) and N5381A/B (right)

Table 2 Direct welding front end differences

For the selection of damping resistors for the soldered probe tip, please refer to the product manual or user manual. For example, E2677A/B has two types: 91Ω and 150Ω, of which 91 provides full bandwidth and 150 provides medium bandwidth capability.

For damping resistors outside the front-end PCB, if you need to reach into the device for testing, the wires between the damping resistor and the test point should be as short as possible to minimize the stub effect. For damping resistors on the front-end PCB, the wires should also be as short as possible.

03 When the testability design and front-end selection are guaranteed, what other factors will affect the signal fidelity of the signal detection link?

Perfectly soldering the probe is also an important step to ensure signal fidelity during the test process. It is also crucial for the reuse of the probe and is a proper protection of assets!

First, choose the appropriate soldering iron tip for different probe front ends:

There are many ways to connect a test instrument probe to a device under test (DUT). One method is soldering, which provides a reliable connection and minimizes parasitic probing effects by keeping cable lengths and connections as short as possible. Many of Keysight's high-performance oscilloscope probes use soldered connections. As probes and devices continue to shrink, using the right tools and soldering skills is critical to avoid damage and connection issues.