Choosing a high bandwidth probe for high-speed digital applications

The growing demand for high-speed digital applications calls for increases across the board for greater system bandwidth and faster signal edge speeds, as well as smaller and more efficient chipset and component design.

In turn, these developments also result in greater complexity for PCB layouts, challenging high bandwidth probes to meet the performance levels required at greater speeds. This blog post takes a look at the issues involved when selecting probes suitable for high speed digital applications, and what obstacles designers may face.

The demands of a high bandwidth probe

For a probe to be efficient and effective, it should provide an accurate reflection of the signal being measured. For high bandwidth probes, loading characteristics can vary much more significantly than in standard probes, and the resultant loading effects can lead to much greater limitations on a probe’s performance than expected.

One key difference in high bandwidth probes is the variation in input impedance. While traditional probes will often have an impedance characteristic of around 50 MΩ, higher bandwidth probes may have much greater impedance characteristic, of up to 100 MΩ.

This can lead to a higher crossover frequency than conventionally found. Combined with fast edge speeds, standard probes can also experience interaction between input impedance and resistors, leading to long time constant effects. To minimize this, a probe with higher impedance across a wider bandwidth is more suitable for usage.

Probe noise

Another factor that can impact a probe’s performance is the inherent noise that is a pre-existing part of it. While many factors affect the noise figure of a reading, the most significant of these is the signal to noise ratio.

A lower attenuation ration will often result in a higher signal to noise ratio with reduced noise, but this also produces a lower input resistance and reduced dynamic range.

In order to get an optimum balance between the different values, a compromise is often needed – one way of doing this is to use the attenuation ratio and probe noise level, and use it to estimate probe noise. In addition, using the vertical range of a scope – the most sensitive part of it – will also reduce unnecessary magnification of the scope’s noise. By choosing a probe with a lower attenuation ratio to start with, it is possible to optimize the signal to noise ratio as much as possible.

Choosing a probe tip

For any probe, its tip is its most important element – but this is also most susceptible to flaws and limitations.  Each component in a measurement system can impact the bandwidth of the overall device – and probes and probe accessories are more prone to loss of bandwidth than any other component.

High bandwidth probes with longer input lead wires at the tip are often more susceptive to greater degradation, and resultant frequency response variation. In addition, that can increase loading and non-flat frequency responses, with greater variation in measurements.

In order to optimize performance of probe tips, they are best kept as small as possible, both with input leads and across the loop area. For single-ended probes, it is more efficient to keep their low inductance ground connection as short and wide as possible.

Do you have any questions about selecting probes for high-bandwidth applications? Share them with us here.


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