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25 January 2021

Situational Awareness: The Impact of the Interconnect

Fig 1. Coaxial cable has little effect on signal rise time,
but that's not true for every connection method.
How you connect a signal to your oscilloscope affects your measurements, and knowing the impact of different connection methods is an important part of your situational awareness.  

Using the same 40 ps fast edge signal we used for the risetime measurements in the last post, we’ll compare connections made using coaxial cable and a 10x passive probe, with and without different accessories.

Coaxial Cable

Using a matched coaxial cable has a very minor effect on the signal rise time. With a 1-meter, 50 Ohm coaxial cable, rise time increases only slightly from 470 ps to about 500 ps (Fig 1). 

Fig 2. Adding a mini-grabber to a coaxial cable
significantly increases rise time and 
can distort the signal. 
But beware adding accessories to your coaxial cable connection, as will often be necessary when probing small interconnects and power rails within embedded systems.  In the case of the 40 ps fast edge signal, modifying the connection by introducing a mini-grabber (Fig 2) causes the rise time to increase to 7.5 ns and distorts the signal.  The higher impedance and inductance of the mini-grabber leads results in visible reflections and an increase in rise time.  Mini-grabbers are best used on signals with greater rise times.

10x Passive Probe

Fig 3. Inductance caused by ground leads
can cause overshoot and ringing.
A properly compensated 10x passive probe with a sprung hook and a ground lead reduces the rise time to 1.8 ns, but the inductance of the ground lead causes some overshoot and ringing (Fig 3).

Many probes come with a number of adapters to reduce ground lead inductance, such as coaxial adapters and spring clips. Using a coaxial adapter with the passive probe replaces the ground lead, and when mated with a BNC connector results in a low inductance connection, which improves measurement fidelity. 

Fig 4. Using a coaxial adapter with a passive
probe minimizes ground lead inductance.

The coaxial adaptor has reduced the signal rise time to 1.4 ns and eliminated the inductive overshoot (Fig 4).  The passive probe bandwidth is then about 320 MHz. 

Fig 5. The ground spring reduces the length of the
ground connection and the lead inductance.
It also reduces antenna pickup.
The ground spring reduces the ground lead inductance, and because it doesn’t require a BNC connector, it can be used for open probing on a circuit board. It also has a good effect on the rise time measurement, reducing it to about 1.3 ns (Fig 5).

We've made numerous related posts on this topic, such as Using 50 Ohm Coax from DUT to Oscilloscope, Secrets of the 10x Passive Probe, Squeezing More Bandwidth from a 10x Passive Probe, 10x Passive Probes and Cable Reflections, The Effects of Passive Probe Ground Leads and more.

Watch Dr. Eric Bogatin demonstrate this concept in the on-demand webinar, SI/PI Measurements on a Budget.

Also see:

Four Measurement Best Practices

Situational Awareness: Testing Oscilloscope Outer Limits

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