Questions on building probe station and pulse measurement

Question

I don't have a strong ECE background, so I may ask many naïve questions.

I am building a probe station to measure pulses in a micro-coil, which looks like the following circuit.

Because I want to measure the current flowing through the coil, but the oscilloscope only measures the voltage, I use channel 1 to measure the voltage across a known resistor and use Ohm's Law to calculate current. Some measurements are attached below.

These 2 graphs show the measurements when probe is landed or lifted with 5V pulses. The sample has a large impedance, so when probe is in contact, the voltage change across the resistor is very small but still distinguishable.

For a 50V pulse, I can clearly see huge oscillations when pulse starts and ends. My first question: is this ringing pulse? If it is ringing, what is the cause and how to prevent it?

My sample is a micro-coil connected with a pair of transmission lines. The transmission lines were designed to have LC=1/w^2, where rise time is about 1ns. I used the following links to calculate the inductance and capacitance of differential stripline, but them may not be the best choice. My second question: is there any better way to determine the dimensions of transmission lines for my case?

https://www.emisoftware.com/calculator/coplanar/

My third question: the pulser I use needs a 50Ohm load, does that mean the total impedance between signal and ground is 50Ohm? Or each piece in the circuit should be 50Ohm? Such as, each cable should be a 50Ohm coax, channel should be 50Ohm, and the probe and the sample should also be 50Ohm?

Answer 1

• all signal paths ought to be match loaded to 50 Ohms.

Since the probe impedance (pF) makes all the errors from tip capacitance and ground lead inductance around 8nH/cm you can eliminate most of this with calibrated probes only using tip &ring.

When both probes are on the same signal with this method A-B ought to give a flat line if both probes are calibrated.

But 300 MHz BW measurements are best done with matched impedance probes to the circuit or using SMA to BNC coax terminated with 50 Ohms . You will find twisting both 10M probes will also reduce CM inductive noise impulses.

As usual if the current sensor is small in R giving 50mV to 100mV max then the differential impedance will be low. Use this as a Rule of thumb for all current sensing. But specific V,I,Z(f) ought to be specified for all including the probe in your equivalent circuit. Impedance loading the coil to 50 Ohms is a good idea if you wish to load it for higher BW. but it depends on the Q you are assuming.

• draw an accurate schematic of all parasitic elements and coil if possible with expected and actual results.

Some images of scope probe

TIP-Barrel test of oscilloscope

Logical schematic of two probes with gnd spring coil and exposed tip matched to same RF squarewave signal in A-B mode with a 0V flat line. This usually works up to 200 MHz. Above this SMA connections with 50 Ohm terminations using attenuation resistor series pads works better.

^{simulate this circuit – Schematic created using CircuitLab}