Testing a control PCB for electrical noise using Scope on High bandwidth mode.

[zenerbjt]

Dear Engineers,

We are doing some test measurements on a Control PCB which will get used in a noisy environment, where several 1kW BLDCs and their drives are being operated. We wish to have tests which can see how noisy the control PCB is.

The control board contains microcontrollers, as well as Radio Transceivers and multiple PWM channels sending speed control PWM signals to the electric drives. Also, there are comms busses carrying data. Also, there are 4 DCDC power modules. The DCDC modules output 5V and there are 4 of these, partly to give redundancy. Vin to the DCDC's is a 48V battery. Total control board power useage is some 10W.

We want to test all the rail voltages on this control PCB for noisiness, and also see how noisy are the PWM speed control signals on this board.

As such, to test for electrical noisiness, we will simply scope the voltage rails (and PWM signals) with a 70MHz scope in AC coupled mode, and without “Bandwidth Limit” being selected. We will use a roll-your-own “coaxial cable scope probe” which is 1:1 and does not have a “dangling ground clip”. We will observe the high frequency noise, and record the peak-to-peak of it, to assess noise.
We will observe the PWM speed control signals in the scope's DC coupled mode, and see how noisy are the mark and space of these pulse trains. Also, we will look out for sharp undershoot and overshoot spikes at the high-to low ( and vice versa) of the PWM signal transitions.

Do you think this is an effective ploy to test for noisiness in the PCB?


Scope is a Keysight DSOX1204A

DSOX1204A Oscilloscope: 70/100/200 MHz, 4 Analog Channels

4 channel model of the InfiniiVision 1000 X-Series entry-level oscilloscopes that can be upgraded from 70 MHz to 100 MHz or 200 MHz.

www.keysight.com

 

[FvM]

You can't probe fast logic signals with a coaxial cable probe without completely distorting it. For low ground inductance connection, you better use a regular high impedance probe with a coaxial jack directly soldered to the to the circuit like this

Cable probe can work for supply voltage measurements, but the cable itself can still pick up common noise in a noisy circuit. Additional means are required to block common noise, e.g. ferrite toroid cores.

 

[zenerbjt]

You can't probe fast logic signals with a coaxial cable probe without completely distorting it.

Thanks, and i wonder if it is OK with a cable probe for fast logic signals if we add a 100k resistor in series with the probe, and add say a 22pF capacitor across the end of the probe near the scope?
...Because i assume you mean the coaxial cable capacitance will distort the signal?