Electronics > Mechanical & Automation Engineering

Assessing electrical noise with 70MHz scope with "Bandwidth limit" deselected.


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

No I don't think so.  Ripple and noise measurement is an art all by itself.  What is the voltage you want to measure?  By that, I mean, between which two points in your equipment?  You want to make a differential measurement that won't depend on whatever you decide to call ground.

What does this acronym BLDC mean, and why do you need to use an acronym?  Too often people use acronyms to convey their presumed knowledge but fail to recognize that we are here to help rather than to praise.

Coaxial cables have to be terminated with their characteristic impedance on both ends, or you will get reflections.

Oscilloscope probes are much more then a simple piece of coax.
For starters, the center conductor is extremely thin. Less circumference means less capacitance. Then they also have a surprising amount of resistance. It can easily be a few hundred ohms. This dampens reflections, and still has a negligible impedance compared to the 1MOhm of the scope itself.

Currents through wires generate an magnetic field around the wire. Changing currents generate changing magnetic fields, which is also noise, as it may induce voltages elsewhere. In a DC-DC converter there is a high AC component between the input filter cap, and the power switch. The voltage may be relatively constant here, so measuring voltages won't help much.

There are magnetic probes for measuring such things.
In a cheap DIY setting you may get some decent results by using a head from a cassette tape recorder.

David Hess:
Noise measurements typically require a differential or isolated probe to keep ground noise between the oscilloscope and DUT from corrupting the measurement.  With some care, you might get away with using two oscilloscope inputs in invert and add mode to make a differential probe but this requires careful setup to match the gain between the two channels.

--- Quote from: Doctorandus_P on August 29, 2020, 05:02:44 am ---Coaxial cables have to be terminated with their characteristic impedance on both ends, or you will get reflections.
--- End quote ---

Point-to-point transmission lines only require termination at one end to prevent reflections.  For example low impedance attenuating probes for oscilloscopes lack source termination.  RF power amplifiers are also not source terminated because this would limit efficiency.


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