How to check for noise with an oscilloscope

[bgolash]

Hi

I just got a oscilloscope for motor tuning my G320 and checking for
noise. At this time I`m more interested in checking for noise. I would
assume the first place to look would be the encoder wires. I`ve never
used a scope before so I need a bit of help getting started.
The scope I have is a Tektronix dual trace at 35 MHz


Barry

[gar]

070515-1008 EST USA

bgolash:

First, learn how to work with your scope. Use a low voltage isolation transformer to provide a low voltage signal. Experiment with triggering and synchronization. Find out what happens when you change different adjustments. Make a free running multivibrator and look at its various waveforms. Feed the scope internal calibration signal to a low pass single stage RC low pass filter, and adjust R and C for various modifications to the square wave calibration signal.

What kind of noise and probability of are you looking for? A scope may not be you best method for troubleshooting noise problems.

A Simpson 260 meter may be your best first test for noise.

When a 260 or 270 meter is in a DC position it reads the average of the input voltage. Thus, a sine wave with 0 DC component will read 0 V. A half wave rectified sine wave with no filtering will read 0.636/2 = 0.318 of the half sine wave peak.

In AC position 12 VDC input will read approximately (0.636/0.707) * 12 = 10.8 on the AC scale. This is because the meter in AC position reads the average value of a full wave rectified sine wave but is calibrated in the RMS value of a sine wave. So you have the ratio of the average value to RMS value.

To remove the DC component move the meter + lead to the output connection. This puts a capacitor in series with the input.

The bandwith of the 260 is fairly flat to around 250 kHz. Whereas Fluke DVMs are more in the range of 25 to 50 kHz.

If you have a continuous noise source, then either the meter or scope may allow you to measure the noise.

If you have random noise it may be much more difficult.

Good design to avoid noise, or provide immunity is your best approach.

.