Unwanted noise in oscilloscope display

Question

Hi I'm a beginner on using an oscilloscope and I'm sorry if my question may be repeated in other posts.

I was working on a circuit with the following materials:

(1) an Al plate (5mm thick)

(2) a disk-shaped piezo transducer with special electrodes (product note)(product link)

(3) two wires

(4) a coaxial cable

(5) a Tektronix DPO4104 oscilloscope

So the PZT was glued on the plate, and wires were soldered on the PZT to connect to the oscilloscope.

I tested my setup by knocking the plate, and ended up with sudden voltage rise/drop for the knocks. But I noticed the continuous milli-volt noise signals throughout the whole process.

Are they actually noise or something else? How should I reduce them to less than 0.1V as maximum amplitude, or even none? Because I would latter want it to detect 0.3-0.4 V.

I've tried scaling, reducing record length, and switching acquisition modes to SAMPLE / PEAK DEFECT / HI RES / ENVELOPE / AVERAGE(with max value possible). But they didn't work.

Fig 1. Output Graph

Fig 2. Screen picture

Fig 3. Parameters of oscilloscope

EDIT:

Just to update, I tried putting a foam plastic plate under the Al plate and half of the "noise" amplitude reduced (though I want to reduce more)

The problem is not solved for the time being.

Answer 1

Looks like it's EMI noise coupling into your measurement from your power supply. Make sure that the power cord for the scope is as far away from your probe as possible.

To confirm it is noise from your power supply check the frequency by measuring the peak to peak time difference and calculating 1/(t1-t2).

Another hug help could be reducing the ground loop of your probe by using the tiny little ground ring at the end of your probe. You could wrap a peice of wire around it or a paperclip.

You ground loop has a current generated in it proportional to the change in magnetic field flowing through it. Smaller loop means less change.

Good luck!!

Also, and amplifier circuit could help scale your piezo output from say 0-0.5V to around 0-5V, while even filtering some noise out for you. Google low-pas filter. Cheers!