Dear members,
I am using a TDS2024C scope and recently encountered the following two issues for which I do not have any satisfactory explanation.
1. I am trying to measure the output of a hydrophone (piezoelectric) using the scope. I connected a BNC to the scope from the output leads of the hydrophone (floating unit not grounded). When the hydrophone is not powered, just connecting the BNC to the scope displays a noisy signal with pk-pk to voltage of 10/20 mV. I understand that connecting the negative signal lead of the circuit to the BNC brings it to scope ground reference, so what is the reason for this noise even when the circuit is dead ?

2. While trying to figure out the above, I observed that if I just touch the center of any input terminal, with a wire while touching the other end of the wire by hand, it displays a periodic waveform in that channel. What is the reason for this?

Any help in this regard will be highly appreciated

 

Dear members,
I am using a TDS2024C scope and recently encountered the following two issues for which I do not have any satisfactory explanation.
1. I am trying to measure the output of a hydrophone (piezoelectric) using the scope. I connected a BNC to the scope from the output leads of the hydrophone (floating unit not grounded). When the hydrophone is not powered, just connecting the BNC to the scope displays a noisy signal with pk-pk to voltage of 10/20 mV. I understand that connecting the negative signal lead of the circuit to the BNC brings it to scope ground reference, so what is the reason for this noise even when the circuit is dead ?

There is nothing unusual about your observations. This is normal.
Disconnect the BNC cable from the input of the oscilloscope. Measure the noise level as indicated by the scope. Most scopes have a noise floor of about 5-10mV.

2. While trying to figure out the above, I observed that if I just touch the center of any input terminal, with a wire while touching the other end of the wire by hand, it displays a periodic waveform in that channel. What is the reason for this?

Again this is normal. Measure the frequency of the periodic waveform you observe on the scope.

 

Measure the frequency of the periodic waveform you observe on the scope.

I was about to suggest the same thing. The frequency of the measured waveform should give you an idea of where it's coming from. I'll give you a hint: Your body is acting like an antenna.

Have fun and keep experimenting!

Regards,
Matt

 

I was about to suggest the same thing. The frequency of the measured waveform should give you an idea of where it's coming from. I'll give you a hint: Your body is acting like an antenna.

Have fun and keep experimenting!

Regards,
Matt

Hmm...I think the AC mains is radiating and my body is acting like an antenna and picking up the noise..the spectra showed 60Hz and multiples..so now it makes sense

 

Try this see if it helps any.

connect 2 probes, one to each channel.
Connect the ground plugs on each probe, to each other.
Use the the 2 main probes on your piezo leads.

set the math menu to subtract

This should clear up some of the noise. I once had a similar issue with measuring piezo's

 

piezio transducers are a very high impedance, shielding and proper grounding is required to prevent hum and noise pickup.