Noise in oscilloscope

Discussion in 'General Electronics Chat' started by samik, Mar 16, 2014.

  1. samik

    Thread Starter Member

    Nov 9, 2009
    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
  2. MrChips


    Oct 2, 2009
    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.

    Again this is normal. Measure the frequency of the periodic waveform you observe on the scope.
  3. DerStrom8

    Well-Known Member

    Feb 20, 2011
    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! ;)

    samik likes this.
  4. samik

    Thread Starter Member

    Nov 9, 2009
    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 now it makes sense

    Well-Known Member

    Mar 1, 2012
    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
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    piezio transducers are a very high impedance, shielding and proper grounding is required to prevent hum and noise pickup.