Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How To Superimpose AC Signal Onto DC Line?

  1. Feb 10, 2012 #1
    Hello all! First post!

    I've built a 7.83 Hertz sine wave oscillator that is fed into a 1,000 wind coil (soft iron rod/bifilar windings/self cancelling) and have tested in out on my oscilliscope and it's working beautifully with a peak voltage of 5.8 VAC (4.1 RMS AC). This device is being used as an environmental experiment to re-establish the Schumann earth resonance.
    The one issue that I've been pondered about the Schumann Resonance is whether it is considered a AC sine wave (positive side alternating to negative side) or if it is more of a pulsing DC sine wave (fluxating amplitude, positive the entire time).
    My device is a simple Wein-Bridge oscillator and I would like to take a 6 volt DC line and superimpose the oscillator on top of it. Theoretically getting a sine wave going in amplitude from .2V to 11.8V. I think this can be accomplished by simply using using a capacitor that acts as a wall for the AC to pass through whilst keeping the DC contained.
    One thought I had was to simply use the same positive supply rail that powers the oscillator and run it through a voltage divider network with resistors (easy) in order to get the 6 volt signal. I then send take that 6 volts and met it up with a T point with the capacitor, and then on to the coil and ground.
    I thought I would see if someone if familiar with this little 'trick'.

    Thanx and Namaste,
  2. jcsd
  3. Feb 10, 2012 #2
    You can bias the AC signal with a pull-up from the DC signal or a resistor divider. This is very common for audio signals.

    Also like you said, you can use an AC coupling capacitor to isolate your AC source from the DC voltage. I did this for a audio cable project once. You still will be biasing the AC signal with a DC voltage and resistor, and you select the resistor to so that the cutoff frequency is not in your bandwidth.

    You can also put the AC and DC voltage sources in series, but I'm not sure if this is actually done in practice.
  4. Feb 10, 2012 #3
    Hello Dragon Petter and thank you for the reply.

    Do you think that since this is a very low frequency that it will need the use of a large capacitor?

    I'm very interested in your mention about the pull up from the DC signal. How would this be done? Perhaps I've already thought of the correct answer but it will be a little bit until I am able to try it out.

  5. Feb 10, 2012 #4


    User Avatar
    Science Advisor

    Your coil probably doesn't have much DC resistance, so if you have a DC component, this small resistance will present a heavy load to whatever is driving it.

    You could get an offset voltage with an opamp, but if the load was almost a short circuit to DC, then the output from an opamp would be low.

    Why are you using an oscillator? Isn't the Schumann effect caused by lightning around the world and your function would be mainly to listen to it with a receiver?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook