Oscilloscope and Function Generator

Discussion in 'General Electronics Chat' started by paul_alan, Nov 24, 2011.

  1. paul_alan

    Thread Starter Member

    Nov 5, 2011
    I'm beginning to work with oscilloscopes and a function generator at my school. Of course, because we're new students, it's kind of basic tutorial type instuction. One thing I'm having trouble with is how to calculate the internal resistance of the function generator by using a 10kΩ load (and various others one at a time: 1k, 470, 100) connected between it and the oscilloscope. We have it set up where the minimum output of the function generator shows a 2V p-p when using the 10k load. I know it says right on the face of the unit itself (50), but we have to show calculations and how we were able to determine the value of the internal resistance. I was thinking that the Voltage Divider equation using the different values we're working with and an assumed value internally for the function generator would be sufficient. And that trial and error with different values would eventually find the Ir. However, any time I try making calculations it always seems to not make sense. If anyone would be able to help me with this, I'd greatly appreciate it.
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Connect scope to signal generator sinewave output 1kHz, ground to ground, probe to output.

    Use scope to measure peak-peak voltage, set scope to lowest volts/div while keeping waveform on screen (probably 0.5V/div). Make sure the volt adjust inner knob if present is set to "cal", and not adjust.

    If using a scope without auto-measure, move the X-Y Pos knob back and forth until one peak and then the other are on the center graticule to ensure top peak is right on the 4th line from center, and then move the position again to ensure the bottom line is on the 4th line from center. If not, use vertical position to make sure it is centered and 2V (2 divisions up from center, 2 divisions down from center line for 1V/div). The more divisions (squares) covered on the screen, the more accurate your voltage reading will be, provided you do not move the center knob from the "Cal" position (it should click and stay there). I suggest not going past the 10% and 90% dotted lines for the initial signal level, some scopes may not read perfectly accurate when near the edges, though a good and recently calibrated one will, schools vary.

    Measure resistance you are going to use as a load, 10k may be 9500-10500 Ω, so write down the measured resistance exactly. Disconnect DMM, and put resistor across output of Signal Generator, with the scope still attached, measure voltage as described below.

    Use same method as above for finding the voltage with the load resistor, but you will have to use the divisions on the center graticule for the slightly shorter waveform. Do not change the volts/div to measure the 10k, but you will most likely need to change the volts/div down to 0.5V/div to measure the lower resistance loads accurately.

    Once you have your voltages across the different resistors measured and written down, the rest is cake.

    Multiply all voltages (including 2V p-p initial by 0.707 or 1\over\sqrt{2}).
    Do it both ways (peak to peak and RMS) and let us know the difference. ;)

    What you created in every case was a voltage divider with different ratios. Divide the measured voltage by the measured resistance to find the current through the resistor. Subtract measured voltage from 2V (open circuit, and that times 1/√2 for second set of calculations), and divide by measured current to find internal resistance.

    When done finding the resistance values, add them all together and find the average. Post your results here, both the average using peak to peak voltages, and the average using RMS voltages. Oh, give the results to your teacher as well.
    Last edited: Nov 24, 2011