Operating instructions

University of Saskatchewan
Electrical Engineering Laboratory Equipment Manual
Tektronix TDS 3012B Oscilloscope
FFT Math Function
This function converts an oscilloscope time-domain waveform into its frequency
spectrum. Waveforms with a D.C. component may cause errors in FFT magnitudes.
Push the Save/Recall button, Recall Factory Setup button and the Confirm button.
Connect the source to the scope and display the time-domain waveform. The acquisition mode
should be set to sample and the horizontal resolution should be set to normal. These are found
in the Acquire menu. The Acquire menu also displays the sample rate. Push the CH 1 and
menu buttons. AC coupling is preferred. The bandwidth can be set to 20 MHz. This filters the
source signal, limiting frequencies below the nyquist frequency. Full band width is 150 MHz.
Use the fine scale button so the waveform uses approx. 95% of the vertical scale. The horizontal
scale knob should be set to display a minimum of five periods. This controls the sampling
frequency and the frequency resolution. The more cycles displayed increases the frequency
resolution and decreases the sampling rate. This is the trade off. The nyquist frequency is half the
sampling frequency. Aliasing may occur if the sampling rate is low. Push the math and FFT
buttons for the frequency spectrum. The rectangular window is the only non-windowing
function. Non-windowing is used for transients. Periodic waveforms require windowing. The
magnification button is used along with the horizontal scale and position knobs. This is used to
magnify and position the FFT spectrum. Magnification is about the center of the display. The
entire waveform record is used to calculate the FFT spectrum. Magnification increases display
resolution only. The FFT spectrum is not recalculated. Recall the factory setup before a new FFT
spectrum is calculated.
Examples:
1GS/s = sample rate for display
Nyquist = 1GHz / 2 = 500 MHz BW
500 MHz / 10 div = 50 MHz / div
freq. resolution = 50 MHz / 50 = 1 MHz.
50 MS/s = sample rate for display.
Nyquist = 50 MHz / 2 = 25 MHz BW
25 MHz / 10 div = 2.5 MHz / div
freq. resolution = 2.5 MHz / 50 = 50 kHz