Application Examples
Taking FFT Measurements
You can take FFT measurements to determine if low -level distortion is
present, or to find the s ou rce of noise in a mixed circuit.
Detecting Distortion
A pure sine wave can be input into an amplifier to measure distortion;
any amplifier distortion will introduce harmonics in the amplifier output.
Viewing the FFT of the outpu t can determine if low-level distortion is
present.
You are using a 20 MHz signal as the amplifier test signal. You would
set the oscilloscope and FFT parameters as listed in the next table.
Control Setting
1 Coupling AC
Acquisition Mode Average 16
Horizontal Resolution
Normal (10k points)
Horizontal Scale
100 ns
FFT Source Ch1
FFT Vert
Scale
dBV
FFT Window Blackman-Harris
In the next figure, the firstcomponentat20MHz(figure label 1) is
the so
urce signal fund amen tal frequency. The FFT waveform also
shows a second-order harmonic at 40 MHz (label 2), and a fourth-order
harmonic at 80 MHz (label 3). The presence of componen ts 2 and
3in
dicate that the system is distorting the signal. The even harmonics
suggest a possible difference in signal gain on half of the signal cycle.
TDS3000C Series O scilloscope User Manual 45
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