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ADC, FFT and Noise. p. 1 ADC, FFT, and Noise Analog to digital conversion and the FFT A LabView program, “Acquire&FFT_Nscans.vi”, is available on your pc which (1) captures a waveform and digitizes it using an analog-to-digital converter (ADC), (2) takes the fast Fourier transform (FFT) of a specified number of samples of the waveform, and (3) displays the Fourier transform on the monitor. For the number of samples always use a number that is a power of 2, i.e. 2 n , because the FFT program works much more efficiently on such a sample. 1. Digitize low-frequency waves from the function generator, sine, triangle, and square. Observe the power spectra on the monitor and compare with expected power spectra. (Note: Because there is no anti-aliasing filter in front of the ADC, some components of the triangle and square are bound to be above the Nyquist frequency (component frequencies greater than half the sample rate) and lead to aliasing. But if the fundamental frequency is low, the components should be small.) 2. Digitize a sine wave with frequency f 0 greater than half the sample rate f SR Compare the frequency of the FFT peak with the calculated frequency of the alias, f SR – f 0 . Noise The best way to build a noise generator is to use a zener diode, reverse biased, and a high-gain amplifier. Figure 1 below shows how. The strength and quality of the noise depends upon the diode and upon the current limiting resistor R 1 . As a rule, high-voltage zeners work best. Because the power supply is 15 volts, a zener with breakdown voltage between 10 and 13 volts is recommended. Different zeners behave differently as noise sources.

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