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www.thinkSRS.com About FFT Spectrum Analyzers Application Note #1 What is an FFT Spectrum Analyzer? frequency points as there are time points. (Remember Nyquist's theorem.) Suppose that you take 1024 samples at FFT Spectrum Analyzers, such as the SR760, SR770, SR780 256 kHz. It takes 4 ms to take this time record. The FFT of this and SR785, take a time varying input signal, like you would record yields 512 frequency pointsbut over what frequency see on an oscilloscope trace, and compute its frequency spectrum. range? The highest frequency will be determined by the period of two time samples or 128 kHz. The lowest frequency is just Fourier's theorem states that any waveform in the time domain the period of the entire record or 1/(4 ms) or 250 Hz. can be represented by the weighted sum of sines and cosines. Everything below 250 Hz is considered to be DC. The output The FFT spectrum analyzer samples the input signal, spectrum thus represents the frequency range from DC to computes the magnitude of its sine and cosine components, 128 kHz, with points every 250 Hz. and displays the spectrum of these measured frequency components. Advantages of FFT Analyzers Why Look at a Signal's Spectrum? The advantage of this technique is its speed. Because FFT spectrum analyzers measure all frequency components at the For one thing, some measurements which are very hard in the same time, the technique offers the possibility of being time domain are very easy in the frequency domain. Consider hundreds of times faster than traditional analog spectrum the measurement of harmonic distortion. It's hard to quantify analyzers. In the case of a 100 kHz span and 400 resolvable the distortion of a sine wave by looking at the signal on an frequency bins, the entire spectrum takes only 4 ms to oscilloscope. When the same signal is displayed on a spectrum measure. To measure the signal with higher resolution, the analyzer, the harmonic frequencies and amplitudes are time record is increased. But again, all frequencies are displayed with amazing clarity. Another example is noise examined simultaneously providing an enormous speed analysis. Looking at an amplifier's output noise on an advantage. oscilloscope basically measures just the total noise amplitude. On a spectrum analyzer, the noise as a function of frequency In order to realize the speed advantages of this technique we is displayed. It may be that the amplifier has a problem only need to do high speed calculations. And, in order to avoid over certai