TABLE OF CONTENTS The oscilloscope is great for observing more or less repetitive signals that are constant in time, but it gives only scant idea of the frequency content of any but the simplest of signals. The spectrum analyzer, on the other hand, gives little information on the time-variations, but a superbly detailed picture of the frequency content. Both are powerful and necessary tools for designing and debugging photo-measurement systems. The spectrum analyzer is used for noise and signal-to-noise measurements, for choosing modulation frequencies, for tracking down spurious signals and interferences. Indeed, there is a lot of merit in investigating all completed designs with a good spectrum analyzer, in order to see problems with spurious emissions and weak or out-of-band instabilities that go unnoticed on a time-domain display. This investigation must extend well beyond the notional signal frequency, as high-frequency instabilities can have a marked and difficult to understand effect on signal performance.
However, there is a lot of confusion about what the spectrum analyzer actually shows, which has been compounded by the new generation of digital analyzers, PC data-acquisition systems, and fast fourier transform (FFT) computing oscilloscopes. Hence it is useful to work though a few simple examples in pedantic detail (the simple ones we all get wrong!).
If a 1V peak (2V pk-pk) sine-wave voltage at 1kHz (Fig. B.1) is applied to a conventional spectrum analyzer, what does the main spectral line read on the ordinate scale? To answer that we need to make...
