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Optical Signal Measurements Using A Real-Time Oscilloscope

Application Notes

Real-Time vs Sampling Oscilloscope

The first thing you may notice when looking at the bandwidth and sample rate characteristics of two different types of oscilloscopes is that a real-time oscilloscope samples at a rate faster than the signal being observed, according to the Nyquist theory saying that the sampling rate should be, at least, twice the highest analog frequency component of input signal to avoid undesirable measurements in the form of aliasing or distortion.

Sampling rate > 2*fmax of signal where fmax is the highest analog frequency component. However, in the sampling oscilloscope, the actual sample rate is much slower than the signal being observed.

The real-time oscilloscope captures the entire waveform in one acquisition cycle and an interpolation technique such as sine x/x is used to fill in the points in between actual sampled points. Therefore, the real-time oscilloscope is excellent for capturing transient, non-repetitive single shot events as well as repetitive signals.

On the other hand, a sampling oscilloscope captures only one sample at each trigger. The next sample is taken at a slight time delay compared to the previous sample, and it goes on and on. Through multiple passes of the signal trigger and sample captures, the waveform can be precisely reconstructed. Because only one sample is taken at each trigger, the A /D process can have a very high resolution with very low noise and jitter.

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