The impact of “classical” Digital Signal Processing (DSP) techniques began to be felt in the design of oscilloscopes starting in the late 1970s, and accelerated throughout the 1980s and beyond. Much of it was hidden to the user; for instance, the use of interpolating digital filters when the scope’s horizontal sweep is expanded beyond the point where there are enough data points to fill the screen.
Some early scopes did offer the choice between Linear and SinX/X interpolation. The latter gets its name from the fact that, for a perfect (“brick wall”) low pass digital filter, the impulse response is a SinX/X function, which has a characteristic appearance of a damped sinusoid in the time domain. The function also plays a key part in the reconstruction of an analog signal from its digital samples.
Scope architectures evolved to include a DSP subsystem, which in turn acquired a hardware component in some instances. Normally, in a digitizing scope, the front end A/D converter is run at the highest sampling rate possible, which means that in slower sweep speeds, there is the potential for some real-time DSP between samples that are stored in acquisition memory. It still takes significant hardware to do this processing, but front-end architectures since the 1980s have incorporated such features as peak detect, digital filtering, and other useful functions affecting waveform display.
An interesting example of real-time DSP is provided by the hi-res acquisition function in some scopes. Here, a simple FIR filter implements a time averaging function at the highest sample rate. The idea is to trade off, for low frequency signals, effective sample rate for vertical resolution. The results are often surprisingly good for such simple hardware.
Do any of you have experience with hi-res mode? The noise reduction and resolution improvement on a low-frequency waveform can look astonishingly good. Typically, no dither needs to be added to the input signal. At high vertical sensitivities there is enough noise in the system to decorrelate the quantization noise at the A/D output.
The bulk of DSP functionality in scopes now is usually in the firmware running on the powerful processor systems found today. Commonly used functions, such as waveform averaging, math operations, and FFT, were the first DSP procedures to become commonplace. Since then, the new opportunities for DSP have included application-specific areas as well as new measurement domains.
Communications in general, and serial data links in particular, have been rich fields for applying the scope’s DSP power. Coupled with huge advances in display and other processing capabilities, the results, in eye diagrams, 3D waveform displays, and constellation displays, are but a few of the advances made by scopes in the last 20 years.
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