Design of a mixed-signal oscilloscope

Hewlett-Packard Journal, April, 1997 by Matthew S. Holcomb, Stuart O. Hall, Warren S. Tustin, Patrick J. Burkart, Steven D. Roach

The HP 54645A/D oscilloscopes combine traditional peak detection with intra-acquisition dithering when the instrument is in peak detection mode. They simultaneously acquire and plot both sampled versions of the incoming signal. The peak detected version highlights the signal extremes (one minimum/maximum pair per pixel column), while the denser conventionally sampled record (six points per pixel column) provides statistical information about the signal.

Smoothing. Yet another approach for decimating the incoming samples is shown in Fig. 3e. Here, rather than store the signal extremes for each interval, the average value is stored. This is logically equivalent to a simple low-pass boxcar filter cascaded with a decimating filter. At the slowest sweep speeds, millions of samples are averaged together for every point drawn on the screen, even on single traces. This display mode is useful for pulling the signal out of the noise.

While smoothing reduces the noise in the signal in a manner similar to conventional averaging, there are some differences. Smoothing works on signals acquired with a single trigger, while averaging requires multiple acquisitions to be effective. Smoothing functions as a low-pass filter with the cutoff frequency depending on the time base setting of the oscilloscope. When possible, the HP 54645A/D oscilloscopes use smoothing decimation when conventional averaging is turned on to provide additional noise reduction.

Logic Channel Decimation (Glitch Detection). The acquisition system of the HP 54645D mixed-signal oscilloscope has 16 logic channels. Decimating the series of 1s and 0s for these logic channels provides its own challenges. Simple decimation techniques would lose narrow glitches, so a peak detection technique (known as glitch detection in the logic analyzer domain) is always used. Two bits are sufficient to encode any sample interval, one value for each state (high and low). Not suffering from the drawbacks of peak detection on oscilloscope traces, logic waveforms are always acquired and plotted using glitch detection encoding regardless of the display mode selected for the oscilloscope channels.

Trigger System Features

Perhaps the most difficult task of the design was the trigger architecture for the mixed-signal oscilloscope. It needed to be a mixture of both analog and digital trigger systems. The traditional oscilloscope triggering modes (rising or falling edges on any channel, with a variety of coupling and noise-reject selections) needed to be coupled with the logic triggering modes (pattern triggering, sequencing, Boolean AND/ OR, and so on). But more significant, all of the cross-domain triggering circuits needed to be defined and developed.

Development of this cross-domain triggering system architecture and choices about which trigger modes to support were guided by two key design objectives. The first of these was to seamlessly integrate the trigger configuration for all channels, whether analog or digital. This allows any channel to be used as a trigger source in any trigger setup. A second objective was to extend this seamless trigger configuration to a full set of trigger features. This feature set extends far beyond traditional single-channel, edge-mode triggering to include functionality essential in a mixed-signal test environment.