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 oscilloscope architecture is designed to optimize the memory depth for the user's requirements. If the user is viewing repeated acquisitions of the signal, the memory depth is decreased if necessary to maintain the maximum update rate. If the user stops the acquisition, the memory depth of the last acquisition is changed to use all of the available acquisition memory. This feature is made possible by the same architectural feature that was designed to maximize the oscilloscope's update rate. During most continuously acquiring configurations, half of the acquisition memory is used to write newly acquired data while the other half is being read into the display. Since the architecture is fundamentally based on a 1-million-point memory system, 500,000 points are dedicated to the next trigger while 500,000 points are being read to the display.

If memory depth has been traded off for update rate, a subset of 500,000 points is used for each trigger during continuous acquisition mode. For example, at 200 [micro] s/div, the acquisition time required to fill the screen with an acquired waveform is 200 [micro] s/div x 10 divisions = 2 ms. The time required to acquire 500,000 points at 200 MSa/s is the sample period times the number of points, or 5.0 x [10.sup.-9] x 500,000 = 2.5 ms. Since the time required to capture 500,000 points is larger than the time required to fill the screen, we choose to reduce the number of acquired points to maintain maximum update rate. In this case, 400,000 points can be acquired in 2 ms (400,000 x 5.0 x [10.sup.-9] = 2 ms), so we acquire 400,000 points. This maintains the theoretical update limitation of 500 waveforms per second (1/0.002 = 500). A more extreme trade-off occurs at 5 [micro] s/div. Only 10,000 points can be captured in the 50-[micro] s acquisition time (10,000 x 5.0 x [10.sup.-9] = 50.0 x [10.sup.-6]).

If the user presses the Run/Stop key while an acquisition is in progress, the assumption is made that this may have been done to examine the last acquisition in more detail. The last acquisition that occurred before the user's action is stored in one half of the memory system. Since it cannot be assumed that more triggers will follow the user's action, this half of the memory may not be written to any more after the Run/ Stop key is pressed. However, since the other half of the memory system now contains defunct data, the acquisition system is reconfigured to use the entire 500,000 points available in this other memory half, and keeps acquiring data, looking for a trigger. Since the primary task to be executed when the user presses the Run/Stop key is to stop acquisition in what appears to be an instantaneous fashion, the system cannot wait forever for an additional trigger. Therefore, a timeout is initiated that will cause the system to cease looking for additional triggers after 100 ms. For the oscilloscope's acquisition system, this is a large window of opportunity to capture a deep-memory record. However, if no additional triggers occur, the acquisition system is brought to a stopped state, and the shorter acquisition obtained on the last trigger is used for postacquisition pan and zoom. Since most applications involve relatively frequently occurring triggers, this methodology will result in most users viewing deep-memory acquisitions every time they stop their oscilloscopes. For applications in which the rate at which triggers occur is uncertain, or in which there is only one trigger event, the user can press the Single key and guarantee a 1-million-sample acquisition for every trigger.