Trigger jitter reduction for an internally triggered real time digital oscilloscope

• Main Authors: Weller, Dennis J, Draving, Steven D, Urban, Ralph
• Format: Patent
• Language: English
• Published: 22.06.2004

Real time digital oscilloscopes operate by digitizing closely spaced consecutively sampled values of an applied waveform, storing the digital values in a memory, and then re-constructing the waveform as a displayable image (the "trace") on a raster-scanned CRT by reading and processing the stored values. We shall call the stored digital values an "acquisition record" and note that its contents correspond to a definite time interval in the history of a signal's behavior. The length of that time interval is largely determined by the number of addressable memory locations devoted to a signal's acquisition (the memory's "depth") and the rate at which the samples are acquired. That sample rate will be high enough to meet Nyquist sampling and bandwidth requirements, but is in all probability not high enough to directly produce a satisfactory reproduction of the waveformn, even though it does determine what that reproduction ought to look like. Accordingly, it is conventional to "fill-in" the "missing" samples before all or some portion of the acquisition record is used, by sending the portion of interest through an interpolation filter before being used by other mechanisms. Trigger jitter in an internally triggered real time digital oscilloscope can be reduced through correcting the horizontal position value obtained from a conventional trigger interpolation mechanism by an error or substitute amount learned from an inspection of the acquisition record that notes where in the acquisition record the signal actually crossed the trigger threshold. The display (and measurement) sub-systems that utilize the selection by panning and zooming of a portion of the acquisition record for viewing (and measurement) are supplied with the acquisition record portion of interest, and with an associated horizontal position value that originates with trigger interpolation and that may be subsequently modified by panning. After the trigger circuit detects the trigger condition for a new acquisition record, the voltage threshold for the trigger level in use by the trigger circuit can be compared to the region proximate the trigger location in the acquisition record to determine by further local interpolation the precise location in the acquisition record where the triggering event should have occurred. This allows the determination of a correct and jitter free horizontal position value. Actual display of the desired portion of each new acquisition record then proceeds by supplying to the display rendering system a corrected associated horizontal position value. This reduction in visible trigger jitter is also beneficial to measurements made on the displayed trace.