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Oscilloscope particular acquisition modes – EDN

Digital oscilloscopes are usually operated in real-time acquisition mode, the place the analog enter is sampled and digitized at a user-selected sampling fee and written constantly into the acquisition reminiscence.

There are, nevertheless, different acquisition modes accessible in most oscilloscopes: random interleaved sampling (RIS), sequence mode, and roll mode (Determine 1).

Determine 1 The sampling mode picks in a typical oscilloscope embody sequence, RIS, and roll along with the usually used real-time mode. Supply: Arthur Pini

 These particular modes are helpful in functions for measuring particular kinds of indicators. RIS mode will increase the efficient sampling of the oscilloscope fee for periodic indicators. Roll mode is helpful in displaying low-frequency indicators having very lengthy durations. Sequence mode reduces the lifeless time between acquisitions and can be utilized to indicators which have low-duty cycles with lengthy lifeless instances between important occasions.

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RIS acquisition mode

Let’s take a look at these acquisition modes,beginning with RIS mode. RIS mode is a type of equivalent-time sampling that permits the oscilloscope to accumulate repetitive indicators at very excessive sampling charges. A number of acquisitions are mixed in RIS to create a composite waveform with a better efficient sampling fee. The set off occasion and the sampling clock aren’t synchronous. The time between the set off and the primary pattern in an acquisition is randomly distributed. Oscilloscopes use a time-to-digital converter (TDC) to measure the time delay between the set off and the closest pattern of every sweep. This delay is named the horizontal offset. The acquisitions are grouped by delay to supply samples spaced by as little as 5 ps, an efficient sampling fee of 200 giga-samples per second (GS/s). Chosen waveforms are added collectively, making a composite waveform, as proven in Determine 2.

Determine 2 Random interleaved sampling creates a composite waveform based mostly on the measured delays between the set off and the closest pattern level. Supply: Arthur Pini

RIS requires a periodic enter waveform with a secure set off. The utmost efficient RIS sampling fee is achieved by making a number of acquisitions and choosing these with horizontal offsets, yielding the specified pattern spacing. The random timing between digitizer sampling instances and the occasion set off gives the time variation. The instrument makes use of a number of triggers to finish an acquisition. The variety of acquisitions required depends upon the efficient pattern fee. The upper the efficient pattern fee, the extra triggers are required. Determine 3 compares real-time and RIS acquisitions of a high-frequency sine wave.

Determine 3 Evaluating the real-time acquisition of a 5 GHz sinewave at 40 GS/s (decrease hint) with a RIS acquisition at 200 GS/s (higher hint). Supply: Arthur Pini

The true-time acquisition of the 5 GHz sine is sampled at 40 GS/s and produces a waveform with 8 samples per cycle. Viewing that waveform utilizing linear show interpolation produces a ‘boxy’ show. The RIS acquisition has an efficient pattern fee of 200 GS/s, yielding 40 samples per cycle and a smoother show. Sin x/x interpolation could possibly be used to clean the waveform, however because the waveform bandwidth approaches the Nyquist frequency, particularly for pulse-like indicators, the potential for interpolator errors comparable to Gibbs ears will increase and the RIS acquisition mode produces a extra correct illustration of the acquired waveform. Within the oscilloscope used, RIS is just accessible for timebase settings of 10 ns/division or much less; related restrictions will apply to all oscilloscopes.

Roll mode

Roll mode operates at gradual sweep speeds, the place the sampled knowledge is acquired at a sufficiently low sampling fee and shows the samples in actual time as they’re acquired. That is necessary at gradual sweep speeds as a result of it eliminates “pre-trigger” acquisition delays. Often, an oscilloscope holds off show till the acquisition is full. In case you are utilizing a one-second per division horizontal scale setting, it’s important to wait no less than ten seconds earlier than you see the acquired waveform. At gradual pattern charges used with lengthy acquisitions, roll mode writes to the show because the pattern turns into accessible, eliminating that delay. In roll mode, the hint strikes slowly to the left within the method of a strip chart recorder show (Determine 4).

Determine 4 The simulated development of a waveform acquired in roll mode. Supply: Arthur Pini

This determine exhibits the development of an electrocardiogram sign acquired in roll mode over time. The waveform is written beginning on the correct and shifting to the left over time. Every grid within the determine, beginning on the higher left, exhibits the waveform at a later time till it fills the show within the decrease proper when a set off occasion is detected and the acquisition is full. Roll mode might be entered relying on the pattern fee. This oscilloscope allows roll mode at sweep speeds better than 100 ms/division (greater if extra acquisition reminiscence is used).

Roll mode is helpful with low-frequency indicators like this electrocardiogram waveform.

Sequence mode

Sequence mode is right for capturing many quick sign occasions in fast succession or for capturing a number of occasions separated by very long time intervals. This mode breaks the acquisition reminiscence into smaller segments and permits a number of acquisitions inside the acquisition reminiscence. Sequence-mode acquisitions decrease lifeless time between acquisitions (sometimes <1 µs) by holding off show till all segments are captured. Every acquired section is time-stamped at its set off time with the real-time clock, the delay between segments, and the elapsed time because the first set off.

Sequence mode has three fundamental functions:

  1. To accumulate knowledge at a excessive sampling fee when the enter waveform has lengthy intervals of lifeless time.
  2. To accumulate knowledge with minimal lifeless time between acquisitions.
  3. To make use of the set off time-stamp desk to know the occasion timing.

For an instance of utilizing sequence mode to research indicators with lengthy lifeless time, take into account capturing a number of packets of an I2C knowledge sign and measuring the time between packets and the rise time of the info (Determine 5).

Determine 5 Analyzing and I2C knowledge sign for the inter-packet delay and the rise time of the info sign. Supply: Arthur Pini

4 knowledge packets had been acquired at 20 ms/division at a sampling fee of 5 MS/s. Cursors measure the delay between packets at 43.5 ms. The obligation cycle of the I2C sign is small. So, there’s quite a lot of reminiscence used to indicate the lifeless time between packets, leading to a low sampling fee. Measurement parameter P2 measures the rise time of the sign. It exhibits an increase time of lower than 291 ns with a yellow warning indicator. The sampling fee is 5 MS/s, or a pattern interval of 200 ns, is the reason for the measurement warning.

Utilizing sequence mode, we will purchase every packet in its personal reminiscence section. The variety of segments is model-dependent and is a perform of the scale of the acquisition reminiscence. Within the oscilloscope used, as much as 2000 segments might be saved. The packets are a lot shorter than the entire knowledge stream, and a better sampling fee can be utilized (Determine 6).

Determine 6 A sequence mode acquisition captures ten knowledge packets at a sampling fee of 500 MS/s. It additionally measures the time between packets utilizing the sequence mode set off time stamps. Supply: Arthur Pini

The variety of segments acquired is user-selected; on this instance, ten packets had been acquired with a pattern fee of 500 MS/s (2 ns sampling interval) horizontal scale of 200 ms/division. A zoom hint, Z1, horizontally expands section 1. The rise time measurement now exhibits 28.19 ns, with the inexperienced test standing icon indicating a sound measurement. The upper sampling fee gives greater than enough time decision for the measurement.

Under the show grids, the sequence mode time stamps learn the time of every set off utilizing the oscilloscope’s real-time clock. It additionally reads the time of every section set off from the primary set off in addition to the time between triggers with a decision of 1 nanosecond. The time between segments is nominally 43.6 ms. On this instance, sequence mode eradicated the very lengthy lifeless time between knowledge packets and improved the measurements.

A second measurement gives an instance of utilizing sequence mode to reduce lifeless time between acquisitions. Wanting on the indicators generated by an ultrasonic vary finder. The vary finder emits a burst of 5 40 kHz ultrasonic pulses every time a measurement is made (Determine 7).

Determine 7 Every measurement of an ultrasonic vary finder emits 5 40-kHz bursts. After every burst, it waits for an echo from the goal earlier than emitting the subsequent transmitter burst, stopping after the fifth burst. Supply: Arthur Pini

When a measurement is began, the vary finder emits a 40-kHz burst. It waits to detect an echo earlier than emitting the subsequent burst and continues the method till 5 ultrasonic pulses have been transmitted. In real-time acquisition mode, attempting to accumulate 5 particular person bursts wouldn’t work. After every acquisition, the oscilloscope would pause to show the info acquired and never be prepared for the subsequent burst within the collection. That’s the place sequence mode has a bonus. It segments the acquisition reminiscence right into a user-set variety of segments and acquires one acquisition into every section with out pausing to show the info till all of the segments are crammed or the acquisition is manually stopped by the person. The latency between acquisitions of every section is lower than a microsecond within the oscilloscope used. In the course of the acquisition, it data the set off time of every section. Determine 8 exhibits an instance of the five-segment acquisition.

Determine 8 The sequence mode acquisition of ultrasonic vary finder sign with every of the bursts in its personal section seems within the higher left grid. Supply: Arthur Pini

Sequence mode waveforms might be displayed in any of 5 other ways. The show proven is named an adjoining show. The segments may also be overlapped for comparability. They are often displayed as a waterfall show, with every show section offset vertically. A perspective show gives a three-dimensional view with segments displayed with each vertical and horizontal offset. The ultimate show sort locations every section in its personal grid and is named a mosaic show.

Every section might be operated on as an impartial waveform. Utilizing zoom traces, every section might be displayed independently of the others. In Determine 8, zoom traces Z1-Z5 present every of the segments. Any of the oscilloscope’s measurements and math features might be utilized to the segments independently. For example, the mathematics hint, F2, applies a 2 kHz bandwidth second-order Butterworth band move filter centered about 40 kHz to section 1. Averaging might be utilized not solely to particular person segments as they’re acquired, but it surely may also be computed throughout all of the segments in a single sequence, as proven in math hint F1.

The sequence mode set off time stamps desk exhibits that ultrasonic bursts happen with a nominal 4.2 ms interval and confirms that none had been missed.

RIS, Roll, and Sequence modes

These examples present proof of the usefulness of RIS, Roll, and Sequence modes. They present how one can lengthen the capabilities of your oscilloscope.

 Arthur Pini is a technical help specialist and electrical engineer with over 50 years of expertise in electronics take a look at and measurement.

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