Digitizing Oscilloscopes

One of the important signal integrity measurement solutions is the digitizing oscilloscope.

Digitizing oscilloscopes have different forms — the sampling oscilloscope, the digital phosphor oscilloscope (DPO) and the digital storage oscilloscope (DSO).

Ideally suited for low-repetition signals with narrow pulse widths or fast edges, the DSO can easily capture transients and one-time events, and is the best solution for multi-channel, high-speed design applications.

The DPO, on the other hand, is the perfect tool for digital troubleshooting, for identifying intermittent signals and for various kinds of mask testing and eye diagram. The extraordinary waveform capture rate of the DPO overlays sweep after sweep of data more easily and quickly than other oscilloscope, offering frequency-of-occurrence details, in intensity and colour, with unrivalled clarity.

The digital sampling oscilloscope is the tool of choice when the real-time oscilloscope’s bandwidth is not enough. It is ideal for capturing repetitive signals with much higher frequency components than the sample rate of the oscilloscope. Measuring signals faster than other oscilloscopes, the digital sampling oscilloscope can also achieve up to 100 GHz bandwidth using sequential equivalent-time sampling of repetitive signals.

In selecting an oscilloscope, users should take note of several performance considerations that affect the quality of signal integrity measurements. These include rise time, bandwidth, record length, waveform capture rate, triggering flexibility and sample rate.

Rise time measurements are important in the digital world. Rise time is the more appropriate consideration when selecting an oscilloscope to measure digital signals such as steps and pulses.

Generally, an oscilloscope with rapid rise time can more accurately capture the fast transition’s critical details. The rise time taken by oscilloscope depends on both the rise time of the oscilloscope and the actual rise time. Faster oscilloscope rise time results to more accurate rise time measurements.

Meanwhile, oscilloscope bandwidth is critical when troubleshooting designs with fast rise time signals or high data rates. The digital signal’s rise time carries higher frequency components than what is implied by its repetition rate. To capture the higher frequency components, the oscilloscope should have adequate bandwidth while showing signal transitions accurately.

The oscilloscope cannot resolve high-frequency changes without sufficient bandwidth, while its special features and strengths will mean nothing.

Record length refers to the number of samples that an oscilloscope can store and digitise in one acquisition. With the oscilloscope storing only a limited number of samples, the length of time captured or the waveform duration will be inversely proportional to the sample rate of the oscilloscope.

Expressed as waveforms per second (wfms/s), the oscilloscope’s waveform capture rate determines how often the oscilloscope captures a signal, while the oscilloscope’s sample rate shows how frequently it samples the input signal in a cycle or waveform.

Oscilloscopes nowadays provide triggers for various analog events, including slew rate conditions and edge levels; low-amplitude events and events width conditions; pulse characteristics; serial data patterns as well as setup and hold time violations.

These trigger types assist engineers in isolating and detecting signal integrity problems. The digitizing oscilloscope also features various combinations of timing, logic triggers and voltage as well as specialty triggers, for applications like serial data compliance testing.

Different types of oscilloscopes

Analog oscilloscopes, which employ cathoderay tubes to display a waveform, were the first oscilloscopes that were used by engineers. The screen’s photoluminescent phosphor illuminates every time an electron hits it. A representation of signal is displayed as successive bits of phosphor light up, while a trigger makes the displayed waveform appear stable. Upon completing an entire trace of the display, the oscilloscope waits until a particular event occurs and begins the trace again.

Since the illuminated phosphor does not disappear immediately, the analog oscilloscope allows users to view several traces overlapping each other, providing a glimpse of the irregularities or glitches in the signal.

However, an analog oscilloscope cannot keep the waveform for a longer period of time, as it does not have the capability to “freeze” waveforms. Thus, a signal is lost once the phosphorus substance deluminates. Users cannot automatically perform measurements on the waveform. Instead oscilloscope users will have to use the grid on the display to make measurements.

Another downside of an analog oscilloscope is the limited type of signals it can display, due to the upper limit on the speed of the vertical and horizontal sweeping of the electron beam. Nowadays, although most people still use analog oscilloscopes, they are, however, not sold very often.

The modern tool of choice, digital storage oscilloscopes, or simply known as DSOs, were developed to remedy the negative aspects of analog oscilloscopes. This type of oscilloscope digitizes input signals through an analog-to-digital converter.

Its attenuator scales the waveform while its vertical amplifier offers additional scaling whenever the waveform passes through the analog-to-digital converter (ADC). The ADC digitizes and samples the incoming signal, and stores the data in the memory. The time-base adjusts the oscilloscope’s time display while the trigger searches for trigger events. Before finally displaying the signal on the oscilloscope, the microprocessor system performs additional postprocessing specified by the user.

With the data in digital format allows the oscilloscope to perform various measurements on the waveform and indefinitely store signals in its memory. Thus, the data can be printed or transferred to a PC through a LAN, flash drive, DVD-RW, or USB.

The advances in digital electronic technology resulted to an increasing demand to simultaneously monitor digital and analog signals. This constrained oscilloscope vendors to come up with another type of oscilloscope — the mixed signal oscilloscopes (MSO). A mixed signal oscilloscope has the ability to trigger on and display both digital and analog signals. It usually features more digital channels than analog channels (two or four).

A portable or handheld oscilloscope is a compact oscilloscope that can be carried around. Lightweight and portable, this type of oscilloscope can quickly turn on and off, and are easy to use. However, it does not to offer much performance as its larger counterparts.

Generally found in university laboratories, economy oscilloscopes are affordable by does not provide as much as performance as high-performance oscilloscopes, which offer the best performance capabilities available.

High-performance oscilloscopes are used by those requiring high bandwidth, fast update and sampling rates, large memory depth and a wide array of measurement capabilities.

Agilent introduces world’s fastest real-time oscilloscope

Agilent Technologies (www.agilent.com) is proud to launch its DSOX93204A high-performance oscilloscope – industry’s best 32GHz true analog bandwidth with 80GSa/s sample rate for two channels and 40GSa/s sample rate for four channels.

The oscilloscope has the highest real-time scope measurement accuracy – the highest true analog bandwidth in the market (twice the analog bandwidth than the leading competitor). It has also the lowest oscilloscope noise floor with 2.31mVrms at 32GHz at 50 mV/div, as well as the lowest jitter measurement floor of <150 fS.

The real-time oscilloscope is the industry’s first 30GHz oscilloscope probing system with fully customized probe amplifier s-parameter characteristics that provide accurate probe correction for every individual probe amplifier. Agilent’s bandwidth-upgradeable probes allow the user to prepare for evolving probing needs.

The oscilloscope is deemed as the most comprehensive application-specific measurement. It has the broadest range of jitter, analysis, trigger and protocol tools and features pre-built compliance testing software based on the company’s expertise on the standards committees. It can support emerging technologies such as SATA 6G, SAS 6G, GDDR5, SAS 12G, QPI, DisplayPort 1.2, PCIe gen 3, 10GBaseKR and more. MATLAB software is also available to allow users make analysis routines and custom measurement, instrument applications or user-defined filters.

“When you’re deploying emerging high speed serial bus technology, identifying spectral content of wide-bandwidth RF signals, or analyzing transient physical phenomena, you need the truest representation of your signals under test. The Agilent Infiniium 90000 X-Series oscilloscopes are engineered for 32 GHz true analog bandwidth that delivers. By investing in a proprietary integrated circuit process, Agilent has enabled high-frequency capability while yielding the industry’s lowest noise floor and jitter measurement floor,” said the press release.