DSOs Tackle The Challenges Of Faster, More Complex Designs

Like an octopus, the oscilloscope on today's design bench must sprout multiple tentacles, reaching out to monitor signals in many parts of a circuit and displaying the measured values all at once—with all events synchronized to a common time base. But with the increasing prevalence of both analog and digital signals in new designs, getting to the heart of a problem quickly and effortlessly has never been more critical.

Also, designers often need to capture events in real time, then scroll through to find where a glitch or other anomaly is stifling performance. That means huge memories. In digital designs, discerning a "high" from a "low" is simply no longer good enough. So oscilloscopes with multiple inputs, on-board data processing, and mixed analog/digital analysis capabilities are a must.

It turns out that oscilloscopes are edging closer to fulfilling these requirements. The digital storage oscilloscope (DSO) has become indispensible. In addition, many oscilloscopes have become de facto PCs and, therefore, highly capable of performing many computations on their own—with the ability to communicate.

A recent example of these trends is the Agilent Technologies 54642D mixed-signal, deep-memory scope (Fig. 1). It's called mixed-signal because it can measure and display two analog and 16 digital channels with all 18 channels time aligned. Consequently, it combines a scope's detailed signal-analysis capability and a logic analyzer's multitiming measurements. Intended for designs with lots of digital signals, it lets users view the complex interrelationships among all displayed signals.

Therefore, the 54642D is well suited for mixed-signal developmental projects, like cell phones, where the input and output to the user is analog while much of the signal processing occurs in the digital domain. Each of its analog channels provides 500 MHz of bandwidth. Its standard acquisition memory captures up to 8 Mbytes. The high-definition display is mapped into 32 levels of intensity that instantaneously disclose subtle details.

Powerful Triggering Necessary: Viewing analog and digital channels on a single instrument requires powerful triggering. When working with mixed analog and digital designs, it's sometimes difficult to trace an anomaly back to its cause unless the user can trigger on it and correlate it with another trace.

Consider a situation that employs the serial I²C (inter-integrated circuit) protocol as a communication channel. I²C is popular because it needs only two I/O lines for full implementation. However, along with the benefits of using two I/O lines comes the hassle of a complicated protocol. So a tool on the bench that eases I²C troubleshooting is certainly a benefit.

If the designer is working with microcontrollers that use PC serial communications, the 54642D's I²C bus trigger mode can be employed to first verify inter-IC communication handshaking. Then the I²C trigger can be used again to ensure that correct data is being transmitted to the desired device. Other triggers provided with the 54642D include CAN, USB, and SPI.

In summary, the deep memory capability on this mixed-signal oscilloscope (MSO) accelerates troubleshooting and verification. It permits the user to see slow analog and fast digital signals simultaneously and sustains a high sample rate, providing excellent resolution. The high definition realized, in turn, makes it easy to view subtle details that the deep memory captures, while serial triggering makes it possible to locate complex data patterns.

In the past, collecting data and transforming it into some type of desired format was a cumbersome job. But Gould Instrument Systems has streamlined this task and automated it in the Ultima 500, bringing the benefits of a sophisticated PC to a high-performance oscilloscope. This DSO provides 500-MHz bandwidth at the probe tip and a 2-Gsample/s sampling rate with 1 million points of data storage per channel. Internal floppy and hard drives—coupled with high-speed 100-Mbit/s Ethernet, USB, and PCI connectivity—enable virtually unlimited data storage and flexible communication. Users can easily transfer data to other computers (Fig. 2).

Automatic Logging: As it's capturing data, the Ultima 500 can transfer data to various analysis packages, such as Excel, for graphing and data logging. If the user wants to perform a peak, mean, or other parameter measurement on that data, it can be readily logged into a file, then graphed later in Excel.

Likewise, whenever the Ultima 500 triggers, the user can take advantage of a Windows feature called Decom Technology. It takes the desired parameters—like a peak, a mean, or a date and time—and transfers them to Excel. As data arrives, all analysis previously set up in the report is updated on-the-fly, behind the scenes. Once measurements have been set up, acquiring data and printing reports become routine.

In dual-video mode, an external monitor is connected to the Ultima 500. Then the user can view two different images simultaneously without having to click back and forth. The image on the scope comes directly from the test probe, while the data on the external monitor is derived from the analysis of collected data that has been massaged by Excel or some other software package.

Long a proponent of deep memories, LeCroy Corp. recently introduced the WaveMaster. This 5-GHz DSO provides the speed and flexibility for analyzing long, complex, high-speed signals and very fast edges (Fig. 3). Its architecture's front end starts with silicon-germanium (SiGe) amplifiers and ADCs that track the arriving signal, digitize it at 10 Gsamples/s on each of its four input channels, and stream data to a fast acquisition memory that can store up to 48 million points arriving at a 10-Gbyte/s data rate.

This acquisition memory performs a variety of operations, including packetizing the data arriving from the ADC, and then transferring the packets in a real-time streaming mode via a high-speed data bus directly into cache memory on the CPU board. Instructions and calculations can be fetched and performed much faster by operating on cache than on data stored in RAM.

After the initial batch of data arrives at the CPU, a number of display, measurement, and analysis routines can be invoked. The CPU can analyze the initial portion of a long, complex waveform, while the remainder of the signal continues to flow through the streaming architecture.

Doubling The Memory: Sometimes an engineer needs to look at just one or two signals while using a four-channel digital oscilloscope. In such instances, the WaveMaster doubles the sampling rate to 20 Gsamples/s and doubles the acquisition memory length when using one or two of the channels. Dual-channel measurements, such as setup-and-hold time, can be characterized in this mode of operation.

Introduced earlier this year, the Tektronix TDS6604 is a second-generation, high-performance SiGe instrument. It lets digital design engineers rapidly and effectively pinpoint faults in emerging serial bus architectures, like InfiniBand, RapidI/O, 3GIO, and HyperTransport. This four-channel, real-time instrument provides a 6-GHz bandwidth as well as simultaneous 20-Gsample/s sample rates on two channels. It can support clock recovery at 2.5-Gbit/s data rates and 1.25-Gbyte serial pattern triggers.

The TDS6604 is the first Tektronix DSO to incorporate an open Windows platform, providing access to industry-standard peripherals, networking elements, and analysis tools. It supports various automated measurement packages for jitter measurements and USB 2.0 compliance testing.

An option offers a portfolio of masks that let users perform conformance testing to electrical standards, such as InfiniBand, Gigabit Ethernet, Fibre Channel, USB, Serial ATA, and IEEE 1394. The scope uses Tektronix's Digital Phosphor Oscilloscope technology.

Traditionally, the oscilloscope and PC have been separate entities that communicate with each other through interface connections like RS-232 and GPIB. Gage Applied Inc. has merged these entities in a line of high-speed digitizers. Called CompuScopes, they plug into a PCI slot in virtually any PC and provide up to 5-Gsample/s sampling speeds.

Each digitizer is normally equipped with two, simultaneous input channels. When multichannel applications are required, additional digitizer cards can be simply plugged into adjoining PC slots for simultaneous sampling on up to 32 channels on a single PC. Though the system comprises physically distinct cards, the ensemble behaves functionally as one multichannel module.

The CompuScope supplies up to 2 Gsamples of onboard acquisition memory. The digitizers are then well suited for communications applications where users want to sample very quickly over human-type time scales of, say, several seconds. Also, the acquisition memory can be segmented for the rapid acquisition of repetitive signals. A user can capture millions of thousand-point radar records in the onboard memory without CPU intervention.

High resolution generally isn't important for probing repetitive, continuous wave-like signals. But for signals with a wide dynamic range, high-digitizer resolution is essential. Though 8-bit is the norm, Compu-Scope can supply 14- and 16-bit digitizers with 16,384 and 65,536 levels, respectively.

A classic application is the acquisition of radar, lidar, or ultrasonic signals. Quite similar, they all comprise large and small echo pulses that result from reflected wavelets. The user can't increase the gain beyond the point where the largest echo almost saturates the digitizer input range. Therefore, vertical resolution sets a limit on the minimum echo amplitude that can be detected. With an 8-bit digitizer, echoes under 1/256 of the largest echo can't be detected. In contrast, with a 12-bit digitizer, the resolution limit is 16 times smaller—or 1/4096 of the largest echo.

Pico Technology has upgraded the PC-based ADC-212/100 DSO/spectrum analyzer's sampling rate. Also known as the PicoScope, it targets repetitive signals with an equivalent sampling rate of 5 Gsamples/s (100 Msamples/s real-time).

This real-time DSO combines the functionality of a conventional benchtop oscilloscope with the benefits of a PC. It requires no external power supply because it draws its power from the PC's parallel port. Measuring 5.5 by 7.5 by 1.77 in., it's well suited for portable use with a laptop.

Traditional benchtop instruments have a fixed set of functions, so they eventually become outdated. But PC-based instruments can easily be updated with software upgrades that enhance functionality and increase useful life.

The equivalent-time sampling function was achieved via software en-hancements to the application running on the host PC and a new firmware configuration within the ADC-212s. This configuration is automatically installed in the resident hardware when PicoScope is run, and it boosts the performance of the scope far beyond its 100-Msample/s real-time capability. The 12-bit resolution ADC-212/100 and the 50-Msample/s version, the ASC-212/50, also transform PCs into spectrum analyzers—with 50- and 25-MHz capabilities, respectively.

The PicoScope's FFT-based spectrum analyzer has the same trigger features as the DSO, making it possible to capture the spectrum of a one-off event. Other features include normal, average, and peak detect modes plus linear/log scales for amplitude and frequency.

Because they use a 12-bit ADC, the PicoScopes provide a basic dc accuracy of 1%. Both scopes have nine input ranges, from ±50 mV to ±20 V. Due to their 1% accuracy, they can be implemented with the same confidence as a conventional digital multimeter. When implemented as a spectrum analyzer, the 12-bit resolution in the Y-axis means that the ADC-212 can deliver a dynamic range of 80 dB and can detect changes as small as 0.024%.

In the handheld space, Fluke Corp. has added color and enhanced the capabilities of its dual-input 100- and 200-MHz ScopeMeters (Fig. 4). Designated the 196C and 199C, respectively, these dual-channel DSOs feature separate high-speed digitizers for each channel. Just 10.1 by 6.6 by 2.5 in. deep and weighing in at 4.3 lbs, these instruments sport bright 4.5- by 3.4-in. displays (320 by 240 pixels).

The color displays make identification of individual waveforms significantly easier. In particular, they let users distinguish individual traces if waveforms fall on top of each other or are too close to one another. Moreover, the ScopeMeters use color for warnings and other onscreen labels. A brighter, high-contrast display enables clear reading under varying light conditions while preserving a full 4 hours of battery life.

The instruments always memorize the last 100 screens. When an anomaly occurs on the screen, a 10-second time window lets the user activate process-and-hold, allowing review. Also, if the user sets up a ScopeMeter for triggering on glitches or intermittent anomalies, the instrument operates in a babysit mode and captures 100 such events.

Where It's Heading: Is the time close at hand when we will run all of our T&M instruments from a mouse? That's not likely. While more and more test and measurement instruments are choosing the PC as the host environment, instrument designers know that customers are quite settled in their tried and proven ways. There would be plenty of customer resistance against abandoning user paradigms, such as knobs, that have become so comfortable.

But DSO development, with ever-more-sophisticated onboard computer capabilities, won't stop. Already, instrument users are initiating closed-loop testing. It will soon be common for designers to use these modern, powerful oscilloscopes for programming tests as well as for monitoring results.

For almost half a decade, design engineers have been looking for scopes that eliminate barriers between themselves and the design under test, freeing them to tackle engineering challenges without interference. That day is at hand.