An Gigasampleper Second Modular Digitizing Oscilloscope System

For the first time, a digitizing sampling oscilloscope achieves single-shot bandwidths exceeding even the fastest laboratory analog oscilloscopes. The HP 54720/10 oscilloscope combines a 2-GSa/s digitizer, plug-in modularity, and software flexibility to provide the application-specific and general-purpose capabilities needed by designers of high-speed digital devices and systems.

by John A. Scharrer

The need to observe low-duty-cycle or single-shot electrical waveforms has been with us for a long time. This need has been greatly intensified with the advent of high-speed digital computer and digital communication circuits and systems. These high-speed systems are prone to glitches, ground bounce, and timing problems. These problems are usually [be result of complex algorithmic processes which by their very nature result in rarely occurring problems that are hard to find and observe, but critical.

Schemes to observe fast single-shol signals have been analog in nature and generally involve storing electron beam traces on a phosphor target in a conventional cathode ray tube. If the phosphor has a fast writing rate and long enough persistence, the image can be photographed with high-speed film. Storage-tube CRT technology allows direct visual observation of the waveforms without a camera. There have been many variations on this theme, but improvements in performance have been very limited in recent years and significant disadvantages to this approach are difficult if not impossible to overcome. Among these are trace blooming, dim traces. CRT wearout, displays burned permanently into the phosphor, and small displays.

Until recently this approach was the only hope of achieving high-bandwidth single-shot capability. Now, with the introduction of the HP 54720 and 54711) digitizing oscilloscope mainframes, performance exceeding that of analog storage oscilloscopes is available. The HP 54720 (Fig, 1) and 54710 use a new high-speed digitizer methodology coupled with major improvements in computing, display technology, and product design to achieve this performance.

Hie HP 54720/10 system is modular. The HP 54720 mainframe provides four digitizing input slots, which accept piug-ins that offer bandwidths from 500 MHz to 2 GHz, sensitivities

Fig. 1. The HP 54720A is a four-channel, modular digitizing oscilloscope capable of sample rates up to 8 GSa/.s. It is shown here with the HP 5471J1A active probe

from 2 itiV/div, and input impedances of 50 ohms arid i megohm. depending on the plug-ins selected. Each plug-in slot is matched with a 2-GSa-s (gigasample per second) analog-to-di git ill convener system, and by choosing a 2-slot-wide or a 4-slot-wide p)ug-iit. sample rates of 4 GSa/s and 8 GSa/s are achieved. The HP ">4710 mainframe provides 1 wo input slots. The HP 54720D and 54710D mainframes have memory deptiis of 64K samples per slot, and with 2-slot-wide or 4-slot-w ide plug-ins. memory depth extends to 128K and 256K samples, respectively. The HP 54720A and 54710A versions of the mainframes have one quarter of the mainframe memory of the D versions.

Consistent with tliis performance, an active probe, the I IP 54701A. provides 0.5-pF input capacitance and 100-kiIolim input impedance while maintaining the bandwidth of the entire system including the probe as high as 1.3 GHz depending on the plug-in selected. The probe itself has a bandwidth of greater than 2,5 GHz and can be powered cither from a plug-in or from an external supply. In the past users have been reluctant to use active probes because of their mechanical fragility and the special precautions required to avoid overvoltage at the input The HP 54701A does not require such precautions because it is protected from damage resulting from static discharge and overvoltage. In addition, it is mechanically rugged, has replaceable tips, and js highly resistant to physical damage.

Important as it is that digit al storage oscilloscopes overcome the problems of analog storage, there are far more compelling reasons to move to digital storage technology. Among these are:

The ability to store antl retrieve waveforms i'or further analysis or for visual observation either in die oscilloscope or other environments such as a workstation The ability to observe pre trigger events Ka.se of use.

In addition, the IIP 54720/10 project focused on providing accuracy and precision in high-band width time-domain measurements and the flexibility lo configure the product in software and hardware so that specific customer application needs can he supported.

Storage and Pretrigger

The IIP 54720/10 acquires waveform data in digital form and stores it in memory. The waveform can be observed, scrolled, and zoomed. Cursors can be used for automatic readout and automatic measurements can he performed on the stored data. Waveform data can be routed to internal waveform memory, to an internal flexible disk drive, or to an external computer or peripheral. Digital bus interfaces provided for this purpose include the HP-IB ( IEEE 488, IPX 625), Centronix, and a parallel expansion port for very1 high-speed data transfer.

The HP-IB port achieves data transfer rates greater than 500 kilobytes per second. Waveforms can also be transferred by flexible disk and are formatted for use by other widely used programs including spreadsheets and graphics programs.

The analog-to-digital and memory systems run continuously. When a trigger occurs, all data in memory is stored. Therefore, data occurring before ihe trigger (negative time) is

Fig. 2. "Hie HP 54720/10 faitlifully reproduces a SOO^Wide smfilr-siiot event.

Fig. 2. "Hie HP 54720/10 faitlifully reproduces a SOO^Wide smfilr-siiot event.

captured. This greatly facilitates troubleshooting and characterizing a system because trigger points before the point of interest are not necessary.

Of greater significance is the single-shot capability, which allows prefault observation of waveform data even if the event occurs only once. Ln the single-shot mode (also called the real-time mode), resolution and faithful waveform representation are determined by Ihe analog-to-digital sample rate.

According to the Nyquist criterion, if the sample rate is I wice the higliest-frequency component of the signal sampled, then the signal can be faithfully reproduced. In reality, the bandwidth of an oscilloscope is down only 3 dB at t he specified bandwidth and frequency components beyond the bandwidth will be sampled, causing aliasing and incorrect waveform display. To avoid this situation, Hewlett-Packard uses 8 rule of 4 times the specified bandwidth for the sample rate required. The HP 54720/10 achieves sample rates of 2, 4. and 8 GSa/S depending on Ihe plug-ins selected, allowing faithful waveform reproduction at 500 MHz, 1 GHz, and 2 GHz band widths, respectively (Fig. 2). The maximum available sample rate is traded off against the number of channels available: 2 GSa/s allows 4 channels. 4 GSa/s allows 2 channels, and 8 GSa/s allows 1 channel.

If tlie signal viewed is repetitive, the equivalent time mode allows the effective sample period lo be as small as one picosecond. This is achieved by accurately relating the time from Ibe trigger event to the samples in the capture memory. Artei each trigger and acquisition, samples are positioned in the waveform record (and onscreen) to build up a highresolution picture of the waveform. For repetitive waveforms, equivalent time sampling can be used and the Nyquist sample rate is not an issue, but the digitizer sample rate does directly effect the throughput, to the display. I'sirtg this technique for repetitive signals with the appropriate plug-ins, four channels can be observed at 2-GHz bandwidth. I 'sing the I IP 54714A dual-channel plug-in, eight channels can be viewed al 400-MHz bandwidth for repetitive signals.

Ease of Use and Flexibility

An oscilloscope with the advanced measurement capability of the HP -"4720/1 ri is often referred to as a laboratory oscilloscope or a high-performance oscilloscope. The use model is focused on troubleshooting and characterizing high-speed digital systems generally in the laboratory or the pre production phase of product development In production this class of instrument is usually used within a system in an automated way. Focus groups of users falling into these classes were used early in the project and results of this exercise indicated the need for familiar controls, high system and display throughput, and application flexibility. These requirements dictated t he system architecture and user interlace.

The user interface is leveraged from the HP §4lxx family of high-performance oscilloscopes with significant improvements derived from user interface studies conducted on the HP .541 xx family, the general-purpose HP 545xx family, and competitive products. The resulting interface uses a pop-up menu scheme with very limited depth of menus, an intuitive graphical user interface, and extensive color. Access to often-used measurements is from the front panel instead of being buried in menus or soft key levels. The setup keys such as time base, trigger, and vertical channels are fixed and always available, rather than using softkeys and hidden levels of additional keys.

Historically, ease of use in digitizing oscilloscopes has been enhanced through the use of extensive automatic pulse parameter measurements and functions such as rise time, delay, and pulse width, to name a few. The list is ever growing and the ability to add features is very powerful in extending the value of the initial investment in the product The fast Fourier transform ('FTT), mask testing, histograms, and applications such as communications and computer design were all added after the IIP 54720/10 was introduced. To accommodate this flexibility the software in the HI* 54720/10 is stored in flash EPIiOM and SRAM and can he loaded from a flexible disk. Add-on labels and shift keys allow upgrades to be marie easily, yet the added features have I he same direct access as existing features. The mechanical modularity provided by plug-ins allows flexibility in configuring the inputs to the digitizers and makes it easy to customize systems for specific applications.

Ease-of-use studies indicate that the responsiveness of the oscilloscope to controls and signal changes is crucial to interpreting data, adjust ing the device tinder test, and promoting confidence in the representation of "viewed signals. The HP 54720/10's high waveform display rate helps avoid visual aliasing and misinterpretation of the waveforms. To achieve a high display rate, a three-processor architecture was chosen for the IIP 54720/10, A CPU controls acquisition hardware and communications, performs automatic measurements, and manages waveform data, while a graphics processor and a cust om display processor present the waveform data.

The ability not only to see changes as they occur hut also to observe the relative frequency and "freshness" of data was a contribution of the HP variable-persistence analog storage oscilloscopes. With this feature, old data gradually fades away while new, brighter data is written onscreen. Until the IIP 54720/10 this feature eluded digitizing oscilloscopes, but

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fig. 3. Variable persistence provides u dynamic picture of waveform changes, fig. 3. Variable persistence provides u dynamic picture of waveform changes, the custom display processor in the IIP 54720/10 accommodates this feature While maintaining a high display throughput (Fig. 3).

Accuracy

Although iis very high digitizing sample rates andbandwidths open the door to high precision and accuracy in voltage and time measurements over a broad band of frequencies, considerable care had to be exercised in the HP 54720/10 system design lo ensure that these benefits were realized. All adjustments for gain, offset, timing, and frequency response are computer-controlled (no manual adjustments) and the necessary calibration routines are automated and use calibration resources resident in the mainframe and plug-ins. Therefore the user has a self-contained accuracy calibration system. If plug-ins and mainframes are intermixed after each has been calibrated, 3% vertical gain accuracy is achieved. If a system best-accuracy calibration is performed, 1% vertical gain accuracy is achieved.

The HP 54720/10 exhibits very low jitter on repetitive singleshot or equivalent time waveform displays because of a new trigger interpolator system. The resolution of the interpolator is 1 ps and jitter on repetitive waveforms is less than <i ps rms. Again, the internal calibration capability ensures ±-30-picosecoud time interval accuracy in equivalent rime mode and ±50-picoseeond accuracy in single-shot mode at a sample rate of 4 GSa/s.

System Design

The HP 54720/10 is a flexible system whose characteristics are defined by the plug-in modules and software installed and the software features selected, A block diagram is shown in Fig, 4.

I'he boards in the main card cage arc interconnected by the system interface bus, which carries address, data, control, and power. The plug-in modules are connected to the syslem via the module interlace bus, which carries address, data, control, interrupts, and power.

Plug-in 1

Plug-in 1

Vertical Acquisition Board

ÏMS34QTI) Display Processor

Cemtonics Port

Display Board

Vertical Acquisition Board

2-GSa/i

Fast

ADC

Memory

Vertical Acquisition Board

2-GSais

Fast

ADC

Memory

2-GSa/i

Fasl

ADC

Time Base Board

Run Conlrul

68020

68882

SIB

CPU

Coprocessor

Buffer

Cemtonics Port

Display Board

ÏMS34QTI) Display Processor

Display Accelerator

Display Accelerator

Tig. -t. Simplified block diagram of the HP 54720/10 modular oscillüscijpe mainframe.

Plug-ins. The plug-ins determine gain, bandwidth, maximum sample ral e, and input characteristics. Their general design includes an input attenuator and overvoltage protection followed by a preamplifief which also supplies a signal split for internal trigger pickoff. In the 2-slot-wide plug-in the signal is split to drive two analog-to-digital converter inputs. This splil allows interleaving two analog-to-digital converters to achieve a -1 GSa/s sample rat e on Iwo channels. A passive four-way splitter in ihe IIP 54722A -1-slot-wide plug-in achieves 8 GSa/s on one channel. The plug-ins also provide probe power and offset voltage for the HP 54701A active probe. The plug-ins are calibrated using the mainframe calibration resources and software, and the calibration factors are stored in the plug-in.

Acquisition System. The main and trigger signals are coupled to the mainframe through very high-bandwidth connectors and are subsequently routed to the analog-to-digital hybrid circuit and the trigger system via semirigid coaxial cable. The analog-to-digital converter hybrid uses a new technique called sample-and-filter, as opposed to sample-and-hold. The sample-and-filler technique is described in the article on page 11.

There are five monolithic integrated circuit chips on the analog-to-digital converter hybrid: a sampler and two analog-to-digital converters are custom HP bipolar chips, and two fasl in, slow Out (F1SO) memories are custom 1 It' CMOS. The analog-to-digital subsystem including very fine-line-geometry filters are constructed on a custom Hewlett-Packard I hick-film multichip module. There are two analog-to-digital hybrids on each of the venieal acquisition boards.

The i rigger signals from all four slots are input to a custom logic trigger chipset, which provides numerous combinational and sequential logic trigger capabilities. The resultant ixiggcr is applied to an interpolator counter system, which determines the exact local ion of ihe input signal in relation to the trigger event. The waveform captured can consist of one full acquisition memory before the t rigger or can lie delayed up lo one second after the trigger. A dual-ramp interpolator system resolves trigger location within 1 ps. The interpolation takes only six microseconds.

A clock distribution board generates a 2-GIIz clock and a phase-related 100-MHz dock signal for each acquisition hybrid. The phase-related 100-.VI Hz clock is also distributed to the time base system.

Computer and Display. The main system CPU is a 68020 with a (>8882 floating-point coprocessor, but with an unusual feature. A state machine controls data Cow on I he CPI' btis such t hat a complete 16-bit data word representing a captured sample can he moved from t he FISO memory t o CPU memory on each bus cycle by direct memory access (DMA). Similarly, data can be moved by DMA to the display processor and external ports. Coupled with the high-speed graphics subsystem this gives the HP 54720/10 a high-throughput, highly interactive display.

The display processor is a TMS34010 coupled to a custom IIP display accelerator. The accelerator does the work of decrementing pixel brightness in variable-persistence mode, erasing the screen, and ch awing lutes.

The CPU RAM is nonvolatile battery-backed SRAM and t he operating code is stored in flash EPROM. This allows completely changing or adding to die operating software usmg die flexible disk. The state of the entire oscilloscope, including all data, is preserved when the instrument is turned off.

Digital interfaces to the HP 54720/10 include the HP-IB, a Centronix printer port, and a high-speed parallel poit which provides direct access to the CPU bus. The two other internal buses—the module interface bus and the system interface bus—interface with the plug-in modules and the internal system boards, respectively.

Software

The software system was a start-from-scratch design, and the size of the task led the design team to consider and use structured design techniques. Indeed, the first half of the software design portion of the project, was design, not coiling. The resulting design has been very low in defects, and adding recent features such as histograms, FFT capability, and application-Specific programs went smoothly and took significantly less time than in less structured designs.

Product Design

The constraints of plug-in flexibility and high-bandwidth performance led to a new modular mechanical design that makes it possible to plug high-bandwidth amplifiers into a mating mainframe with little or no signal degradation. Plug-in modularity from the front is complemented by card modularity in the rear of the mainframe. The HP 54720/10 package is the same physical size as the previous IIP 541xx high-performance digitizing oscilloscopes.

Manufacturing

Formal concurrent engineering may require elaborate discipline and tools. The simpler concept of developing manufacturing test tools concurrently with the system and circuit design not only sufficed for this project but was indispensable in achieving the project goals. There tire four analog-to-digital mtdtichip modules per HP 54720/10 acquisition system and therefore a high loaded-hybrid yield is imperative. The hybrid test system was developed in parallel with the hybrid and ensures close t.o 100% loaded hybrid yield. The test system was ready in time to help develop and evaluate early prototypes. Similarly, plug-in and board test tools were defined at project inception and developed concurrently with the HP 54720/10 circuits.

Acknowledgments

A project of this size had a large number of contributors and they are acknowledged in the accompanying articles. In addition, recognition must go to the original concept definition team and management st;dT consisting of Lyiuie Camp, Bill Escovitz, Mike Karin, Dave Long, Fred Rampey, Ken Rush, and John Wilson. The manufacturing engineering and process team consisted of Mike Manley, Mike Van Grouw, Van Martin, Angus Foster, Jerry Townsend, Mike Kinney. Pat Ciuba, and Don Hanlon.

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