Today’s high-end test equipment is reaching an astonishing level of capability. Consider the latest addition to LeCroy’s LabMaster line, the 10Zi, which sports a real-time bandwidth of 60 GHz, and scopes like Agilent’s Infiniium 90000X-Series, with bandwidths of up to 32 GHz. These instruments bring a wealth of measurement capabilities to the testbench. They can reveal minute details of signals with lightning-quick rise and fall times.
But these scopes wouldn’t be as capable as they are without equally capable probing systems. The last thing test engineers want or need is a probe that’s going to influence their measurements or fail to deliver the full bandwidth that’s available to them on the scope.
Fortunately, today’s high-end probes are constructed to sidestep these issues. And it’s not only probes that make these instruments tick, but also software that adds functionality and other accessories.
Probing Bandwidths On The Rise
In the case of Agilent’s Infiniium 90000X-Series scopes, the InfiniiMax III (N2800A-N2803A) is the company’s third-generation InfiniiMax high-performance probing system (Fig. 1). Agilent offers a choice of four probe-amplifier models with bandwidths from 16 to 30 GHz. A wide range of probe heads allows connection using a browser, zero-insertion-force (ZIF) tip, 2.92-mm or 3.5-mm surface-mount assembly (SMA) cable, or solder-in tips.
What is driving the rise in probe-amplifier bandwidths? Engineers working with emerging wireline communication standards and high-speed serial data links such as USB, SAS, or PCI Express use scopes such as the Infiniium 90000X-Series to capture fast, single-shot events and to make critical measurements like jitter, while ensuring compliance to industry standards for interoperability. With data rates in the next few years extending beyond 10 Gbits/s, probe-head bandwidths are a critical element of making these higher-bandwidth measurements.
Agilent used a proprietary indium-phosphide (InP) process and state-of-the-art thick-film packaging technology to achieve these probes’ performance within tight geometric constraints. The InfiniiMax III browser uses a “crisscross” blade grounding system for lower inductance grounding, a poly-iron wrap of coax tips to reduce standing waves, and very low parasitic replaceable resistor tips to achieve 30-GHz performance.
Power measurement requirements are becoming increasingly challenging in a broad range of applications such as electric and hybrid automobiles, industrial applications, lighting, solar energy, and computer servers. These new power technologies require probes that address measurement of both high and low voltages. Often, the probes supplied with a given scope are not suited for the high-voltage part of the equation.
To that end, Tektronix recently rolled out four high-voltage probes and significant upgrades to three existing probe offerings to match market requirements in terms of bandwidth, dynamic range, and input impedance characteristics. Tek’s THDP, TMDP, and P52XXA series high-voltage probes meet these requirements across three main dimensions (Fig. 2).
For one thing, today’s power-supply designs are using faster switching devices such as insulated gate bipolar transistors (IGBTs). Higher-bandwidth probes, such as the THDP0200 and TMDP0200, can capture signals with fast slew rates, making it easier for designers to accurately measure these newer designs.
For another, these probes address the dynamic range deficiencies of older probes, allowing designers to measure high-voltage signals and the associated noise and ripple components with the same probe. The THDP0100 has the largest dynamic range in the family with ±6000-V differential and common-mode specifications.
Finally, when probes are connected to circuits, they potentially may alter the circuit’s behavior and mask problems. Tek’s high-voltage probes minimize this problem by providing the highest resistive and lowest capacitive loading available on a high-voltage probe. For instance, the upgraded P5210A and the new THDP0100 both offer 40 M? of resistance and less than 2.5-pF capacitive loading.
The TMDP and THDP connect to Tektronix oscilloscopes through the Tektronix Versatile Probe Interface (VPI) architecture used on most Tektronix mid-range oscilloscopes. The VPI architecture enables intelligent, bi-directional communication between the scope and probes. To maximize user safety in high-voltage applications, they all comply with EN61010-031 requirements.
Activity In Active Probes
Engineers should avail themselves of a wide range of probes, both passive and active, to make the most of their oscilloscopes. Making decisions about which probe is best for a given application isn’t necessarily easy (see “Choose The Best Passive And Active Oscilloscope Probes For Your Task” at electronicdesign.com). But Agilent offers a new generation of 1- to 2-GHz single-ended active probes in its N2795A/N2796A series (Fig. 3).
The N2795A/96A are supplied with the AutoProbe interface (compatible with Agilent’s InfiniVision and Infiniium scopes). These probes integrate many of the characteristics needed for general-purpose, high-speed probing, especially in digital system design, component design/characterization, and educational research applications.
An important characteristic of these probes is their 1-M? input resistance and extremely low input capacitance of 1 pF, which makes for ultra-low loading of the device under test (DUT). Combined with very high signal fidelity, this makes the probes suited for most of today’s digital logic voltages. They also deliver wide dynamic range (±8 V) and offset range (±12 V for N2796A, ±8 V for N2795A).
Adapting To Circumstances
Scope manufacturers would generally prefer that you pair their own probes with their scopes, but this isn’t always possible or even desirable. To that end, the test-equipment vendors will generally provide adapters to enable you to use one’s probes with another’s scope.
An example is Agilent’s N2744A T2A Tektronix-to-Agilent probe adapter (Fig. 4). With this device, engineers may connect Tektronix TekProbe-BNC Level 2 probes to Agilent’s Infiniium and InfiniiVision oscilloscopes. It provides the necessary probe power, calibration, and offset control to the Tektronix probes. Engineers who already own Tektronix active probes can use Agilent oscilloscopes without having to purchase new probes.
Often, circumstances will dictate that measurements be taken in locations that are difficult to reach. For such occasions, there’s Agilent’s N2887A 36-channel Pro and N2888A 18-channel half-channel InfiniiMax soft touch probe heads (Fig. 5). These probe heads connect Agilent soft-touch connectorless probes to the input connectors of the Agilent InfiniiMax I and II Series probe amplifiers.
The probe heads enable engineers to probe high-density signals with up to 4 GHz of bandwidth. The probes also allow engineers to make the multichannel measurements commonly required in double-data rate (DDR) memory testing and other high-speed applications where space is tight.
Designers also should have a current probe in their toolbox. The Agilent N2893A is a 100-MHz, 15-A ac-dc current probe with an AutoProbe interface for the InfiniiVision and Infiniium oscilloscopes. It features auto degauss (demagnetization) and auto calibration to remove residual magnetism and unwanted dc offset in the probe so engineers can make more accurate low-level dc current measurements. The N2893A current probe also excels in accurately capturing transient or steady-state currents, a must for testing and debugging power electronic devices.
Sense The Power
Continuing on the topic of power measurements, Anritsu’s MA24105A standalone, compact, and highly accurate in-line peak power sensor provides a wide range of power measurement capability from 350 MHz to 4 GHz (Fig. 6). Featuring a dynamic range of 2 mW to 150 W and a combination of forward and reverse measurement functionality, the MA24105A can be used by both manufacturing and field engineers in a variety of commercial cellular, land mobile radio, and general-purpose military/defense RF applications.
The MA24105A is compatible with Anritsu’s S3xxE series Site Master cable and antenna analyzers, MS271xE and MS272xB Spectrum Master handheld spectrum analyzers, MT8212E Cell Master, MT822xB BTS Master, and MS202xA/B and MS203xA VNA Master. It also can be used with the MS271xB economy bench top spectrum analyzers.
Highly accurate average power measurements can be made with the MA24105A. The wide dynamic range eliminates the need for lower-power sensors, reducing setup and measurement times. Accuracy is ensured because the calibration data is stored directly in the sensor and all necessary corrections are made inside the microprocessor of the sensor. Also, return loss and directivity are optimized to maintain high accuracy.
A “dual path” architecture in the MA24105A enables true-RMS measurements over the entire frequency range. Coupled with the dynamic range, this enables users to measure continuous wave (CW), multi-tone, and digitally modulated signals, such as GSM/EDGE, CDMA/EV-DO, W-CDMA/HSPA+, WiMAX, and TD-SCDMA.
A forward direction path includes a 4-MHz bandwidth channel that has peak and comparator/integrator circuits that add measurement functions, such as peak envelope power, crest factor, complementary cumulative distribution function (CCDF), and burst average power. Another detection circuit on the reverse direction adds reverse power measurement capabilities, including reverse power, reflection coefficient, return-loss, and standing-wave ratio (SWR). The presence of a microcontroller, along with signal conditioning circuitry, an analog-to-digital converter (ADC), and a power supply in the sensor, make the MA24105A a complete miniature power meter.
Versatile, the MA24105A can be used in numerous applications. Its good match and low insertion loss suit the sensor well for continuous power monitoring of transmitter systems and antennas. Additionally, the 350-MHz bottom-end frequency makes the sensor perfect for testing P25 and TETRA networks.
The Software Side
Instrument vendors have long been in the business of developing application-specific software packages to hone a given instrument’s focus on a particular market segment. A current example is LeCroy’s recent release of three such packages for the automotive market.
Automobile and chip manufacturers, such as Broadcom Corp. and partners of the MOST Cooperation, have been the driving forces behind LeCroy’s two QualiPHY (QPHY) automated compliance test packages. These packages enable developers to offer lower-cost, lower-power, highly versatile, and robust systems for entertainment and infotainment applications. Further, LeCroy’s Vehicle Bus Analyzer (VBA) test solution has been expanded to operate on the WaveRunner 6 Zi, WavePro 7 Zi, and WaveMaster 8 Zi oscilloscope platforms.
The QPHY package provides an automated test script that allows quick and reliable testing of both the MOST 50 ePHY and 150 oPHY signals. MOST (Media Oriented Systems Transport) is a widely used standard for multimedia and infotainment networking in the automotive market. These standards have emerged in response to the rapidly increasing customer demand for devices connected to video displays and car infotainment systems.
This demand has grown immensely over the past five years, creating more test challenges for automotive designers to ensure that the various devices—video displays; GPS navigation; audio, DVD, CD, and satellite radio; Bluetooth connectivity; and voice microphone systems—all interact on the same network without any interruption.
Broadcom’s recent BroadR-Reach technology targets 100-Mbit/s Ethernet connectivity over unshielded single twisted-pair wiring. BroadR-Reach is optimized for multiple in-car applications and supports a variety of connectivity options for external devices. LeCroy’s physical-layer (PHY) test approach for BroadR-Reach includes QPHY-BroadR-Reach, an automated QualiPHY compliance test package, as well as the required test fixtures and cables.
In addition, the ubiquitous CAN, LIN, and FlexRay automotive system protocols have assumed key roles in every automobile in use today. LeCroy’s VBA test bundle combines CAN, LIN, and FlexRay trigger, decoding, and analysis solutions in one option, with the addition of PHY testing for FlexRay.
The VBA solution provides the unique capability to decode CAN protocol signals into Symbolic (application layer) text using the vehicle .dbc database file, allowing users to view the full range of CAN protocol stack information, and the ability to view additional in-circuit electrical signals that influence the CAN bus.
The additional capability to extract data from serial protocol message streams and graphically plot that data on the oscilloscope display makes the VBA a required option for any automotive design engineer. This full feature set is now available on the WaveRunner 6 Zi, WavePro 7 Zi, and WaveMaster 8 Zi oscilloscope platforms.
Automatic Probe Calibration
Instrument makers always go to great lengths to minimize channel losses in their probing systems, though cables and probes are inherently lossy. Depending on the configuration, the loss can be substantial. At other times it can be just enough to cause variations in measurements, making results inconsistent. Plus, frequency response and phase characteristics can vary from probe to probe, so each probe and cable must be characterized and accounted for to ensure the truest representation of the signal.
To ensure that users are getting the most out of their scopes and probes, Agilent Technologies offers PrecisionProbe software for its Infiniium 90000 X-Series and 90000A Series oscilloscopes. The software, which works with built-in hardware in the Infiniium scopes, lets engineers automatically characterize and correct the response of any path to a scope input using no external equipment.
The analysis provided by PrecisionProbe software improves measurement margins so engineers can make the most accurate measurements possible. The added margins are especially valuable in situations where the probe setups consume measurement margins without it being apparent to the user.
Agilent’s N2809A PrecisionProbe software not only quickly corrects for cable and channel insertion losses, it corrects probing issues such as phase linearity and magnitude flatness as well. The software matches frequency response and phase of every cable or probe on a circuit. It also characterizes and compensates for losses on channel paths such as switches without adding additional equipment.
According to Agilent, PrecisionProbe is the first software on a real-time oscilloscope to provide full ac calibration for probes, not just dc calibration and skew correction. Also, it does not depend on externally generated s-parameter characterization files.
These files, commonly generated from other instruments such as time-domain reflectometers or vector network analyzers, can be time-consuming to set up and require expertise to generate accurate and consistent results. Instead, PrecisionProbe uses a built-in signal source on the oscilloscope to automatically generate the files. A software setup wizard quickly moves engineers through the setup and characterizing of channel elements such as probes, cables, and switches using PrecisionProbe.