Early in the first chapter of his microwave textbook, my former undergraduate professor Dr. Peter Rizzi introduced me and my fellow classmates to some of the anomalies in this field of study by making comparisons to its low frequency and optical counterparts. Along with the electrical impact of skin depth and component size relative to wavelength, Dr. Rizzi wrote that another “unique characteristic of microwave work was in the area of measurement technique.”
“At low frequencies, it is common to measure voltage and current when studying the properties of a circuit or system. In microwave circuits, however, the voltage and current are not, as a rule, uniquely defined. Furthermore, the direct measurement of microwave voltages is usually accompanied by considerable noise fluctuations, resulting in poor sensitivity, low dynamic range and significant errors. As a result, microwave measurements usually involve the accurate determination of impedance and power rather than voltage and current.”
Well, that was the early eighties. Flash forward twenty-plus years and the advancement of RF/microwave measurement techniques and supporting semiconductor and software technology has progressed to the point where direct, time-domain measurements of voltages and currents operating in the microwave range is not only reality, it is becoming critical to design and debugging. And so the ultimate “time machine” for measuring IV waveforms—the oscilloscope—has renewed interest among RF engineers. In this special report, MWJ editors spoke with technologists from Agilent, Le-Croy, Rohde & Schwarz (R&S) and Tektronix about the state of multi-GHz oscilloscopes, their enabling technologies, performance, features, applications and what to look for from the perspective of a user. This special report details our findings.
Digital oscilloscopes can be divided into real-time including digital storage oscilloscopes (DSO), digital phosphor oscilloscopes (DPO) and mixed signal oscilloscopes (MSO) based on the over-sampling methodology and (equivalent time) sampling scopes based on the sequential-sampling methodology. Equivalent time sampling scopes take advantage of the fact that many natural and man-made events are repetitive in nature and therefore samples may be acquired over many repetitions of the signal. These types of scopes typically target the very high-end bandwidth (currently >100 GHz), traditionally with lower sampling rate Analog to Digital Converters (ADC). As a result, only repetitive signals can be captured by means of under-sampling. Example applications include characterization of the signal integrity of high-speed communication signals.
Guido Schulze, Product Manager of Oscilloscopes at Rohde & Schwarz, contends that the bulk of digital oscilloscopes are real-time oscilloscopes. While sampling oscilloscopes offer high bandwidth, the tradeoff compared to DSOs is limited dynamic range and the inability to capture intermittent events.
Figure 1 Improved resolution shown of a fast rising edge on a Tektronix MS)72004C real time oscilloscope at 100 GS/s (white trace) vs. 50 GS/s sample rate (yellow trace) using linear interpolation.
One of the most important elements of a real-time oscilloscope is sample rate, which refers to how frequently it takes a snapshot or samples a signal. The faster an oscilloscope samples, the greater the resolution and detail of the displayed waveform and the less likely it is that critical information or events will be lost, as shown in Figure 1. The sample rate/bandwidth ratio has to fulfill the Nyquist criteria of equal to or greater than 2. In practice, a factor above 2.5 to 5 is required; for example, 2 GHz bandwidth requires a 5 Gsample/s maximum sampling rate.
With over-sampling applied, such as the 100G S/s capability found in the Tektronix 70000C Series Oscilloscopes, engineers get more data points to accurately understand rise time performance on signals with ~20 picosecond rise time. “With the 5× oversampling on our latest introduction, Tektronix is making sure that leading-edge signal fidelity designers have what they need to characterize very fast design speeds,” says Chris Loberg, Senior Technical Marketing Manager, Tektronix.
An interesting trend of the real-time oscilloscopes is that more and more functionality is being added to the original time domain analysis. Examples are additional digital channels (1 bit comparator instead of 8 bit ADC) for logical analysis, protocol-based trigger and decoding for debugging of protocol-based interfaces, or advanced FFT capabilities for a spectrum display. All these new capabilities support faster debugging of highly integrated designs (embedded designs).
David Graef, Chief Technical Officer at LeCroy, agrees that many signals nowadays are not and cannot be made to be repetitive. For this reason, a broad band (DC to multi-GHz) digitizing oscilloscope is the most generally useful. However, with the very broad band signal path comes noise. Many signals can be characterized using a digitizing oscilloscope with 35 to 40 dB SNR and less than 1 percent THD. However, there are classes of signals that require better than that. Signal analyzers generally employ a down-converter in front of a higher resolution ADC. The total bandwidth covered can be quite wide. The instantaneous bandwidth (bandwidth that is actually digitized for analysis) is quite narrow, generally in the range of a couple of hundred MHz. The SNR and THD in that narrow band can be quite good so signals with very wide dynamic ranges can be analyzed. This is very useful for band-limited signals.
High-bandwidth Oscilloscopes and their Enabling Technologies
One of the biggest changes to oscilloscope technology recently has been the shift to higher frequencies, allowing them to operate in the > 20 GHz range. What key enabling technologies have allowed oscilloscopes to operate in the multi-GHz range? This capability has been the result of many engineering innovations and investment in faster chip processes by leading scope vendors. According to Brig Asay of Agilent Technologies, “The cutting edge of process speeds is necessary in order to keep up with the bandwidth demands of engineers. A side benefit of these new processes has been better noise and jitter performance.”
Figure 2 Interleaving in the time domain involves upsampling the acquired waveforms by inserting intermediate zeros and then adding them together with a half sample period delay for digitizer B's samples.
LeCroy’s Graef credits a few key technologies with enabling oscilloscopes to operate in the multi-GHz range. “There are a number of facets to this... the signal path (front-end amplifier—ADC—memory), stable sample clock generation and triggering. The availability of very high speed processes—SiGe and InP—have allowed much faster front-end amplifiers and ADCs. The ability to achieve the required sample rate using massively interleaved CMOS ADCs as an alternative to faster SiGe ADCs has also allowed high frequencies to be reached. LeCroy has an additional technology—Digital Bandwidth Interleave (DBI)—that allows them to go beyond the limits of raw silicon performance and multiply that by a factor of two or three thereby achieving even higher bandwidths” (see Figure 2).
Both Tektronix and LeCroy currently utilize IBM’s 8HP process, which offers a significant speed improvement over the previous 7HP process. Using its frequency interleaving technique, LeCroy has been able to push the operating range of its scopes to greater than 20 GHz.
According to Graef, there have been many advances in frequency generation technologies that allow sample clock generation circuits to be built with extremely low jitter. This is required for the oscilloscope acquisition system (ADC) to have sufficiently good performance at very high input frequencies. The availability of low noise, high quality amplifiers—many designed on a SiGe process—and frequency generation techniques (fractional-N synthesizers) along with high quality oscillators at reasonable costs has enabled the oscilloscope time base to be built. Loberg points out that low noise performance becomes even more critical as oscilloscope bandwidths rise. “With the timing performance of a rising edge being characterized in today’s labs with jitter tolerances in the femtosecond range, the slightest amount of instrument-based noise from the amplifiers or other components in the oscilloscope can have a dramatic effect on signal integrity. Tektronix advises its customers to consider bandwidth needs alongside the ability to minimize noise in the instrument to achieve accurate measurement data that reflects the device’s performance alone.”
Figure 3 Agilent's DSO-X front-end module with preamp based on the HB2B InP HBT process also contains the trigger and sampler ICs.
The same processes that enable the signal path also enable the triggering. SiGe and InP circuits can be designed with sufficient bandwidth to allow triggering the oscilloscope up to bandwidths approaching the bandwidth of the scope. In contrast to the 8HP process favored by LeCroy and Tektronix, Agilent has developed a proprietary Indium Phosphide process with a transistor cut-off frequency of greater than 200 GHz (see Figure 3). The use of InP allows Agilent to achieve its performance without the use of interleaving, a technique that uses down-conversion to be able to achieve the bandwidth. According to Agilent, there are some tradeoffs with using this technique, which include higher noise density and mixing errors, but it is a technology that allows for faster bandwidth.
The Mid-bandwidth Oscilloscope Market
Oscilloscope models ranging from 350 MHz to 2 GHz are the single most popular segment for oscilloscopes and are used to develop and verify products in a broad range of industries and applications. Typically priced under $20,000, mid-range oscilloscopes are becoming increasingly capable. For instance, the new MSO/DPO5000 Series from Tektronix incorporates many of the most popular features from the company’s leading performance oscilloscopes at a much lower price. Features include a >250,000 wfm/s maximum waveform capture rate, fast segmented memory acquisition and long record length to capture long time periods at high resolution, extensive built-in tools for in-depth analysis of complex designs, and a Windows-based user interface.
According to Guido Schulze from Rohde & Schwarz, his company’s recently introduced RTO digital oscilloscope family uses standard components in building scopes dedicated to the mid-bandwidth oscilloscope market. Schulze attributes the measurement performance of the product family to the in-house designed front-end components. The R&S RTO front-end is based on a low noise amplifier stage and a single-core 10 GHz ADC. Beyond about 350 to 500 MHz, digital oscilloscopes are possible only through customized amplifiers, ADCs and ASICs.
The front-end amplifier used in the R&S instrument provides low noise characteristics and high sensitivity, supporting a vertical range down to 1 mV/div without bandwidth reduction. The respective ASICs are realized in SiGe BiCMOS process and for bandwidth reasons are mounted on a ceramic board. The 10 GHz ADC is built up in a true monolithic single-core design. This approach avoids distortions caused by interleaving of several slower ADCs and enables a high measurement accuracy of > 7 effective number of bits (ENOB).
The processing ASIC is the heart of the R&S RTO oscilloscope. It includes several innovations concerning fast acquisition and analysis and the triggering concept. A main challenge for using a “digital oscilloscope” as a debug tool is the fast processing of the high amount of sample data (e.g. a 10 GHz 8 bit ADC outputs 80 Gsample/s). An acquisition cycle consists of a typically short acquisition time (e.g. 1000 samples at 10 Gsample/s result in 100 ns acquisition time) followed by a relative long processing time where the oscilloscope does not capture the test signal. Schulze (R&S) claims that typical digital oscilloscopes are in most cases more than 99.9 percent blind.
To improve on this, R&S uses a processing ASIC that includes multiple processing paths for the acquisition and post-processing steps. This enables a maximum acquisition and analysis rate of greater than one million waveforms per second. The high acquisition rate enables the fast detection of rare signal anomalies. The fast processing on the other hand allows more waveforms in the analysis and therefore provides higher statistical confidence in the results.
The second part of the processing ASIC relates to the trigger system. The R&S RTO includes a real-time capable digital trigger system (Schulze comments that the trigger jitter of analog oscilloscopes can be reduced with SW-based post-processing, which has the disadvantage of extending the blind time). The trigger system of the R&S instrument operates with the same sample data as the acquisition unit. Consistency between acquisition and trigger system results in trigger jitter below 1 ps RMS, which is similar to other mid-range oscilloscopes on the market.
Figure 4 Simplified block diagram of the Tektronix DPX-based display system.
Advanced waveform display technology in a mid-range oscilloscope has been an important part of the Tektronix DPO family of oscilloscopes for many years (see Figure 4). Available on its recently introduced MSO/DPO5000 Series, Digital Phosphor technology (DPX) utilizes a patented parallel processing for a continuous waveform to capture, display, store and analyze complex signals in real-time, using three dimensions of signal information—amplitude, time and distribution of amplitude over time—resulting in an intensity graded display.
Another advancement in mid-range oscilloscopes has been the sophistication of triggering systems. An intelligent, responsive trigger system dramatically speeds up debugging by “arming” the designer with a wider array of methods to capture a signal of interest. As an example, the Tektronix MSO/DPO5000 Series Oscilloscope comes equipped with an A/B Event trigger that can trigger on one of up to 350 different trigger conditions. Triggering could be setup to look at a digital circuit’s setup and hold violation or even look for a serial bus protocol word or addressing scheme and trigger when that “word” is found.
Asked about how the typical RF designer uses an oscilloscope to analyze circuit performance, oscilloscope manufacturers mentioned that a typical traditional RF engineer tends to think in the frequency domain more than the time domain. As Graef of LeCroy puts it, “For a die-hard, old school RF engineer a VNA and a spectrum analyzer would be more the tools of choice. However, many engineers today are working in the RF domain and are required to solve time domain problems.” Serial data standards are now extending in frequency to at least 12.5 Gb/s for common third generation standards. This is traditional RF territory for digital data. Oscilloscopes are very useful for being able to look at and make detailed measurements on jitter or many other characteristics of the signals. Modern high performance sampling or equivalent-time oscilloscopes can also use S-parameter descriptions of a transmission path to de-embed that path from the measurement. Oscilloscopes can also do demodulation of signals so the engineer can look at the information as well as the modulated carrier.
Schulze responded that many RF designers have to deal with analog and digital components in their designs. “Here the basic debug tool is again a digital oscilloscope. An oscilloscope becomes interesting for RF interfaces if the bandwidth requirements exceed the maximum bandwidth of current generations of spectrum analyzers. In such cases, however, the measurement dynamic of the oscilloscope is the limiting parameter (oscilloscopes typically have 8 bit ADCs, spectrum analyzers rather 12-14 bit ADCs).” It is worth noting, of course, that it is not the number of bits that define dynamic range; spurious-free dynamic range (SFDR) is the key figure of merit for RF designers. SFDR needs to be specified by fractional bandwidth and at the frequency of interest when using a scope, says Loberg.
A good example for when to use an oscilloscope is a cellular phone chip. It includes all kinds of RF, digital and mixed signal interfaces. An oscilloscope is useful for tests on digital interfaces, e.g. signal integrity or data content. On the RF interface, the oscilloscope, along with spectrum analyzers are used. An interesting combination of the spectrum analyzer and the scope would be the correlation of the data content, signal quality or signal errors between the RF interface and the digital interface (e.g. DigRF).
Typical applications for digital, analog and mixed signal components include standard amplitude and time measurements like voltage swing, rise time, frequency, delay, etc., advanced measurements on histograms and data eyes (e.g. jitter), and protocol decoding (e.g. low speed serial interfaces as I2C or SPI). For RF components, typical applications include spectrum display and measurements and sometimes more sophisticated measurements like demodulation, etc.
An emerging segment for more sophisticated test and debugging is time-correlated, multi-domain RF analysis using an MSO together with spectrum analysis software, suggests Tektronix’s Loberg. The integration of the logic state triggers enables time correlated acquisitions from the digital domain to the analog domain with a timing accuracy of 80 ps when using a high-performance MSO. This allows the designer to easily validate wideband RF and microwave designs and characterize wideband spectral events.
Asay of Agilent believes that wide bandwidth digital oscilloscopes can help the RF designer track down and debug issues with wide bandwidth RF and microwave transmitters when they occur. For example, Asay considered a transmitter design that may have been designed to meet a system-level metric like Error Vector Magnitude (EVM) for a wide bandwidth modulation application (e.g. SatCom) or multi-channel application (e.g. MIMO). If it is not meeting its EVM specification, it can be difficult to tell where in the transmitter chain the waveform is being most impacted (e.g. power amplifier, pre-amplifier, LO phase noise, mixer impairments, IQ gain/phase imbalance, or a combination of all of the above). A wide bandwidth digital oscilloscope with Vector Signal Analyzer (VSA) software can help the RF designer evaluate the EVM performance at different stages along the transmitter chain from analog baseband to carrier frequencies up to 32 GHz. This can help gain insight into where issues are occurring, and help quantify the impact of the different stages on the transmitter’s system-level performance.
Therefore, one key RF/microwave application is to perform measurements on wide bandwidth X-, Ku- and Ka-band radar and SatCom transmitters—directly up to 32 GHz, without the use of custom external down-converters. This is important because down-converters can introduce their own impairments into the test signal by adding LO phase noise, amplifier nonlinear distortions and filter group delay. These down-converter impairments could potentially mask the true performance of the transmitter DUT design, and make it difficult to determine how much of the measurement result is actually from the transmitter DUT, or from the external down-converter, or from a combination of both.
That said, many of these problems are the result of using improperly designed or calibrated frequency down-converters. A customizable standard down-converter such as those available with Tektronix oscilloscopes can go a long way toward expanding the role of oscilloscopes for such applications as wideband radar characterization and broadband satellite signal analysis. Advantages of down-converters include the ability to optimize down-converted signals to the optimum SFDR of an oscilloscope, the ability to increase ENOB, better time stability for microwave measurements by making measurements at down-converted frequencies, and the ability to use a less expensive scope. In the case of the latter, down-conversion of multi-GHz signals from 32 to 3 GHz using a mid-range scope is much cheaper than direct Ka-band signal analysis.
Key radar transmitter measurements include evaluating the pulse envelope characteristics (e.g. rise time, fall time, pulse width), and the pulse modulation characteristics (e.g. linear FM chirped frequency and phase). Oscilloscope features such as segmented memory acquisition can be used to evaluate time domain measurements such as rise and fall time. To gain additional insight into the transmitter’s performance, VSA software can be used to perform frequency domain measurements such as chirped frequency and phase. It can also be used to view frequency hopping characteristics with a spectrogram measurement.
To illustrate the value of today’s digital oscilloscopes for radar applications, Asay (Agilent) used the example of a 2 GHz wide LFM chirp at 10 GHz (X-band). The pulse envelope characteristics (rise time, fall time and pulse width) can be evaluated in the time domain by placing oscilloscope pre-configured measurements on the pulsed radar RF envelope. Segmented memory can also be used to further optimize the number of radar pulses captured and analyzed within the oscilloscope’s capture memory (2 GSa). Taking this example a step further, consider that the engineer would like to evaluate the spectrum and chirp modulation characteristics. The VSA software can be used on the oscilloscope to evaluate these frequency domain characteristics and perform other key RF/microwave measurements.
Loberg of Tektronix notes that traditional rise/fall time measurements of pulsed signals are very different than the traditional rise/fall time measurements in the default menu of an oscilloscope since it is not the carrier response, but the envelope response that needs to be measured. For pulse measurements, engineers can select from a full set of over 27 automated scalar and vector measurements that cover individual pulse frame measurements and multi-frame measurements. This includes trending statistics, histograms and FFT of measurement on-time.
Key measurements for SatCom transmitters include constellation measurements, EVM and spectrum measurements. Oscilloscopes with VSA software can perform these types of measurements for QAM signal formats, as well as advanced formats such as orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) signal formats. As an example, take a 1 GHz wide 16 QAM modulated waveform at X-, Ku-, or Ka-band. The VSA software can be used on the oscilloscope to demodulate the waveform up to 32 GHz (Ka-band) to measure the microwave transmitter’s EVM. Many different signal formats can be measured and demodulated, including OFDM and OFDMA commercial-off-the-shelf (COTS) formats.
For MIMO applications, evaluating the EVM and antenna crosstalk of multi-channel MIMO RF transmitters is important. Multi-channel phase-coherent oscilloscopes with VSA software can be used to evaluate the EVM of each channel, as well as the antenna crosstalk between the four channels.
An interesting application Loberg highlights concerns the design of frequency hopping radios or radars in which traditional RF analysis tools require the signal of analysis to be located in the center of the RF analysis window. To perform this, engineers are often challenged to capture the signal-of-interest when it occurs exactly in the analysis window or must use a special (non-hopping) test mode to guarantee that a signal is captured. Unfortunately, the most difficult behaviors to troubleshoot occur when hopping systems are actually hopping, behavior that may not be represented in the test mode. Discovering timing and settling issues are not fully vetted with static non-hopping test modes. An MSO together with integrated VSA software can handle these types of measurement challenges.
Loberg adds that Tektronix takes a different approach to the integration of Vector Signal Analysis software on oscilloscopes. Rather than implementing the measurement software at the Virtual Instrument Software Interface (VISA) layer, which limits the size of data transfer for modulation and FFT measurements, Tektronix implements the software directly into the acquisition engine. This makes it easier to setup RF functions like frequency span, resolution bandwidth and acquisition time as independent variables, and allows use of the entire memory of the oscilloscope for measurements. This enables designers to take narrow resolution bandwidth views of very wideband RF signals and not be limited to a fixed number of points for FFT processing.
What are some of the key traits an engineer should look for in an oscilloscope and why? We asked the major oscilloscope providers to outline the key factors and to highlight noteworthy capabilities that each company’s scopes bring to the RF test & measurement market. Speed and accuracy along with functionality, such as data processing and display technology, were among the top criteria cited.
Asay (Agilent) underscores the versatility of the oscilloscope by responding, “The answer is that it depends. This depends on what the engineer is doing with the oscilloscope. An engineer using the oscilloscope for RF measurements needs to really look at the noise, effective number of bits and the edge trigger bandwidth of the oscilloscope. All of these specifications make a big impact on applications such as radar. For instance, if you are looking at a radar application of 20 GHz, and only have 15 GHz on the edge trigger, you have the potential of missing key trigger events as scopes are forced to use software to expand the trigger.”
Noise is always a concern. The higher the noise of the oscilloscope, the more noise contribution the oscilloscope makes in a measurement. Engineers want to see what they are really measuring, not what the scope contributes. Noise is one of the easiest and most effective ways to evaluate different scopes.
Bandwidth, on the other hand, is limited to the frequency domain, not the time domain, and therefore does not describe how a particular instrument would fare at capturing spectral components of a complex signal, Loberg points out. In fact, two oscilloscopes that have the same bandwidth performance can have different rise times, amplitude and phase response. Therefore, knowing only the bandwidth of an oscilloscope will not fully describe its measurement capability or its ability to accurately capture complex signals. It is important to consider other factors such as instrument-based noise, frequency flatness and roll-off, the rise-time response, etc., to better determine the instrument’s ability to make accurate measurements.
Graef (LeCroy) believes that the signal to noise ratio is important. However, baseline noise (noise on a scope with no input signal) is easy to look at but does not tell the whole story. The signal-to-noise ratio with a signal applied is a much better indication of what the user will see in real applications. Effective number of bits is a figure of merit that includes all of the error terms in the digitization chain and that is often used for comparison. However, ENOB alone is not sufficient. The very next question after one learns that the ENOB is 6.2 bits should be, “Is that primarily noise or distortion?” The two parameters SNR and THD should tell the user a great deal about the acquisition performance.
Once the characteristics of the acquisition system are deemed to be sufficient, the next traits an engineer should look for are the capabilities to measure and debug the circuit under test. Different scope manufacturers have different approaches to how they make measurements. For example, if you ask for a rise-time measurement, LeCroy will measure every rise-time in an acquisition. “There could be thousands or even millions of them in a single acquisition. Other manufacturers measure only a few or, in some cases, only one per acquisition. LeCroy can also show a histogram very easily and even allow the user to find the outliers using the company’s WaveScan technology. This can all be done on very long memory acquisitions while keeping the responsiveness of the scope very good. Many other manufacturers bog down with long memory causing the user to lose sight of what they are trying to accomplish and end up focused on making the tool do what they want it to do. Highly responsive scopes with good tools for debugging problems can help the engineer get their job done more quickly with less frustration,” said Graef.
Schulze (R&S) commented that it is worth looking into the details of an oscilloscope’s specification before a purchase decision is made, listing the following areas to consider:
a) Front-end - Probably the most popular and the most obvious decision criteria is the maximum bandwidth that is supported by an instrument. Other important parameters include:
- Smallest input sensitivity without SW magnification or bandwidth limitations: Many applications use low voltage swings. With probes having attenuations of typically 10:1, a 100 mV signal requires 1 mV/div for full display over 10 divisions
- Intrinsic noise (RMS noise floor) of the oscilloscope at the various amplifier ranges
- Effective number of bits (ENOB): This determines the measurement dynamic
- Channel-to-channel isolation: For measurements with multiple input channels also this parameter should be evaluated
b) Acquisition - The capability to “look” at the test signal many times per second is an important performance parameter, i.e. the waveform acquisition rate matters. A comprehensive triggering system with accurate trigger (low jitter) capability is also an important aspect of any oscilloscope. Another important question concerning the data acquisition is the maximum available memory. On certain instruments one channel can use the memory of the other channel if they are switched off (sometimes only one neighboring channel, sometimes all channels).
Combined with longer memory is the ability to find what has been captured in memory. Without good signal processing search tools like the Wave Inspector featured on many of the Tektronix mid-range oscilloscopes, it may not be helpful to capture and store important events if they cannot be found quickly and easily. With Wave Inspector, engineers can search, mark and navigate long record lengths to find all occurrences of your event.
Another consideration on acquisition is probing, says Loberg. A well-matched probing system that provides the ability to accurately transport the signal from the device under test to the front-end of the oscilloscope is as important as the front-end of the instrument itself.
c) Processing - Certainly there are different processing functions like simple and advanced math functions (e.g. add or sub versus FFT), analysis functions (e.g. mask test, histogram) and measurement functions (e.g. cursor, automated measurements). Besides the question whether an individual function is available or not, again the acquisition and analyzing rate is a crucial differentiator. Schulze (R&S) believes a histogram test that can evaluate 1,000,000 waveforms per second is much more powerful than a histogram test at 100 waveforms per second (TTM, TTY, etc.).
d) Usability - The operating concept can make a big difference—a big screen is not everything. As everybody has his own preferences a user should always try to get access to a demo unit to try an instrument upfront. Size and weight also should be considered, carrying around 10 kg versus 20 kg really matters.
As mentioned above, the signal acquisition path needs to have the level of performance required for the application or the scope will not be useful. LeCroy’s scopes have excellent signal fidelity that allows the signals to be faithfully acquired. Beyond that, LeCroy’s speed of processing and the depth of the “toolbox” are two of the primary advantages in its scopes. According to Graef, the company’s patented X-Stream technology can process waveforms up to 100× faster than competitive instruments.
This capability keeps the scope lively and interactive even with long memories and many complex measurements turned on. Features in LeCroy scopes allow the user to identify the source of a problem—WaveScan, TriggerScan and Histicons are capabilities that allow the user to identify and characterize intermittent or rarely occurring problems. LeCroy’s ability to decode many different standards—PCIe, SAS, SATA, USB—to name a few, allows the engineer to look at the physical layer characteristics while looking at the protocol layer messages being sent. Then when an error occurs the error message can be correlated with what is happening at the physical layer. With the ProtoSync capability engineers can get a full protocol layer view and “cross probe” to the physical layer signal that resulted in that error. This is extremely useful for bringing up new designs where it is not clear what is working and what is not. In addition, LeCroy scopes have the ability to allow users to insert their own processing algorithms into the processing stream of the scope. Programs can be written in many different languages including MatLab or C++ and added to the scope’s already deep toolbox.
Asay considers accuracy as a leading advantage for the Agilent oscilloscopes. The 90000 X-Series (which was introduced in 2010) has the industry’s lowest noise floor along with the highest pre-amplifier bandwidth. Because of the InP-based chips, the scope is able to achieve high bandwidths without design trade-offs, resulting in a highly accurate test instrument. In addition to lower noise, higher effective bits and lower jitter, the 90000 X-Series also has higher probe bandwidth and higher edge trigger bandwidth, which makes it a great instrument for RF measurements.
Loberg emphasizes the importance of sampling rates and signal integrity as well as accurate capture or triggering capability. The company’s DPO/DSA/MSO70000 Series claims the most accurate real-time performance of any oscilloscope on the market and features the industry’s highest sampling rate performance—100 GS/s—for low noise and high margin visibility. For applications such as wideband radar system verification, the Tektronix oscilloscopes feature a high stability time-base that maintains the same modulation characteristics from pulse to pulse.
A key differentiator for Tektronix is the availability of digital phosphor displays that Tektronix invented. The DPO architecture dedicates unique ASIC hardware to acquire waveform images, delivering high waveform capture rates that result in a higher level of signal visualization. This performance increases the probability of witnessing transient events, where DPOs are suitable for viewing high and low frequencies, repetitive waveforms, transients, and signal variations in real time.
Figure 5 R&S rapid acquisition capability provides a "persistence" view of rare signal behaviors in this example of a random runt pulse.
On a very high level, Schulze characterizes the R&S RTO family with the attributes of speed, ease-of-use and accuracy. For speed, Schulze points to the acquisition and analysis rate of up to 1 million waveforms per second. The highly integrated processing ASIC is the key enabler for that standard mode capability. “Looking” at the signal up to one million times per second has the advantage that the user can detect signal faults much faster and achieves more reliable results with his analysis tools based on a high number of acquired waveforms (see Figure 5). Now, for the first time, a histogram with up to 1,000,000 wfms/s or a mask test with greater than 600,000 wfms/s is possible. A fast FFT display and HW accelerated math and measurement examples are other key advantages.
The RTO’s accuracy is achieved through the low-noise highly sensitive front-end, a single-core ADC with an ENOB >7 and a precise digital trigger system that enables trigger jitter of <1 ps at maximum acquisition rate. High measurement accuracy is also supported by the low noise Rohde & Schwarz active probes as well as the high bandwidth probe interface.
Test and measurement equipment manufacturers have made great strides in bringing the classic time-domain circuit/system debugging tool, aka the oscilloscope, into the realm of microwave engineering. This has been made possible by numerous innovations in signal path technology, scope architecture, software and especially in the semiconductor technology employed in the front-end, ADCs and ASICs. Microwave engineers may still have to worry about skin depth and the physical size of their circuit designs relative to wavelength, but with the new class of multi-GHz oscilloscopes, studying circuit behavior through direct observation of IV waveforms, is no longer outside our domain.