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High-performance Oscilloscopes with Bandwidths Up to 2 GHz

August 7, 2010
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Figure 1 The R&S RTO offers high signal fidelity, good measurement dynamics, an acquisition rate of 1 M waveforms/s and a digital trigger.

The new Rohde & Schwarz RTO oscilloscopes are available as two- and four-channel models with a bandwidth of 1 or 2 GHz and a sampling rate of 10 Gsample/s per channel (see Figure 1). Crucial factors for optimum use of an oscilloscope include how quickly it helps find errors and how accurately it displays the signal trace. This fact makes this new family of R&S scopes of particular interest as their very short 'blind times' make it possible, for the first time, to analyze and display one million waveforms per second.


BLIND AMBITION
A digital oscilloscope measures signals in two steps. First, it samples the signal for a defined period and stores the acquired data. It then processes the data and displays the waveform. During this signal processing phase, digital oscilloscopes are 'blind' to the measurement signal. Any errors that arise at the test point during this period go undetected (as illustrated in Figure 2). The effects of this blind time are the most critical at the highest sampling rate. When measuring at a sample rate of 10 Gsample/s and a record length of 1,000 samples, conventional oscilloscopes are blind for 99.5 percent of the acquisition cycle.

Figure 2 Compared to a conventional oscilloscope, the blind time of R&S RTO oscilloscopes is up to 20 times shorter.

Consequently, measurements only take place during less than 0.5 percent of the cycle. These new oscilloscopes, on the other hand, look at the signal 20 times more often. This is made possible by the instruments' highly integrated ASIC. The use of multiple parallel processing paths drastically reduces blind time. The result is a speed of one million displayed waveforms per second. Simultaneously, all instrument settings and measurement analysis functions remain available.

Digital trigger system
With an analog trigger system, the trigger path runs parallel to the signal acquisition path. Due to the different characteristics of the paths, a time and amplitude offset arises in the signal display at the trigger point. This causes measurement inaccuracies (trigger jitter), which post-processing can only partially correct. In what is believed to be the world's first purely digital trigger architecture, the trigger and measurement data share a common signal path, and thus have the same time base. The result is very low trigger jitter in real time and precise allocation of the signal to the trigger point.

In addition to trigger jitter, analog trigger systems also have to cope with long 're-arm' times. During this period, the system does not react to additional trigger events. This means that signal characteristics that users want to trigger on are masked. The digital trigger system, on the other hand, has no re-arm mechanism, and therefore responds reliably to trigger events that occur in rapid succession. No events are lost.

Signal Fidelity
The accuracy of the measurement signal's representation depends heavily on the front-end's bandwidth and noise floor. Consequently, Rohde & Schwarz has developed BNC-compatible broadband inputs, very low-noise front-ends and precise analog-to-digital converters (ADC). The result is a very low inherent noise.

Stringent accuracy requirements are especially needed in the case of low signal amplitudes for digital interfaces and signal analysis in the frequency domain. The accuracy of the signal digitization depends on the ADC's effective number of bits (ENOB). Typically, the ADCs used in digital oscilloscopes consist of several interleaved, time-delayed, slow converters. The higher the number of such interleaved components, the larger the errors caused by the non-uniform behavior of the individual converters. For this reason, a monolithic 8-bit ADC with a sampling rate of 10 Gsample/s has been developed. This module's single-core architecture minimizes signal distortion. With more than seven effective bits, it achieves excellent measurement dynamics, significantly improving accuracy.

Even at low vertical resolution (down to 1 mV/div), the new scopes are highly accurate, because their sensitivity levels are not implemented with software-based zooming; they are implemented with switchable amplifiers in the front-end. For accurate measurements, the full measurement bandwidth is available in all sensitivity ranges down to 1 mV/div.

Compensation for the amplifiers and attenuator pads in the front-end is very precise, and a sophisticated temperature control system ensures excellent temperature stability within the instrument. Furthermore, the channel-to-channel isolation of more than 60 dB up to 2 GHz ensures that the measurement signal from one channel has the lowest possible influence on signals from the other channels.

Figure 3 All signal processing takes place in the recording trigger control ASIC.

Hardware-accelerated signal processing
A highly integrated ASIC handles all signal processing. This ASIC also contains hardware support for measurement and analysis functions, such as spectrum display, mathematical operations, cursor measurements, histograms and mask tests. Even during complex signal analysis, the ASIC's high degree of parallel processing ensures a high acquisition rate (as illustrated in Figure 3).

The various methods for reducing the number of samples—such as Sample, Peak Detect, High Res and RMS as well as arithmetical operations on waveforms, such as Envelope and Average functions—are important tools for signal analysis and troubleshooting. The new oscilloscopes simultaneously display up to three waveforms per measurement channel in different ways. It is possible to combine the type of data decimation and the waveform arithmetic.

Fast Fourier Transformation (FFT) is particularly fast with these oscilloscopes, with the high acquisition rate conveying the impression of a live spectrum on the screen. Combined with the persistence mode, it is possible to see even rarely occurring events in the spectrum. For R&S RTO instruments, the mask test was implemented in the ASIC so that the acquisition rate remains at a very high level (more than 600,000 waveforms/s). The otherwise very time-consuming mask test can be performed quickly.

User-friendly interface
An intelligent user interface with convenient touch screen operation makes work processes easier; the user is able to maintain a clear overview of what is going on, even during complex measurements. The user-friendly screen design with semi-transparent dialog boxes, signal icons to preview waveforms in real time and a configurable toolbar help users perform even complex test and measurement tasks quickly.

The handy, compact instrument comes with a 10.4 inch touch screen and the straightforward menus at the bottom of the screen make it possible to reach any of the settings with no more than two clicks. The flat menu structures and cross-links to logically associated settings simplify navigation. Signal flow diagrams in the dialog boxes visualize the progress of signal processing, and the toolbar at the top of the screen offers fast access to frequently used functions, such as Zoom, Undo/Redo, Histogram and FFT.

Control using a probe
Suitable active and passive probes for use with the new R&S RTO oscilloscopes are also offered. With an input resistance of 1 MΩ, the R&S active probes put only a minimum load on the signal source's operating point. The vertical dynamic range is very large even at high frequencies (for instance, 16 V peak-to-peak at 1 GHz), preventing signal distortion.

Measurements also do not have to be interrupted for compensation processes, because the offset and gain errors of the probes are almost completely independent of temperature. For example, the zero error is less than 90 µV/°C. In addition to solid test and measurement characteristics, the active probes feature two innovations: The micro button on the probe, which can be assigned various functions, such as Run/Stop or Autoset, enabling the user to control the oscilloscope directly from the probe. And the R&S ProbeMeter, an integrated voltmeter that allows precise measurements of the DC voltage regardless of the oscilloscope's current channel settings.

Conclusion
By encompassing digital technology, the new R&S RTO oscilloscopes bring a new dimension to the market. The instruments' highly integrated ASIC and the use of multiple parallel processing paths drastically reduces blind time facilitating a speed of one million displayed waveforms per second. Rohde & Schwarz developed a monolithic 8-bit ADC with a sampling rate of 10 Gsample/s the digital trigger architecture, providing very low trigger jitter in real time and precise allocation of the signal to the trigger point. Other key features include low inherent noise and a user friendly touch-screen interface.

Rohde & Schwarz,
Munich, Germany,
Tel: +49 89 4129 13774,
E-mail: customersupport@rohde-schwarz.com,
www.rohde-schwarz.com
RS No. 305

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