Real-Time Digital Triggers, Logic Channels Buoy Scopes’ Utility

Sept. 6, 2011
Analog triggering can hinder full disclosure on signal glitches, but the digital triggering in the RTO Series scopes promises to shed light on what's really happening in your signals of interest.

Real-time digital triggering enables the RTO Series scopes to capture much more detail in signals than do instruments with analog triggering.

Most digital oscilloscopes come with an immediate handicap, one that hampers them right out of the box and can skew measurements. That handicap is triggering in the analog domain, which brings inherent jitter because the analog trigger and signal acquisition paths run in parallel. These separate circuits, with their separate characteristics, cause the signal to be displayed with time and amplitude offset relative to the actual trigger point.

In its RTO Series digital oscilloscopes (see the figure), Rohde & Schwarz sidesteps this issue by using real-time digital triggering (see a video demonstration of the RTO Series here). In this system, the trigger and the capture data share the same signal path and a common timebase. As a result, trigger jitter is held to exceptionally low levels. Moreover, analog triggers typically come with a long re-arm cycle. While the trigger re-arms, the scope cannot react to trigger events, which means that signal properties that should be setting off triggers are masked. The digital triggers in the RTO Series scopes do not have to be re-armed; thus, every sample can trigger an acquisition.

Now available in a new 600-MHz model in addition to earlier 1-GHz and 2-GHz versions, the RTO Series scopes’ utility is further enhanced by the addition of 16 logic channels with a maximum signaling rate of 400 MHz on the digital lines. The logic analyzer pod is a self-contained part of the scope built on an FPGA that includes acquisition triggering memory and display processing. Thus, when you run logic-analysis channels, the scope’s native processing resources are left untapped and scope performance doesn’t suffer.

The logic-analysis module itself can acquire at 200,000 waveforms/s. Most logic-analysis attachments for scopes are in the 100,000-waveform/s range. Moreover, the module samples at 5 Gsamples/s, so it oversamples the logic inputs. That enables users to see events of very short duration. That sampling rate holds for the entire 200-Msample acquisition memory.

Performance in the RTO Series scopes gains plenty from a custom ASIC that performs real-time signal processing of digitized measurement results. Fast-Fourier transforms are implemented in that digital hardware, making screen updates much snappier. According to Mike Schnecker, business development manager at Rohde & Schwarz, the scopes can analyze 1 million waveforms/s. “Other scopes can update at this rate but can’t take a measurement at the same time,” says Schnecker.

In a live demonstration, the RTO 1024, the flagship 2-GHz model, exhibits no spurs in its internal noise across the full 2-GHz bandwidth. The scopes use a single-core analog-to-digital converter (ADC) design, so there’s no interleaving. Operating at 10 Gsamples/s, the ADC achieves a high dynamic range of more than seven effective bits. Moreover, while operating at 1 mV/division, the scope still displays full bandwidth. “Other scopes trade off bandwidth for resolution. We use an extra gain stage to compensate for this,” says Schnecker.

The instruments’ frequency response is designed to be nearly true Gaussian in nature. “Most digital scopes today don’t do this because it means trading off sample rate for bandwidth,” explains Schnecker. Instead, they use a response with sharp roll-off, such as a high-order Bessel response, to achieve the rejection they need at a give frequency so they can sample. “On ringing and overshoot specifications, youll see 10% overshoot on other scopes, whereas ours is about 1%. You want to measure the signal in, not the scope itself,” says Schnecker.

Rohde & Schwarz’s designers paid a lot of attention to the user interface in the RTO Series instruments. The 10.4-inch touchscreen gives users a more interactive means of using the scopes. Menus are positioned at the bottom of the screen and pop up rather than being pulled down. That means that the menus don’t block the waveform display and neither do the user’s hands, because they can stay largely below the screen. Users select measurements from menus and can have as many active at a time as they need. You can park measurement windows anywhere you want on screen for a fully flexible and customizable display.

The RTO Series digital oscilloscopes are available now.

Rohde & Schwarz Inc.

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