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20 July 2017

The Periodic Table of Oscilloscope Tools: Analyze (Part II)

The Analysis tools in an oscilloscope lend it debug power
Figure 1: The Analysis
tools in an oscilloscope
lend it debug power
Oscilloscopes are central to many engineering tasks, but perhaps to none more so than debugging. Something is going on with your design but you don't know what it is. However, armed with an oscilloscope with the sort of sophisticated analysis tools found in Teledyne LeCroy's instruments, even Mr. Jones can get to the bottom of the problem. Let's continue our survey of the Periodic Table of Oscilloscope Tools with more on analysis tools.

In this post, we'll review three more columns of the table: Serial Message Analysis, Clock and Timing Jitter, and Serial Data Jitter. The first of these three groups builds on the Serial Decode tools covered in our last installment.
  • Intuitive, semi-transparent color-coded overlays are included as part of every serial decoder software option (more than 20 are available). These color-coded overlays enhance understanding of the decoded physical layer serial data signal by clearly identifying the beginning and ending of the relevant portions of the protocol layer, such as ID, DATA, CRC, R/W, and so on. Certain protocol errors are highlighted in red to flag their presence. The color-coded overlays gracefully compact on long acquisitions to avoid display clutter, but expand again in a Zoom trace for quick understanding of the protocol behaviors.
  • Serial decoder options include a Protocol Table, similar to that found in a protocol analyzer, for convenient viewing of the decoded serial data (in addition to the Color Overlays). The Protocol Table is user-configurable for columns and rows, and intuitively summarizes protocol error behaviors, and can be easily exported as an Excel document. 
  • Search & Zoom enhances the Symbol, Protocol Layer, Application Layer, and ProtoSync serial data decode and Protocol Table capabilities. Simply touching a protocol table row creates a zoom trace which is appropriately scaled for easy viewing of the transparent Color Overlays decode information. Use the Protocol Table to location information of interest, and then Search & Zoom to view the physical layer signal, or vice versa.
  • Serial Bus Parameters are provided as part of the various serial bus TDME or TDMP and ProtoBus MAG options and can measure various generic bus activities, such as message bit-rate, bus load, or number of serial messages in an acquisition. When combined with the Track capability as described in Tracks/Trends, they provide a convenient method to measure bus activities and correlate unusual bus activities with causal events, e.g. correlate periods of high serial message density with receiving errors, or fluctuations in bit rates with power-supply disturbances.
  • Serial Bus Timing Parameters are provided as part of the various serial bus TDME or TDMP and ProtoBus MAG options and can measure timing between different combinations of serial data messages and analog signals, e.g. Message to Analog, Analog to Message, Message to Message, (Trigger) Time to Message, or Delta Message (Time). These timing parameters greatly improve debug capability by allowing faster validation of timing conditions (e.g., when a certain I2C message is sent an analog power rail, and Pass/Fail Actions and Boolean Compare. With All Instance measurement capability, every instance in the acquisition is measured to ensure faster validation, debug, and correlation to other events. More than a dozen serial protocol decoders are supported (including, among others, CAN, LIN, FlexRay, I2C, SPI, UART, RS-232, DigRF, and USB2).
  • Serial digital-to-analog converted (DAC) Waveforms may be created using various serial bus TDME or TDMP and ProtoBus MAG options. This capability permits the extraction of digital (serial) data from a user-defined message and the conversion of the digital value to a properly scaled and unitized analog value, per the user definition. This analog value then exists as a measurement parameter value, and Statistics, Histicons/Histograms, Pass/Fail Actions, and Boolean Compare all can be used for further analysis. With All Instance measurement capability, every instance in the acquisition is measured, and all of these values can be displayed as a pseudo-analog "synthesized" waveform using a variation of Track (see Tracks / Trends), which completes the conversion of digital data to an analog waveform. Doing so provides an intuitive view of the underlying embedded serial data that cannot be achieved in any other way. An example is to view sensor data (e.g. steering angle information) embedded in CAN data, or temperature information embedded in I2C messages. More than a dozen serial protocol decoders are supported.
At the heart of nearly any debugging effort is analysis of clock and timing jitter:
  • Clock and timing jitter measurements for some JEDEC standards historically indicated the quantity of measurements to include in the measurement, e.g., 10,000 measurements for cycle-cycle, period, and half-period jitter. Measure Gate in the JITKIT software option provides a simple and intuitive method to define the measurement area and gate all of the measurement results to the specified area for validation to the standard or to understand worst-case performance areas for correlation to causal events. Measure Gates may also be applied to any measurement parameter to limit the measurement area.
  • In the days of analog oscilloscopes, jitter was viewed as the (horizontal) width of a persisted trace at the trigger location, with the maximum width representing the peak-peak jitter for that number of acquisitions. This provided a crude but intuitive way to understand the jitter. Jitter Overlay improves on this technique by providing a software determination of edge placement (to eliminate the impact of oscilloscope hardware trigger jitter), a phosphor-like display of overlaid edges, and selection to view jitter activity as period, half period, cycle-cycle, time interval error (TIE), setup, hold, and so on.
  • Measurement statistics provide a partial story of jitter behavior. A Jitter Track provides a fuller picture with a time-correlated waveform that displays jitter values on the vertical axis vs. time on the horizontal axis, with the time scale identical to that of other acquired or processed waveforms. This is a unique and intuitive view of the jitter measurement behavior, and it makes it simple to correlate high measured jitter values with other events, such as power supply noise, crosstalk, or other disturbing events. Jitter Tracks may be further processed by a full-memory FFT to create a resulting Jitter Spectrum.
  • A Jitter Histogram displays all of the data values so that measurement modality can be observed and used to debug improper behaviors. A variety of histogram measurements can be applied to objectively measure the histogram mean, peak-peak, mode, full-width half-maximum, and more. This is a unique and intuitive view of the jitter measurement behavior, making it simple to understand the statistical "story" behind the numerical measurement set.
  • Jitter Spectrums utilize Teledyne LeCroy X-Stream processing speed and full-memory FFT capabilities to calculate jitter spectrums and identify spectral peaks at very low frequencies - lower than possible with any other instrument. Any identifiable clock jitter quantity (period, half-period, cycle-cycle, duty cycle, etc.) can be processed into a spectral view. Jitter spectrums are also extensively used in modified form in serial data analysis in Rj+BUj Views, Pj Spectral Views, and Noise Analysis.
  • JitKit Quick View provides an instant setup of four views of jitter—statistical Jitter Histogram, Time variation (Jitter Track), persisted Jitter Overlay, and spectral (Jitter Spectrum) for multiple jitter parameters (period, frequency, cycle-cycle, TIE, and others) all at one time.
Equally important for debugging is analysis of serial-data jitter:
  • Serial data jitter is most intuitively understood when displayed as logical 1s or 0s with corresponding transitions overlayed as a persisted waveform that resembles an open human eye. Teledyne LeCroy invented the modern Eye Diagram as utilized in a real-time oscilloscope by combining long acquisition memory, software clock recovery (using a user-definable software PLL), and advanced techniques to mathematically "slice" and overlay the bits to resemble a traditional sampling (equivalent-time) oscilloscope and hardware clock-recovery persisted eye diagram, but without the additive effects of trigger jitter. Teledyne LeCroy's Eye Diagram capability is available in numerous software options and works with very low-speed to very high-speed and continuous NRZ or PAM-4 signals (see PAM-4 Analysis), or bursted NRZ signals (e.g. using DDR Multi-Eye View or the DDR Debug Toolkit, or SPI, UART, CAN, FlexRay, ARINC, etc. using the various serial bus TDME or TDMP options for each low-speed serial protocol). The Eye Diagram can be rendered in various analog or color-intensified views, and is highly-optimized for extremely fast display. Some options permit concurrent numerical jitter calculations and Tj, Rj, Dj for serial data or DDR Tj, Rj, Dj deconvolution, and further post-processing of the Eye Diagram data to provide subjective measurements or additional views of the quality of the Eye Diagram, such as Bathtub Curve IsoBER, DDj + ISI Views, vertical Noise Analysis, or Crosstalk Analysis views. More advanced options (e.g. SDAIII) provide a variety of eye parameters for height, levels, width, extinction ratio, etc., with selectable slide width.
  • Rj, Dj, and Tj: Serial data standards require that jitter be understood as Random (Rj) and Deterministic (Dj) components that add to a Total Jitter (Tj) value that would accurately represent expected performance in real-world systems. Teledyne LeCroy's SDAIII Serial Data Analysis package performs this complex jitter decomposition and Tj calculation. Clock recovery is accomplished in software to eliminate hardware trigger jitter, with user-selectable PLL including custom two-pole PLL filter settings. Rj method is user-definable to use one of three dual-Dirac models—Spectral Rj Direct, Spectral Rj+Dj CDF Fit, or NQ-Scale—depending on user requirements or test specifications. Bit error rate (BER) for Tj calculation is user-definable to 10e-21. All decomposed jitter values may be displayed in time or unit intervals. As-measured time interval error (TIE) values may also be displayed along with associated TIE Jitter Histogram, TIE Jitter Track, and TIE Jitter Spectrum views. Eye Width can be calculated and displayed as one serial data unit interval minus Tj at user-selectable BER.
  • The Bathtub Curve estimates bit error rate (BER) and provides an indication for timing margin in the system, and appears as a cross-section of a bathtub when the BER is very low. It is calculated from the probability distribution function (PDF) for the serial data transitions at a user-defined crossing level (either in percent or absolute voltage). A Cumulative Distribution Function (CDF) and a Q-Fit as described in Rj + BUj Views can be displayed with the Bathtub Curve for better understanding of system behaviors. An IsoBER provides a different way of understanding the Bathtub Curve information by displaying Eye Diagrams extrapolated to a user-defined BER.
  • IsoBER displays show lines of constant extrapolated bit error rate (BER) within the Eye Diagrams, similar to how an isobar shows lines of constant atmospheric pressure on a weather map. BER settings for "from" and "to" are user-definable, as is BER step size. If you're using Crosstalk Eyes, the IsoBER lines inside the Eye Diagram will match the Crosstalk Eye lines on the inside of the Eye Diagram.
  • The Jitter Simulator can simulate a clock or Sinusoid, non-return to zero (NRZ), return to zero (RZ), and bipolar return to zero (bpRZ) serial data signals. It also can simulate phase amplitude-modulated four-level (PAM-4), or pulse-width modulated (PWM) signals. The above provides user-definable clock frequency or bit rate, amplitude, offset, rise/fall time, and frequency cutoff. Serial data signals can be defined with varying pattern sequences and lengths. User-defined (timing) jitter and (vertical) noise and shape attributes may be added to the selected signal. The Jitter Simulator is an ideal tool for learning how to use the Teledyne LeCroy serial data analysis toolsets, such as Eye Diagrams; Tj, Rj, Dj, Rj + BUj Views; Bathtub Curve; DDj + ISI Views; IsoBER; Pj Spectral Views; Noise Analysis; and Crosstalk Analysis.
  • Eye Doctor II is a complete set of tools that allows the full-range of fixture, serial data channel and probe de-embedding, transmitter pre- and de-emphasis and serial data channel emulation, and receiver equalization (CTLE, FFE, or DFE) emulation on full record lengths with little impact on waveform processing time. Eye Doctor II provides real-time results on the acquired or simulated (with the Jitter Simulator) waveforms, with post-processing for Eye Diagrams, Jitter Analysis, and other Teledyne LeCroy tools. Industry-standard S-parameter measurements and Touchstone 1.1 files are used to describe all embedded or emulated objects. Virtual probing (VP) may be enabled via Teledyne LeCroy's Processing Web. 
Next time, we'll finish our tour of the Analysis portion of the Periodic Table of Oscilloscope Tools and also cover the final portion (Document).

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