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Spread spectrum clocking (SSC) allows for clocked devices to reduce radiated emissions by frequency-modulating the signal. This controlled modulation distributes the energy of the carrier across a wider frequency range, thereby reducing peak power. As device frequencies (such as microprocessor chip and system board signals) are now microwave-ranged RF signals, the electromagnetic interference (EMI) generated by these mainstream devices may commonly cause cross-talk and coupling into nearby devices if SSC is not enabled. Anticipating this, special interest groups (SIGs) overseeing standards currently in use such as PCI Express and Serial ATA, have required that devices must be SSC-capable in order to be compliant.

Verifying the modulation profile of SSC has historically been challenging because it involves a frequency shift as a function of time. The typical view provided by a spectrum analyzer displays frequency (Hz) on the X-axis, and magnitude (dB) on the Y-axis. This view can verify the range of SSC frequencies, but cannot show the time order in which they occurred. The typical view provided by an oscilloscope displays time on the X-axis and magnitude (V) on the Y-axis. This view can verify waveshape but does not discern the rate of cyclical frequency shifts. What is needed is a view that displays frequency as a function of time.

Using the track math operator, oscilloscopes are able to plot a parametric measurement as a function of time. When applied to a modulated signal, the track function can demodulate the waveform. By tracking the frequency measurement parameter, an oscilloscope can display the SSC modulation profile as a function of frequency vs. time. As shown in Figure 1, the triangular SSC modulation profile of a serial ATA signal is displayed as a waveform trace. In addition to providing a qualitative view (the SSC has triangular modulation with sharp peaks and linear transition regions), accurate quantitative measurements can also be performed. Because the track operator is a waveform, standard oscilloscope measurements can be applied. These standard measurements, however, take on a special meaning when applied to the SSC modulation profile.

Shown in Figure 1, a 3 Gbps serial ATA Gen 2 waveform (white) is acquired at a sample rate of 40 Gsps with an eight million-sample point record. P1 measures the instantaneous frequency of each of the 149,643 data transitions in the waveform. A frequency track waveform (blue) plots frequency as a function of time, revealing the triangular modulation shape of SSC. In this view, modulation is separated from the carrier and is displayed as an independent waveform. The P2 measurement parameter, applied to the track, is a direct measurement of SSC modulation frequency. Measurement parameters P3 through P5 measure the clock frequency deviation of SSC, down-spread from the corresponding nominal frequency. P3 and P4 show the highest and lowest instantaneous frequency values, while P5 shows the peak-to-peak frequency deviation. This quantitative analysis allows for rapid determination of compliance and rapid detection of any frequency errors occurring within the SSC.

Qualitative information is also available from the SSC modulation profile shape. In some cases, this information can be as valuable, or even more valuable than the quantitative information. Acquired during an actual debugging session of a Serial ATA Gen 2 device, the green waveform in Figure 2 shows a series of non-linear transitions. These “stair-step” transitions resulted in large deterministic jitter values during testing, but the cause was unknown. Only after plotting the SSC modulation profile did the designers determine that their jitter problem was due to a faulty digital encoder in the SSC modulation chipset. A qualitative view of the SSC modulation profile made this diagnosis an easy determination. Without a qualitative view of the modulation profile, discovering the root cause of this real-world jitter problem would have been difficult.

Verifying SSC performance is now possible using powerful parameter tracking capabilities to display a waveform consisting of instantaneous frequency as a function of time. Applying measurement parameters to the track results in new and meaningful measurements such as SSC frequency and SSC frequency deviation. In addition to quantitative SSC-specific measurements, the qualitative view of the frequency vs. time modulation profile can identify modulation anomalies and provide insight into the source of SSC modulation errors.

Mike Hertz is a field applications engineer with LeCroy Corp. in Michigan. He has three U.S. patents pending in oscilloscope measurement design. Hertz received a BSEE from Iowa State University and an MSEE from the University of Arizona.

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