In Figure 2, the persistence trace mean (green waveform) shows the main mode of the frequency track (blue waveform), clearly revealing the spread-spectrum clock modulation of the 1.5-Gbit/s Serial ATA waveform (brown waveform). Parametric measurements show the minimum frequency (1.495 GHz), maximum frequency (1.500 GHz), and peak-to-peak frequency deviation (4.8 MHz) of the 300,000 data cycles acquired (4 million sample points).
Frequency-at-level measurement of the track reveals that the SSC modulation frequency is 29.8 kHz. These time-domain measurements, when applied in a frequency-versus-time mode, provide both quantitative and qualitative insight into SSC behavior, which had never been available previously.
Filtering is the traditional method for reducing high-frequency noise. With the advent of digital signal processing, digital filters were incorporated into real-time oscilloscopes. Today, filter types such as low pass, high pass, bandpass, and band stop are commonly used to filter input signals. Applying this digital filtering technique to a frequency track, instead of applying digital filters to the input waveform, can reduce or eliminate the high frequencies and spurious noise residing in the frequency-track modulation.
Using digital filters to lower track noise is the most versatile method of the three, because exact filter cutoff coefficients can be specified to include or exclude known sources of modulation noise. The digital-filtering-package (DFP) technique gives users extensive control and accuracy in varying low- and high-frequency cutoffs, filter taps, stop-band attenuation, and pass-band ripple. Yet it also requires the greatest processing time of the three methods.
Combining sparsing with enhanced resolution is the third alternative for lowering track noise. Because the track contains many high-frequency fluctuations, the use of the sparse function will decimate the track, leaving N out of every M points remaining. The SSC modulation occurs slowly compared to the frequency of the carrier.
Therefore, decimating the extra frequency track values will leave a sparsed SSC structure intact. The shape of this structure is then further smoothed with enhanced resolution. Combining sparse and enhanced resolution is straightforward to implement and lets users view the intermediate steps involved in this implementation. It's faster than DFP and more flexible than persistence trace mean.
In Figure 3, the blue waveform shows the frequency track of the 4-Mpoint, 2.5-Gbit/s PCI Express serial bitstream (brown waveform) with SSC. The green waveform performs a 75:1 sparsing of the track, and the red waveform performs a Gaussian, 3-bit, low-pass filter enhancement of the sparse. Note how clean the red waveform is compared with the green and blue waveforms.
This combination of sparse and enhanced resolution can reveal the SSC modulation separated from the carrier. Parametric measurements applied to the track show a minimum (2.4858 GHz), maximum (2.5017 GHz), and peak-to-peak frequency deviation (15.9 MHz) of the 500,000 data cycles acquired (4 million sample points), while the track's frequency-at-level measurement reveals an SSC modulation frequency of 31.3 kHz.
The fast Fourier transform (FFT) can be applied to the input waveform of both a normal and an SSC-modulated waveform. Overlapping the FFT spectra on a single grid shows the reduction in peak power levels due to spectral spreading. Measurements determine the relative power difference between the normal and SSC inputs.
The FFT can also be applied to the persistence trace mean, DFP, or enhanced resolution sparse trace to display the frequency content of the track. This would reveal any multitonal effects and determine relative strength of the SSC. When the FFT is applied to the track, persistence trace mean, eres, or DFP output, then the frequency-domain view displayed shows the frequency content of the modulation, separated from the carrier. This way, the effects of SSC are completely isolated from the waveform itself, and the full analysis capability of the scope is applied directly to the SSC.
In Figure 2, the persistence trace mean (green waveform) shows the main mode of the frequency track (blue waveform), clearly revealing the spread-spectrum clock modulation of the 1.5-Gbit/s Serial ATA waveform (brown waveform). Parametric measurements show the minimum frequency (1.495 GHz), maximum frequency (1.500 GHz), and peak-to-peak frequency deviation (4.8 MHz) of the 300,000 data cycles acquired (4 million sample points).
Frequency-at-level measurement of the track reveals that the SSC modulation frequency is 29.8 kHz. These time-domain measurements, when applied in a frequency-versus-time mode, provide both quantitative and qualitative insight into SSC behavior, which had never been available previously.
Filtering is the traditional method for reducing high-frequency noise. With the advent of digital signal processing, digital filters were incorporated into real-time oscilloscopes. Today, filter types such as low pass, high pass, bandpass, and band stop are commonly used to filter input signals. Applying this digital filtering technique to a frequency track, instead of applying digital filters to the input waveform, can reduce or eliminate the high frequencies and spurious noise residing in the frequency-track modulation.
Using digital filters to lower track noise is the most versatile method of the three, because exact filter cutoff coefficients can be specified to include or exclude known sources of modulation noise. The digital-filtering-package (DFP) technique gives users extensive control and accuracy in varying low- and high-frequency cutoffs, filter taps, stop-band attenuation, and pass-band ripple. Yet it also requires the greatest processing time of the three methods.
Combining sparsing with enhanced resolution is the third alternative for lowering track noise. Because the track contains many high-frequency fluctuations, the use of the sparse function will decimate the track, leaving N out of every M points remaining. The SSC modulation occurs slowly compared to the frequency of the carrier.
Therefore, decimating the extra frequency track values will leave a sparsed SSC structure intact. The shape of this structure is then further smoothed with enhanced resolution. Combining sparse and enhanced resolution is straightforward to implement and lets users view the intermediate steps involved in this implementation. It's faster than DFP and more flexible than persistence trace mean.
In Figure 3, the blue waveform shows the frequency track of the 4-Mpoint, 2.5-Gbit/s PCI Express serial bitstream (brown waveform) with SSC. The green waveform performs a 75:1 sparsing of the track, and the red waveform performs a Gaussian, 3-bit, low-pass filter enhancement of the sparse. Note how clean the red waveform is compared with the green and blue waveforms.
This combination of sparse and enhanced resolution can reveal the SSC modulation separated from the carrier. Parametric measurements applied to the track show a minimum (2.4858 GHz), maximum (2.5017 GHz), and peak-to-peak frequency deviation (15.9 MHz) of the 500,000 data cycles acquired (4 million sample points), while the track's frequency-at-level measurement reveals an SSC modulation frequency of 31.3 kHz.
The fast Fourier transform (FFT) can be applied to the input waveform of both a normal and an SSC-modulated waveform. Overlapping the FFT spectra on a single grid shows the reduction in peak power levels due to spectral spreading. Measurements determine the relative power difference between the normal and SSC inputs.
The FFT can also be applied to the persistence trace mean, DFP, or enhanced resolution sparse trace to display the frequency content of the track. This would reveal any multitonal effects and determine relative strength of the SSC. When the FFT is applied to the track, persistence trace mean, eres, or DFP output, then the frequency-domain view displayed shows the frequency content of the modulation, separated from the carrier. This way, the effects of SSC are completely isolated from the waveform itself, and the full analysis capability of the scope is applied directly to the SSC.