Spread Spectrum Clock Generator:SSCG
Summary
What is SSCG?
SSCG ＝ Spread Spectrum Clock Generator
- ElectroMagnetic interference (EMI:Radiation of electromagnetic ray) becomes a problem with the speed-up/high-density of electronic hardware.
- When the clock generator in an electronic hardware outputs the single frequency, radiation at the frequency and high-frequency becomes large.
- SSCG can suppress the peak lower by oscillation with slightly modulated frequency (Frequency modulation).
Features
1. Large effects on reducing the EMI
SSCG can suppress the peak of unnecessary radiation by modulating the frequency.
It is effective for not only the oscillation frequency of clock (basic wave) but also higher harmonic wave.
2. Low cost & downsizing
The other parts for EMI such as bypass capacitor, choke coil, ferrite bead and shield parts etc. can be reduced by using SSCG because it can suppress the unnecessary radiation. In result of that, you can reduce the total developing cost.
Moreover, clock system can be covered by single oscillator by using multiple outputs products (MB88182). It becomes the same frequency's crystal in the each customer's board by using programmable SSCG product (MB88R157A).
3. Reducing man-hour
Generally, for measuring EMI, simulation of electromagnetic radiated emission and the system evaluation/analysis are performed first. Then, EMI parts are inserted/added to the required area. Lastly, the simulation and analysis are repeated. However, as the SSCG can suppress the unnecessary radiation, the evaluation/analysis man-power and the cost to investigate faults and mount the EMI parts can be reduced if the architect is processed on the basis that SSCG is mounted at the early planning stages.
Effects on cost down
| Details |
Effects |
| Ferrite bead, choke coil, capacitor |
Several cents - several dozens of cents |
| Shield parts |
Several cents - Several dollars |
| Shield parts mounting cost |
Several cents - several dozens of cents |
| Research/measurement cost for EMI |
Several thousand dollars or more (Few mans month) |
Technology
The accurate frequency modulation technology by current control
Fujitsu's original method
- The ideal modulated waveform is enabled with the digital control by using current D/A converter. And it reduces 16dB (Modulate +/- 1.5%) of EMI.
-> Dramatic effect on reducing EMI is achieved. The influence on Cycle-Cycle Jitter is small.
Problem of voltage control modulation
- In the conventional example, modulation performed by voltage control causes distortion and blunting occur in the modulated waveform (see above figure).
-> The peak of 3dB occurs in spectrum waveform.
- The peak occurs at the interval of modulated cycle as the single modulated cycle is used.
Composite modulation for more EMI reduction effect
Conventional example：Modulation with single cycle
New method： Compound at modulation cycle
Fujitsu original method
- The peak is distributed by composing the modulation cycles, and it enables more flat spectrum diffusion.
- Thus, 19dB reduction in total is achieved (at modulation depth ± 1.5%)!
About Spread
<Center spread>
Spectrum is spread (modified) focusing around input frequency.
When the usage frequency is within the frequency of ASIC/Microcontroller that is connected to SSCG IC, Center spread is suitable.
<Down spread>
Spectrum is spread (modified) when usage frequency is lower than input frequency.
When the usage frequency is borderline of the frequency of ASIC/Microcontroller that is connected to SSCG IC, down spread is suitable.
Roadmap
