Frequency synthesizers, crystal oscillators, clock signal generation ICs, although essential to the proper performance of many electronic devices, are the principal sources of EMI in electronic circuitry. EMI reduction is therefore a major issue for designers of electronic appliances that use components generating frequency and clock signals.

While conventional EMI reduction methods (shielding, special coating, filtering components, etc.) are still common practice, the tightness of EMI regulations and the cost sensitivity of their impact have led to the development of alternative and less expensive solutions. Our SST (Spread Spectrum Technology) based components provide efficient and cost optimal solutions for EMI reduction.

Spread Spectrum reduces Peak Emissions
In addition to generating the desired electrical signal it is intended to, a frequency generator will also radiate electromagnetic waves over the frequency spectrum of the electrical signal it generates. These electromagnetic waves will have a frequency to which other devices may be sensitive. In this case, this may prevent these devices from functioning properly. Such electromagnetic emissions are considered electromagnetic interference (EMI). The higher the energy radiated by the signal, the worse the interference. EMI is therefore characterized by the energy radiated by the frequency source, usually in terms of the resulting electric field measured in dBµV/m at a given frequency.

It is important to note that EMI emissions will be generated over the entire frequency spectrum of the signal causing the EMI. Therefore, when considering the EMI emissions over a given frequency spectrum, one may distinguish peak emissions from average emissions. Peak emissions are defined as the highest dBµV/m level reached over the frequency spectrum of the signal, while average emissions are defined as the average dBµV/m level radiated over the frequency spectrum of the signal.

In terms of EMI, existing regulations are essentially concerned about preventing interference at any given frequency. Therefore, regulatory bodies limit peak emissions, rather than average emissions.

For a given signal source, the radiated energy (in other words, the EMI emissions) is concentrated within the frequency spectrum of the signal. Seen from another angle, this means that if the power of the signal is kept constant, the peak emissions will be reduced whenever the frequency spectrum of the signal can be spread across a broader bandwidth. The application of this principle is called spread spectrum.

Spread Spectrum ICs reduce the EMI and costs easely and importantly
For applications where EMI emissions are critical, it is recommended to use PhaseLink’s frequency synthesizers and clock generators using the SST (Spread Spectrum Technology) feature, thus reducing EMI emissions at their source, rather than apply a posteriori methods (such as shielding, coating or additional filtering) to limit their propagation. By making other EMI reduction methods and components redundant, SST will often create important cost saving opportunities. For example, SST clock chips and SST frequency generators will typically permit the designer to reduce the number of components, simplify the preparation and assembly of the enclosure, or even permit the use of a less expensive enclosure material. Industry experience has demonstrated that replacing a traditional metal enclosure to lower EMI emissions with an advance Spread Spectrum Technology could achieve cost savings of almost 10$ per unit.

The SST ICs are available in lead free QFN, SSOP8 and SSOP16 packages providing to the user additional cost savings in terms of flexibility (the frequencies and spread modes can be re-programmed at every time, also during a running production), very fast time-to-market (important reduction of development time and costs).

The very deep product range of our Spread Spectrum Timing ICs allows us to offer the optimal solution for every application.

Spread Spectrum Technology: How it works
Applying a slight frequency modulation to the signal causing the EMI is comparable to enriching the frequency spectrum of this signal, which means that the frequency spectrum is spread over a larger bandwidth, thus achieving the desired “spread” of the EMI emissions spectrum. Using PhaseLink’s SST (Spread Spectrum Technology) low EMI frequency synthesizers and clock generators, EMI reduction of 10 to 20dB and more can be achieved.

Figure 1 shows the triangular modulation profile used to generate PhaseLink’s proprietary SST, one can see the modulation frequency (sweep rate) used are extremely low compared to the frequency of the clock signal on which the SST is applied (typical sweep rates vary from 30kHz to 60kHz, depending on components).

Figure 2 shows the frequency spectrum of the EMI emissions for a typical non-modulated square wave signal of a nominal frequency F_o and the corresponding frequency spectrum of the same signal after SST modulation. In comparison with the modulated signal, the relative narrow band of the frequency spectrum results in a corresponding peak in the EMI emissions at frequency F_o.

The modulation is implemented by varying the output frequency up and down around the nominal frequency. This variation is characterized by an upward frequency deviation (∆F_up) and a downward frequency deviation (∆F_down). Using very small upward (or downward) increments, the signal is modulated in frequency until the maximum frequency F_max (or minimum frequency F_min) is reached, at which point the reverse process is engaged. The instantaneous output frequency will therefore always vary between:

F_max = F_o + ∆F_up and F_min = F_o — ∆F_down

We can thus define the magnitude of the modulation as the difference between F_max and F_min:

F_max — F_min = ∆F_magn = ∆F_up + ∆F_down

The magnitude of the modulation and the upward and downward frequency deviations are usually stated as a percentage (%) of the nominal frequency.

The greater the magnitude of the modulation, the larger the spread in spectrum, and the more reduction in EMI. In other terms, a greater modulation percentage will reduce EMI emissions more.

Down Spread and asymmetric Spread avoid over-clocking
The modulated signal shown on figure 2 results from a modulation centered around the nominal frequency (F_o) called “center spread”. It is called center spread because in this case, ΔF_up = ΔF_down = 0.5%, thus positioning F_o at the center of the modulation. As mentioned above, this results in a maximal instantaneous frequency of F_max = F_o + ΔF_up, which is obviously greater than the nominal frequency. Even though the maximum frequency deviations used to implement SST are less than 4% of the nominal frequency (with typical values less than 1%), certain systems do not tolerate clock or signals above the nominal frequency. Operating beyond the nominal frequency is called over-clocking.

In a situation where SST is desired but where over-clocking is not tolerated or has to be limited, down spread and asymmetric spread should be considered.

Down spread is defined as an SST modulation for which the maximum instantaneous output frequency (F_max) is limited to the nominal frequency (F_o), thus avoiding over-clocking. This implies that ΔF_up is set to zero, and that ΔF_magn = ΔF_down. There is no difference with the center spread modulation in terms of implementation, except that the signal will be modulated around a center frequency (F_c) that is below the nominal frequency (F_o). The spectrum of a signal modulated with a down spread SST is shown on figure 3. The drawback of down spread modulation is that the average output frequency will be below the output frequency (in fact it will be equal to F_c). Therefore, a trade-off exists between average output frequency, maximum tolerated over-clocking and maximum SST modulation amplitude.

Half way between the down spread and the center spread modulations is the asymmetric spread. In the context of limiting over-clocking, asymmetric spread is defined as an SST modulation for which the ΔF_up is selected less than ΔF_down. This permits to maintain the magnitude of the modulation that will permit the required EMI reduction, while having a maximum frequency (F_max) less than what it would have been with a center spread modulation having the same magnitude.

Negligible impact on cycle-to-cycle jitter
If we consider the modulated signal in the time domain, variations in the instantaneous frequency will result in changes in the cycle-to-cycle period of the output signal. One may therefore wonder about how badly the SST modulation will impact on the jitter performance of the frequency synthesizer or clock generator. In fact, the difference in the cycle-to-cycle period due to the SST modulation will contribute to less than 0.05% of the cycle-to-cycle jitter without SST. This is why SST frequency synthesizers and clock generators have the same cycle-to-cycle jitter performance as non-spread spectrum clock components.

Detailed product-information:

Spread Spectrum ICs Series PLL701