A practical guide to the theory, operation and selection of
Spread Spectrum Clocks (continued)

HOW DOES SPREAD SPECTRUM WORK

A Spread Spectrum Clock (SSC) takes advantage of the fact that a frequency-modulated carrier will have lower peak energy than a non-modulated carrier. By frequency modulating the carrier, the energy is spread out over a wider spectrum of frequencies, thereby reducing the peak energy contained at any one frequency. Comparing a modulated clock to a non-modulated clock on a spectrum analyzer, it can be seen that the peaks of the modulated clock fundamental and harmonic frequencies are lower in relative strength. The difference in relative strength of energy between the modulated clock and non-modulated clock is measured in dB. One way to visualize the effects of a modulated clock is shown in figure 3 below. The total amount of energy that was originally in the un-modulated clock does not magically disappear but rather is distributed over a wider band of frequencies.

Figure 3.

An actual spectrum analyzer scan of a modulated and non-modulated clock is shown is figure 4 below:

Figure 4.

Sweeping the frequency of the fundamental clock back and forth at some rate will cause a reduction in peak energy. The simple formula shown below demonstrates the relevance of a modulated clock to a non-modulated clock in terms of dB. This formula assumes an ideal clock with a 50% duty cycle and therefore only predicts the EMI reduction of odd harmonics and is intended to be used as a guideline to determining how much EMI reduction might be needed. Other circumstances such as non-ideal clock and ground noise will affect the actual dB reduction. The formula is as follows:

dB = 6.5 + 9(Log10 (F)) + 9(Log10 (BW))

Where; F = Frequency in MHz, BW = total % spread (2.5% = .025)

Using the above formula at a specific frequency, the following dB reduction curves can be plotted.

Figure 5.

Using a 66.6 MHz clock with a 2.0% spread, the theoretical dB reduction would be;

dB @ 66.6 MHz (Fund) = 7.62

dB @ 199.8 MHz (3rd) = 11.91

dB @ 333.0 MHz (11th) =13.91

Sweeping the VCO with a specific modulation envelope or profile creates the modulated clock. The modulation profile of the clock determines the relationship between time and frequency of the clock. As the clock is swept over its band of frequencies, it is stepped from one frequency to the next. The profile of the clock dictates how much time the clock will spend at any one frequency. A modulation profile is shown in figure 6 below.

Figure 6.

Several attributes of the modulation profile are exhibited in this scan using a Time Domain Analyzer. The reference frequency of the clock in figure 6 is 5.00 MHz. First we see that the frequency of the modulation envelope is 39.046 kHz. The frequency of the modulation profile should be above the audio range or greater than about 15 – 20 kHz. Some Low EMI clocks have fixed modulation rates and others determine the modulation rate as a function of the input frequency. In the case of the profile in figure 5, the modulation rate is determined by dividing the input frequency by fixed integer value.

Further inspection of the profile in figure 6 shows that the sweep rate increases when the frequency is nearing the peak frequency limits. This is done because the VCO must go through the same frequency twice in a shorter period of time. The most important observation to be made in figure 6 is the Peak-to-Peak spread of this clock generator. Figure 5 indicates that the Pk-Pk spread is 144.2 kHz. This means that the percentage spread of this clock is 144.2 kHz/5.00 MHz = 2.88 %.

Actually, this clock is spreading negative 2.88 % since the reference frequency is 5.0 MHz and all output frequencies of this clock are below 5.0 MHz and never go higher than 5.0 MHz. There are 2 fundamental types of Low EMI clock generators, center spread and down-spread clocks. A center-spread clock is one that spreads equally, plus and minus, about the reference frequency. If figure 6 were a center-spread clock, the output frequency would be centered on 5.00 MHz and sweep to 5.00 MHz + 1.44%, as well as sweep to 5.00 MHz – 1.44% of the reference frequency.

The second most important reason for using a down-spread clock would be when the original clock frequency of the system could not be increased without risking system failure. Spread Spectrum clocks with excessive amounts of bandwidth can be as harmful to a system as over-clocking.