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*F Page 4 of 12 [+] Feedback FS781/82/84 Table 4. Modulation Rate Divider Ratios S1 S0 Input Frequency Range (MHz) Modulation Divider Number 0 0 6 to 16 120 0 1 16 to 32 240 1 0 32 to 66 480 1 1 66 to 82 720 SSCG Modulation Profile The digital control inputs S0 and S1 determine the modulation frequency of FS781/2/4 products. The input frequency is divided by a fixed number, depending on the operating range that is selected. The modulation frequency of the FS78x can be determined from Table 4. To compute the modulation frequency, determine the values of S0 and S1, and find the modulation divider number in Table 4. Theory of Operation The FS781/82/84 devices are phase-locked loop-(PLL)-type clock generators using Direct Digital Synthesis (DDS). ‘By precisely controlling the bandwidth of the output clock, the FS781/2/4 products become a low-EMI clock generator. The theory and detailed operation of these products will be discussed in the following sections. EMI All clocks generate unwanted energy in their harmonics. Conventional digital clocks are square waves with a duty cycle that is very close to 50%. Because of the 50/50 duty cycle, digital clocks generate most of their harmonic energy in the odd harmonics (e.g., third, fifth, seventh). It is possible to reduce the amount of energy contained in the fundamental and harmonics by increasing the bandwidth of the fundamental clock frequency. Conventional digital clocks have a very high Q factor, which means that all of the energy at that frequency is concentrated in a very narrow bandwidth, conse quently, higher energy peaks. Regulatory agencies test electronic equipment by the amount of peak energy radiated from the equipment. By reducing the peak energy at the fundamental and harmonic frequencies, the equipment under test is able to satisfy agency requirements for EMI. Conventional methods of reducing EMI have been to use shielding, filtering, multi-layer PCBs, etc. These FS781/2 and 4 reduce the peak energy in the clock by increasing the clock bandwidth and lowering the Q of the clock. SSCG The FS781/82/84 products use a unique method of modulating the clock over a very narrow bandwidth and controlled rate of change, both peak to peak and cycle to cycle. The FS78x products take a narrow band digital reference clock in the range of 6–82 MHz and produce a clock that sweeps between a controlled start and stop frequency and precise rate of change. To understand what happens to an SSCG clock, consider that we have a 20-MHz clock with a 50% duty cycle. From a 20-MHz clock we know the following: Clock Frequency = Fc = 20 MHz. Clock Period = Tc = 1/20 MHz = 50 ns. Consider that this 20-MHz clock is applied to the XIN input of the FS78x as either an externally driven clock or the result of a parallel resonant crystal connected to pins 1 and 2 of the FS78x. Also consider that the products are operating from a 5V DC power supply and the loop filter is set for a total bandwidth spread of 2%. Refer to Figure 2. Xin + .5% - .5% 1.0% Total TIME (microseconds) Figure 1. Frequency Profile in Time Domain[5] Note: 5. With the correct loop filter connected to Pin 4, the following profile will provide the best EMI reduction. This profile can be seen on a Time Domain Analyzer. Document #: 38-07029 Rev. *F Page 5 of 12 [+] Feedback 50% 50% Tc = 50 ns. 50% 50% Tc = 50 ns. Figure 2. 20-MHz Unmodulated Clock From the above parameters, the output clock at FSOUT will be sweeping symmetrically around a center frequency of 20 MHz. The minimum and maximum extremes of this clock will be +200 kHz and –200 kHz. So we have a clock that is sweeping from 19.8 MHz to 20.2 MHz and back again. If we were to look at this clock on a spectrum analyzer we would see the picture in Figure 3. Keep in mind that this is a drawing of a perfect clock with no noise. Fc = 20 MHz Fmin = 19.8 MHz Fmax = 20.2 MHz Figure 3. Spectrum Analysis of 19.8–20.2 MHz Clock We see that the original 20-MHz reference clock is at the center frequency (Cf), and the min. and max. extremes are positioned symmetrically about the center frequency. This type of modulation is called Center-Spread. Figure 4 shows a 20-MHz clock as it would be seen on an oscilloscope. The top trace is the non-modulated reference clock. The bottom trace is the modulated clock at pin 6. From this comparison chart you can see that the frequency is decreasing and the period of each successive clock is increasing. The Tc measurements on the left and right of the bottom trace indicate the max. and min. extremes of the clock. Intermediate clock changes are small and accumulate to achieve the total period deviation. The reverse of this figure would show the clock going from minimum extreme back to the high extreme. FS781/82/84 Tc = 50.50 nTc =49.50 ns. Figure 4. Period Comparison Chart Looking at Figure 3, you will note that the peak amplitude of the 20-MHz non-modulated clock is higher than the wideband modulated clock. This difference in peak amplitudes betwe...
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