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 = | Fmax = |
19.8 MHz | 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.
Tc =49.50 ns. | Tc = 50.50 n |
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 between modulated and unmodulated clocks is the reason why SSCG clocks are so effective in digital systems. This figure refers to the fundamental frequency of a clock. A very important charac- teristic of the SSCG clock is that the bandwidth of the funda- mental frequency is multiplied by the harmonic number. In other words, if the bandwidth of a 20-MHz clock is 200 kHz, the bandwidth of the third harmonic will be 3 × 200, or 600 kHz. The amount of bandwidth is relative to the amount of energy in the clock. Consequently, the wider the bandwidth, the greater the energy reduction of the clock.
Most applications will not have a problem meeting agency specifications at the fundamental frequency. It is the higher harmonics that usually cause the most problems. With an SSCG clock, the bandwidth and peak energy reduction increases with the harmonic number. Consider that the eleventh harmonic of a 20-MHz clock is 220 MHz. With a total spread of 200 kHz at 20 MHz, the spread at the eleventh harmonic would be 2.20 MHz, which greatly reduces the peak energy content. It is typical to see as much as 12- to 18-dB reduction at the higher harmonics, due to a modulated clock.
The difference in the peak energy of the modulated clock and the non-modulated clock in typical applications will see a 2 – 3 dB reduction at the fundamental and as much as 8 – 10 dB reduction at the intermediate harmonics: third, fifth, seventh, etc. At the higher harmonics, it is quite possible to reduce the peak harmonic energy, compared to the unmodu- lated clock, by as much as 12 to 18 dB.
Application Notes and Schematic
Figure 5 is configured for the following parameters: Package selected = FS781.
XIN = 20-MHz crystal
FSOUT = 20 MHz (S0 = 1 and S1 = 0).
Bandwidth of the FSOUT clock is determined by the values of the loop filter connected to pin 4.
