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• Measured Y1: 48 MHz

• Y0 is 27 MHz (XTAL buffered ,loaded by 50O)

• Y2 is 56.448 MHz (loaded by 50O)

• Y3 is 33.33 MHz (loaded by 50O)

• Y4, Y5 tri−stated

carrier

48MHz

spur at

27MHz&54MHz

carrier

48MHz

carrier

48MHz

spurs at

47MHz&49MHz

carrier

48MHz

spurs at

47MHz&49MHz

spur 47MHz and

fundamental at 1MHz

w Measured Y0: 48 MHz

w Y1, Y2, Y3, Y4 & Y5 tri−stated

w Inserted 30mV 1MHz @ Vcc = 3.3V

Phase Noise Characteristic

CDCE906

SCAS814 – NOVEMBER 2005

Figure 25. Noise Coupling and Spur Suppression

Figure 26. Power Supply Noise Rejection

In high-speed communication systems, the phase noise characteristic of the PLL frequency synthesizer is of high

interest. Phase noise describes the stability of the clock signal in the frequency domain, similar to the jitter

specification in the time domain.

Phase noise is a result of random and discrete noise causing a broad slope and spurious peaks. The discrete

spurious components could be caused by known clock frequencies in the signal source, power line interference,

and mixer products. The broadening caused by random noise fluctuation is due to phase noise. It can be the

result of thermal noise, shot noise and/or flicker noise in active and passive devices.

Important factor for PLL synthesizer is the loop bandwidth (–3 dB cut-off frequency) — large LBW results in fast

transient response but have less reference spur attenuation. The LBW of the CDCE906 is about 100 kHz to 150

kHz, dependent on selected PLL parameter.

For the CDCE906, two phase noise characteristics are of interest: The phase noise of the crystal-input stage and

the phase noise of the internal PLL (VCO). The following Figure shows the respective phase noise characteristic.

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