Phase locked loop (PLL)

AN2352

16/20

Device noise of the circuit in the PLL

Long term jitter is inversely proportional to the bandwidth of the PLL: The wider the loop

bandwidth, the lower the jitter due to noise in the loop. Moreover, long term jitter is

practically independent of the multiplication factor.

The most noise sensitive circuit in the PLL circuit is definitely the VCO (voltage controlled

oscillator). There are two main sources of noise: thermal (random noise, frequency

independent thus practically white noise) and flicker (low frequency noise). These two noise

sources lead to a jitter as illustrated in

Figure 5. This figure shows three distinctive zones:

–

the first zone in which the R.M.S. value of the accumulated jitter is

proportional to

time interval,

–

the second zone in which the R.M.S. value of the accumulated jitter is

proportional

–

the third zone where for a large value of N, a saturation effect is evident, so the

jitter does not grow anymore when considering a longer time interval (jitter stable

increasing the number of clock periods N). The saturation value corresponds to

what has been called self referred long term jitter of the PLL. In

Figure 5 the

maximum jitter trend versus the number of clock periods N (for some typical CPU

frequencies) is reported: The curves represent the very worst case, computed

taking into account all corners of temperature, power supply and process

variations; the real jitter is always measured well below the given worst case

values.

Noise in supply and substrate

Digital supply noise adds determining elements to PLL output jitter, independent of the

multiplication factor. Its effect is strongly reduced thanks to particular care used in the

physical implementation and integration of the PLL module inside the device. In any case,

the contribution of digital noise to global jitter is widely taken into account in the curves

provided in

Figure 5.

Figure 5.

ST10F27x PLL jitter

N (CPU clock periods)

200

400

±5

±1

800

600

1000

1200

0

0

±2

±3

±4

24 MHz

40 MHz

32 MHz

16 MHz

TJIT

64 MHz

1400