ST10F276 datasheet(199/229 Pages) STMICROELECTRONICS

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ST10F276

Electrical characteristics

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Example 2

–FXTAL = 8 MHz

–

P0(15:13) = ‘100’ (multiplication by 5)

–

PLL input frequency = 2 MHz

–

VCO frequency = 80 MHz

–

PLL output frequency = 40 MHz (VCO frequency divided by 2)

–FCPU = 40 MHz (no effect of output prescaler)

23.8.9

PLL Jitter

Two kinds of PLL jitter are defined:

●

Self referred single period jitter

Also called "Period Jitter", it can be defined as the difference of the Tmax and Tmin,

where Tmax is the maximum time period of the PLL output clock and Tmin is the

minimum time period of the PLL output clock.

●

Self referred long term jitter

Also called "N period jitter", it can be defined as the difference of Tmax and Tmin, where

Tmax is the maximum time difference between N + 1 clock rising edges and Tmin is the

minimum time difference between N + 1 clock rising edges. Here N should be kept

sufficiently large to have the long term jitter. For N = 1, this becomes the single period

jitter.

Jitter at the PLL output is caused by:

●

Jitter in the input clock

●

Noise in the PLL loop

23.8.10

Jitter in the input clock

PLL acts like a low pass filter for any jitter in the input clock. Input Clock jitter with the

frequencies within the PLL loop bandwidth is passed to the PLL output and higher frequency

jitter (frequency > PLL bandwidth) is attenuated at 20dB/decade.

23.8.11

Noise in the PLL loop

This condition again is attributed to the following sources:

●

Device noise of the circuit in the PLL

●

Noise in supply and substrate

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, 1/f). For the

frequency characteristics of the VCO circuitry, the effect of the thermal noise results in a 1/f2

region in the output noise spectrum, while the flicker noise in a 1/f3. Assuming a noiseless

PLL input and supposing that the VCO is dominated by its 1/f2 noise, the R.M.S. value of the

accumulated jitter is proportional to the square root of N, where N is the number of clock

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