STCD2400 datasheet(15/39 Pages) STMICROELECTRONICS

15 / 39 page

STCD22x0, STCD23x0, STCD24x0

Application information

Doc ID 15400 Rev 2

15/39

4.5

Phase noise

Phase noise is a frequency domain phenomenon and is a critical specification in reference

clocks. It is illustrated by a continuous spreading of the energy of the wave mainly caused by

random noise. The phase noise is normally specified with a unit of dBc/Hz at a given offset

in frequency (for example, 10 kHz) from the carrier wave (for example, 26 MHz). The value

of the phase noise is the difference of the power contained within 1 Hz bandwidth of the

offset frequency to the power at the carrier frequency. The total phase noise of the clock tree

is obtained by adding the additive phase noise of STCD22x0, STCD23x0 and STCD24x0

and the phase noise of the clock source (for example, TCXO) in power which is illustrated in

Equation 1.

Equation 1

where:

PNT is the total phase noise in dBc/Hz

PNC is the additive phase noise of STCD22x0, STCD23x0 and STCD24x0 and

PNX is the phase noise of clock source

Make sure the total phase noise is kept within the phase noise requirement of each

application PNA. The user should choose the right TCXO with proper phase noise to meet

the requirement.

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