ICS8545I

LOW SKEW, 1-TO-4, LVCMOS/LVTTL-TO-LVDS FANOUT BUFFER

IDT™ / ICS™ LVDS FANOUT BUFFER

6

ICS8545BGI REV. B MARCH 02, 2009

Additive Phase Jitter

The spectral purity in a band at a specific offset from the

fundamental compared to the power of the fundamental is called

the dBc Phase Noise. This value is normally expressed using a

Phase noise plot and is most often the specified plot in many

applications. Phase noise is defined as the ratio of the noise power

present in a 1Hz band at a specified offset from the fundamental

frequency to the power value of the fundamental. This ratio is

expressed in decibels (dBm) or a ratio of the power in the 1Hz band

to the power in the fundamental. When the required offset is

specified, the phase noise is called a dBc value, which simply

means dBm at a specified offset from the fundamental. By

investigating jitter in the frequency domain, we get a better

understanding of its effects on the desired application over the

entire time record of the signal. It is mathematically possible to

calculate an expected bit error rate given a phase noise plot.

As with most timing specifications, phase noise measurements

have issues. The primary issue relates to the limitations of the

equipment. Often the noise floor of the equipment is higher than

the noise floor of the device. This is illustrated above. The device

meets the noise floor of what is shown, but can actually be lower.

The phase noise is dependant on the input source and

measurement equipment.

Offset Frequency (Hz)