E
vo
C
LOCK
HIGH PERFORMANCE CLOCK GENERATOR
REVISION P
r
B – DECEMBER 2011
Copyright © 2011, M2Tech Srl
9
M2Tech
www.m2tech.biz
5. Jitter sources in a digital system
Jitter is the main detriment factor in a digital system. Even asynchronous connections
based on precision oscillators may fail to achieve the ultimate performance theoretically at
their hand due to jitter which is injected in the system.
Jitter has various causes. One is the inherent phase noise in every oscillator. Different
kinds of crystal oscillators allow for different phase noise, thus different jitter levels. The
most basic crystal oscillator is sensitive to many phase noise sources: power supply,
coupled EMI, internal instabilities of the quartz, random temperature changes. Precision
XO have an internal compensation for the power supply noise, while TCXO (temperature
compensated crystal oscillators) have a reduced temperature sensitivity thanks to specific
compensation circuits. OCXO (oven compensated crystal oscillators) theoretically allow for
the lowest phase noise.
In commercial circuits, an OCXO is a waste of money (they are very expensive), as the
environmental EMI and the power supplies which are generally used inject far more noise
then the theoretical device’s noise floor.
A good TCXO, with very low phase noise, coupled to low noise supplies in a well-shielded
metal cabinet is the best choice for a cost-effective, high performance clock generator.
The
E
vo
C
LOCK
is designed keeping this guidelines in mind: very low phase noise custom
made TCXO’s are used, along with special low noise regulators to achieve very high
performance.
