SD-A2980 Datasheet, PDF (9/13 Pages) Nel Frequency Controls,inc – Differential Positive ECL (DPECL 3.3V)

The Challenges of High Speed Digital Clock Design
In high speed applications,
the faster the signal moves
through the transition
region, the less jitter will
be produced.
Designing clock generation and distribution
systems for today’s high speed digital electronic
devices poses numerous challenges to the design
community. At higher speeds, transmission lines
and their components behave differently than
they do at lower speeds, generating such signal
integrity problems as jitter, noise, reflections, and
crosstalk if not properly specified and configured.
Therefore, when designers approach a project
that will have a high speed digital application,
they must factor in a variety of signal integrity
provisions that are not necessary in lower speed
applications.
Key challenges of planning a high speed digital
project include:
Minimizing timing jitter. It is critical for high
speed, high frequency electronics to have low
timing jitter. Poor jitter characteristics not only
affect data error, but also could cause failures in
phase lock loops using this source as a reference.
FAST TRANSITION TIME REDUCES SYSTEM JITTER
THRESHOLD REGION
THRESHOLD REGION
If the source is to be used as a display clock
reference, the result will be a blurry display.
As a rule, the faster the signal moves through
the transition region, the less system jitter will
be produced (see Fig. 1).
Reducing emissions. In high speed applications,
the likelihood of generating electromagnetic inter-
ference (EMI) increases dramatically. FCC regula-
tions regarding EMI noise reduction are becoming
more stringent with faster digital speeds. Designers
need to address such characteristics as transmission
lines, differential signals, signal amplitude, and
harmonic content in order to maximize the energy
that will be delivered to the load, thus reducing
the amount of energy emissions.
Ensuring stability. In general, the higher the
specified operating frequency of the electronic
system you are designing for, the more critical
the clock stability is. Unstable clock performance
can cause an increased bit error rate, erroneous
data, or missed data in digital systems, whether
they are local or wide area systems.
Transmission line impedance matching. The
impedance and length of the entire transmission
line must be measured and matched with each
termination. If impedance matching is overlooked,
emissions, crosstalk, and reflections can occur.
SLOW TRANSITION TIME
Fig. 1
FAST TRANSITION TIME
Power supply considerations. The prime consid-
eration here is to make sure that the clock is
noise-free. Low power supply consumption
requirements are also increasing with today’s
higher speed systems.
An Effective Methodology
The key to achieving optimum system performance
in a high speed application starts with an effective
design methodology for clock generation and dis-
tribution. Put simply, the designer should adopt a
methodology that addresses the various clock gen-
eration and distribution components as a complete
solution, not as individual parts. Careful attention
to the selection of the appropriate components and
circuit distribution method should be given at the
outset of the project, keeping in mind the interrela-
tion of the components to one another. Further,
it is important to consider the characteristic imped-
ance of all active and passive components at the
frequency of operation as the design progresses.
Proper selection of the following clock generation
and distribution components is essential (see Fig. 2):
1. The crystal oscillator and its output logic
2. The clock driver, which in some cases will
contain enable functions
3. Translators to CMOS at 5V or 3V supply
4. The transmission line (twisted pair, coax,
PCB traces)
This white paper is intended to help you make
informed decisions about these clock generation
and distribution components as you approach your
next high speed digital system design.