SD-A2980 Datasheet, PDF (11/13 Pages) Nel Frequency Controls,inc – Differential Positive ECL (DPECL 3.3V)

The differential output
provided by PECL technology
rejects common mode noise in
the transmission line, thereby
minimizing emissions.
or falling edge. Otherwise further attenuation
may be generated rather than signal buffering.
3 Translator Requirements
If CMOS is to be driven, PECL-to-
CMOS conversions will be needed. A dedi-
cated translator should be used for each
application-specific point. It is important
to locate each translator as close to the load
point as possible in order to maintain good
signal integrity.
4 Transmission Line Considerations
For any high speed and high frequency
clock distribution system, properly config-
ured and terminated transmission lines are
a requirement. Following are some of the
basic characteristics required for a good
transmission line.
• The source, transmission line, and load
impedances need to match as closely
as possible to minimize losses in the
transmission line.
• The transmission line should be free
of discontinuities which can cause poor
performance.
• The transmission line should not have
stubs or branches which can result in
poor performance.
• The load must be at the opposite end
of the transmission line from the source.
• The source must be capable of driving
the transmission line impedance.
Differential Schemes
A differential transmission scheme should be
adopted which makes use of the differential
nature of both the output of PECL drivers
and the input of PECL receivers.
The differentially transmitted signals will have
a very large common mode rejection range
(to capacitively and inductively coupled noise
signals) and will be insensitive to supply volt-
age and temperature variations. The resulting
transmission system will be quite immune
to external noise sources, and will therefore
minimize emissions.
Termination and Layout
Proper terminations are necessary to maximize
power transfer while preventing signal reflec-
tions (bouncebacks) and noise.
Termination methods cause an undue amount
of confusion. The following tips can help
avoid confusion and will help meet the
requirements of complex digital systems.
1. Print the highest impedance level line that
can be manufactured on the PCB with rea-
sonable repeatability. Usually this will be
100 ohms, and it will reduce the power
demand for termination, if compared to
a 50 ohm scheme, by a factor of two.
2. If group delay is a concern, run the two
traces on the top of the PCB, or within
the PCB (propagation delay is a function
of dielectric constant).
3. The width between each output trace
should be five times the width of each trace,
in order to minimize crosstalk. Route the
traces directly to the various tap points—
stubs should be avoided, since they generate
noise. If the tap point is a translator to
CMOS, place it as close as is practical to
the CMOS logic to be driven.
4. Each termination should be built as a
source-terminated input to the transmission
line or the destination-termination should
be structured as the Thevenin equivalent
of the characteristic line impedance (50-
100 ohms). Either method requires two
resistors as a termination but eliminates
the need for termination supply voltage
(and its distribution plane).
5. At 3.3V, the termination may be a
single 100 ohm resistor to ground, if
100 ohms lines are used.
Resistor Considerations
A distinction should be made between pull-
down resistors, which simply provide a load
current for the open emitter follower, and
termination resistors.
For very short connections of low parasitic
capacitance, 2 kohms to GND may be practi-
cal. If, however, a longer distance must be
bridged by the interconnect, this interconnect
will have to be terminated by its characteristic
impedance in order to maximize power trans-
fer and minimize reflections. The characteris-
tic impedance of such interconnects generally
lies between 40 and 120 ohms.
Given this range and the output drive
capability of the emitter followers, the
termination resistor can also serve the
purpose of the pull-down resistor, but
in several termination schemes, they are
indeed kept as separate components.