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SD-A2989-FREQ Datasheet(PDF) 10 Page - Nel Frequency Controls,inc

No. de Pieza. SD-A2989-FREQ
Descripción  Differential Positive ECL (DPECL 3.3V)
Descarga  13 Pages
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Fabricante  NEL [Nel Frequency Controls,inc]
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SD-A2989-FREQ Datasheet(HTML) 10 Page - Nel Frequency Controls,inc

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1Crystal Oscillator and Logic Selection
Selecting the appropriate crystal oscillator is of
the utmost importance in a high speed application,
since it will provide the clock reference for the
entire clock distribution system.
Stringent crystal oscillator applications typically
require a frequency stability of ±20 ppm, fast rise
and fall times of less than 600 picoseconds, low
characteristic jitter, and a Positive Emitter Coupled
Logic (PECL) differential output. The frequency
stability will provide a reliable system reference,
while fast rise and fall times of the waveform will
result in low system jitter. (Although saturating the
transition with fast rise and fall times can introduce
unwanted noise, this noise will be cancelled out by
the use of differential signals.)
Logic Selection: PECL Advantages
Using a PECL logic output provides critical
advantages over CMOS logic output technology
in high speed applications. Unlike CMOS technol-
ogy, PECL technology features a differential out-
put, which is essential for reducing emissions. Yet,
like CMOS, PECL obtains its operating power
from a positive power supply (rather than the neg-
ative power supply voltage that powers ECL logic
technology), enabling the necessary compatibility
with CMOS logic interfaces at the load points.
In addition, PECL technology allows voltage
compensation for further rejection of noise on
the positive voltage supply. All modern PECL
devices contain on-chip bandgap regulators that
provide voltage compensation for noise margins
with variations in the supply voltage, as well as
in junction and ambient temperatures. Because
PECL circuits consist of supply-regulated current
sources which are switched via steering logic to
load resistors, the designer benefits in two ways:
1) the supply current remains unchanged with
operating frequency
2) AC performance remains unchanged with
voltage, temperature, and frequency
Residual sensitivities of less than 1mV/V for levels,
threshold and noise margins with respect to supply
voltage may be achieved.
For junction temperatures, residual sensitivities
of less than 0.1mV/C are achieved for the same
Crystal Oscillator Quality
In addition to ensuring low jitter in the waveform,
designers should be sure that jitter is minimized
in the oscillator itself. This is achieved by selecting
an oscillator containing a very high Q crystal.
Further, the crystal should be tuned to the
oscillator circuit for optimization by the oscillator
manufacturer. Use of a PLL synthesizer in the
oscillator design should be avoided, since jitter
is created by the noise in the phase lock loop.
Other Oscillator Considerations
In PECL systems, all oscillator circuits should
have the power supply well decoupled at the
oscillator. PECL devices need to have this
addressed aggressively, since PECL is referenced
only to the most positive side of the power
supply. Thus, for PECL the Vcc needs to be
as noise-free as possible. Because oscillator
characteristics do change with load impedance
and load bias voltage, it is important to specify
the actual load being used and communicate
this to the oscillator vendor.
The chosen oscillator should have a tight symme-
try of at least 45% minimum and 55% maximum,
and should produce repeatable waveforms to
ensure signal consistency. A ground plane should
be used (see “Design Subtleties” section).
2 Clock Driver/Distribution Considerations
The clock driver should be a PECL differen-
tial device—with differential inputs to receive
the oscillator signals, and differential outputs to
distribute the signals on the PCB. If clock gating
is desired, there should be an Enable pin, probably
single ended.
Another aspect of clock driving should be struc-
tural symmetry of the device, which will reflect in
better overall signal integrity.
Regeneration buffering may be required when trace
length and/or attenuation demand it. It is impor-
tant to structure regeneration such that received sig-
nals have settled out and are not still on their rising
An effective methodology for
achieving optimum system
performance addresses the
various clock generation
and distribution components
as a complete solution.
Careful selection of the
appropriate components
should take place at the
outset of the project, keep-
ing in mind the components’
interrelation to one another.
Transmission Load
Transmission Load
Transmission Load
Transmission Load
Transmission Load
Transmission Load
Transmission Load
Transmission Load
Clock Generation and
Distribution Component Considerations
Fig. 2

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