SY89429A
FREQUENCY
SYNTHESIS
APPLICATION NOTE
AN-06
Introduction
Power Supply Requirements
Micrel-Synergy's SY89429A frequency synthesizer is
SY89429A is designed to operate with a single +5V
designed to be used in various clock subsystems. The supply. The FOUT and /FOUT (the differential PECL
Primary function of the product is to synthesize clock outputs) will interface to PECL inputs of any +5 volt
frequencies required for systems needing a high quality, system. However, SY89429A can also be used in split
low jitter clock source.
supply (+5V and +3.3V) systems as well as true ECL
The cost of other clock sources, either crystal or SAW systems. In a split supply application, the main VCC lines,
oscillators, increase dramatically as precision/frequency (VCC1, VCC_QUIET, and VCC_TTL) are connected to +5V.
requirements of digital systems push into the 100+ MHz The VCC_OUT can be connected to either +3.3V or +5.0V.
arena. Many low cost CMOS frequency synthesizers When applying +3.3V to VCC_OUT, the differential PECL
appeared in the market in the last few years. outputs will interface to PECL inputs of any +3.3V device.
Unfortunately, these products have relatively high jitter The GND and GND (TTL) pins are connected to the
and limited operating frequency range. Therefore, their system ground in both cases. For split supply system,
applications are limited to lower precision/lower please refer to the section titled “Split Supply Design.”
frequencies.
For the ECL system, please refer to the section titled
SY89429A, designed with Micrel-Synergy's high “True ECL Design.”
performance ASSET Bipolar technology and differential
Power Supply Filtering Techniques
ECL circuit technology throughout, is a perfect low cost
alternative to the expensive crystal or SAW oscillators.
Unlike other frequency synthesizers, SY89429A has
extremely low jitter and high supply noise rejection that
ECL is famous for.
Because the device is programmable between 25MHz
to 400MHz using a 16MHz crystal, different system
frequency requirements can all use the same device.
This may dramatically reduce inventory costs and
management of additional products otherwise required
to achieve these various frequencies. This
programmability also makes board/system speed grading
possible as part of the normal production flow without
multiple oscillators. This provides higher overall yield and
lower manufacturing cost.
As in any high speed integrated circuits, power supply
filtering is very important. A 0.1µF high frequency by-
pass capacitor should be used between all separate
power supply pins and ground. VCC1, VCC_QUIET,
VCC_TTL and VCC_OUT should be individually connected
to the power supply plane through vias, and a by-pass
capacitor should be used for each pin. To achieve
optimum jitter performance, better power supply isolation
is required. In this case a ferrite bead along with a 1µF
and a 0.01µF by-pass capacitor should be connected to
each power supply pin. Figure 4 shows the connections
of the power supply filtering using ferrite beads.
Termination For PECL Outputs
The differential PECL outputs, FOUT and /FOUT, are
open emitter outputs. Therefore, terminating resistors or
current sources must be used for functionality. These
outputs are designed to drive 50Ω transmission lines.
Matched impedance techniques should be used to
maximize operating frequency and minimize wave-form
distortion. There are a few simple termination schemes.
Figure 1 shows 3 simple termination circuits for a +5V
system.
In addition to cost savings, there are many other
benefits to using SY89429A. Normal system production
testing can incorporate frequency margining that is
unavailable to fixed frequency designs as in crystal or
SAW oscillators. This capability leads directly to higher
product quality and reliability. Furthermore, SY89429A
can be programmed in small steps (1MHz steps with a
16.000MHz crystal). Other precise frequencies can be
programmed as well. See section titled “Advance
Frequency Control Applications.” This ability to provide
any frequency eliminates the need for the high cost
custom oscillator alternatives.
Interface For Inputs
The SY89429A is designed to interface with TTL
compatible signals. All inputs except XTAL1 and XTAL2
are TTL compatible. These inputs have internal pull up
resistors. Therefore, any inputs can be left open—open
inputs are logical “1” state. Although inputs can be left
open, it is recommended that open inputs be connected
to a power supply line. These inputs can be connected
to a power supply line (VCC for a logical “1”) or a ground
line (VEE for a logical “0”) directly or through series
resistors. Alternatively, these inputs can also be driven
directly from any TTL compatible signals.
General Requirements
Operating the SY89429A is very simple. Very few low
cost external components are required. These low cost
external components provide the tuning capability needed
to optimize and minimize jitter characteristics in each
individual system application. To achieve the best
possible performance in jitter and power supply noise
rejection, basic high speed design guide lines should be
followed.
Rev.: G
Amendment:/0
Issue Date: August, 1999
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