CN-0294 Datasheet, PDF (2/5 Pages) Analog Devices – Increasing the Number of Outputs from a Clock Source Using Low Jitter LVPECL Fanout Buffers

CN-0294
Modern digital systems often require many high quality clocks
at logic levels that are different from the logic level of the clock
source. Extra buffering may be required to guarantee accurate
distribution to other circuit components without loss of integrity.
The interface between the ADF4351clock source ADCLK948
clock fanout buffer is described, and measurements show that the
additive jitter associated with the clock fanout buffer is 75 fs rms.
CIRCUIT DESCRIPTION
The ADF4351 is a wideband PLL and VCO consisting of
three separate multiband VCOs. Each VCO covers a range
of approximately 700 MHz (with some overlap between the
frequencies of the VCO). This permits a fundamental VCO
frequency range of between 2.2 GHz to 4.4 GHz. Frequencies
lower than 2.2 GHz can be generated using internal dividers
within the ADF4351.
For clock generation, the ADF4351 PLL and VCO must be
enabled, and the desired output frequency must be programmed.
The output frequency of the ADF4351 is available at the open-
collector outputs at the RF_OUT pins, which require a shunt
inductor (or resistor), plus a dc blocking capacitor.
The ADCLK948 is a SiGe low jitter clock fanout buffer that is
ideally suited for use with the ADF4351, because its maximum
input frequency (4.5 GHz) is just above that of the ADF4351
(4.4 GHz). Broadband rms additive jitter is 75 fs.
It is necessary to add a dc common-mode bias level of 1.65 V to
the CLK inputs of the ADCLK948 to mimic LVPECL logic levels.
This is accomplished by the use of a resistor bias network.
Omission of the dc bias circuit results in degraded signal
integrity at the ADCLK948 outputs.
COMMON VARIATIONS
Other possible synthesizers with integrated VCOs are the
ADF4350 fractional N (137 MHz to 4400 MHz) and the
ADF4360 integer N series.
Other possible clock fanout buffers in the same family as
the ADCLK948 are the ADCLK946 (6 LVPECL outputs),
ADCLK950 (10 LVPECL outputs), and the ADCLK954
(12 LVPECL outputs).
CIRCUIT EVALUATION AND TEST
The circuit is evaluated using the EVAL-ADF4351EB1Z board
for a clock source, with some minor modifications. The EVAL-
ADF4351EB1Z board uses the standard ADF4351 programming
software contained on the CD that accompanies the evaluation
board. The ADCLK948/PCBZ is also required and can be used
out of the box without modification.
Circuit Note
Equipment Needed
The following equipment is needed:
• The EVAL-ADF4351EB1Z evaluation board kit with
programming software
• The ADCLK948PCBZ evaluation board
• A 3.3 V power supply
• Two cables to connect the 3.3 V supply to the
ADCLK948PCBZ
• Two short equal length SMA coaxial cables
• A high speed oscilloscope (2 GHz bandwidth) or an
equivalent
• The R&S FSUP26 spectrum analyzer or an equivalent
• A PC with Windows® XP, Windows, Vista (32-bit), or
Windows 7 (32-bit)
The SMA coaxial cable is required to connect the RF_OUTA+ and
RF_OUTA− pins of the EVAL-ADF4351EB1Z to CLK0 and CLK0
pins of the ADCLK948PCBZ.
Functional Block Diagram
For this experiment, the ADCLK948PCBZ and the EVAL-
ADF4351EB1Z are used. The boards are connected via a SMA
cable to the ADCLK948PCBZ, as shown in Figure 1.
POWER
SUPPLY
ADF4351
EVALUATION BOARD
(EVAL-ADF4351EB1Z)
RFOUTA+
RFOUTA−
3.3V
COM
T7
CLK0
J4
ADCLK948/PCBZ
CLK0 ^{EVALUATION BOARD}
J2
USB
OUT2
OUT2
HIGH-SPEED
OSCILLOSCOPE
PC
R&S RTO1024
Figure 2. ADF4351 Logic Level Measurement Setup
Getting Started
The UG-435 user guide details the installation and use of the
EVAL-ADF4351EB1Z evaluation software. UG-435 also
contains board setup instructions and the board schematic,
layout, and bill of materials. Necessary modifications to the board
are the insertion of 100 Ω resistors after the dc blocking capacitor.
The resistors are connected to 3.3 V and to GND. This should
be done to both the RF_OUTA+ and RF_OUTA− pins to provide a
common-mode voltage of 1.65 V (above the minimum required
1.5 V). This may necessitate scraping off the solder mask near
these transmission lines.
The UG-068 user guide contains similar information relevant to
the operation of the ADCLK948/PCBZ evaluation board
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