
84330AV-02
www.idt.com
REV. B JULY 25, 2010
12
ICS84330-02
700MHZ, LOW JITTER, CRYSTAL-TO-3.3V
DIFFERENTIAL LVPECL FREQUENCY SYNTHESIZER
If the FREF_EXT input is driven by a 3.3V LVCMOS driver, the
jitter performance can be improved by reducing the amplitude
swing and slowing down the edge rate. Figure 7A shows an
amplitude reduction approach for a long trace. The swing will
be approximately 0.85V for logic low and 2.5V for logic high
R2
100
VDD
R1
100
Ro ~ 7 Ohm
Driver_LVCMOS
Zo = 50 Ohm
Td
RS
43
VDD
GND
TEST_CLK
VDD
GND
TEST_CLK
R1
200
RS
100
Ro ~ 7 Ohm
Driver_LVCMOS
R2
200
VDD
R1
400
R2
400
Ro ~ 7 Ohm
Driver_LVCMOS
RS
200
VDD
GND
TEST_CLK
JITTER REDUCTION FOR FREF_EXT SINGLE END INPUT
(instead of 0V to 3.3V). Figure 7B shows amplitude reduction
approach for a short trace. The circuit shown in Figure 7C
reduces amplitude swing and also slows down the edge rate
by increasing the resistor value.
FIGURE 7C. EDGE RATE REDUCTION BY INCREASING THE RESISTOR VALUE
FIGURE 7A. AMPLITUDE REDUCTION FOR A LONG TRACE
FIGURE 7B. AMPLITUDE REDUCTION FOR A SHORT TRACE
FREF_EXT