Datasheet

Data Sheet AD9523
Rev. C | Page 21 of 60
COMPONENT BLOCKS—INPUT PLL (PLL1)
PLL1 General Description
Fundamentally, the input PLL (referred to as PLL1) consists of
a phase-frequency detector (PFD), charge pump, passive loop
filter, and an external VCXO operating in a closed loop.
PLL1 has the flexibility to operate with a loop bandwidth of
approximately 10 Hz to 100 Hz. This relatively narrow loop
bandwidth gives the AD9523 the ability to suppress jitter that
appears on the input references (REFA and REFB). The output
of PLL1 then becomes a low jitter phase-locked version of the
reference input system clock.
PLL1 Reference Clock Inputs
The AD9523 features two separate differential reference clock
inputs, REFA and REFB. These inputs can be configured to
operate in full differential mode or single-ended CMOS mode.
In differential mode, these pins are internally self-biased. If
REFA or REFB is driven single-ended, the unused side (
REFA
,
REFB
) should be decoupled via a suitable capacitor to a quiet
ground.
Figure 21 shows the equivalent circuit of REFA or REFB.
It is possible to dc-couple to these inputs, but the dc operation
point should be set as specified in the Specifications tables.
To operate either the REFA or the REFB inputs in 3.3 V CMOS
mode, the user must set Bit 5 or Bit 6, respectively, in Register
0x01A (see Table 41). The single-ended inputs can be driven by
either a dc-coupled CMOS level signal or an ac-coupled sine
wave or square wave.
The differential reference input receiver is powered down when
the differential reference input is not selected, or when the PLL
is powered down. The single-ended buffers power-down when
the PLL is powered down, when their respective individual power-
down registers are set, or when the differential receiver is selected.
The REFB R divider uses the same value as the REFA R divider
unless Bit 7, the enable REFB R divider independent division
control bit in Register 0x01C, is programmed as shown in Table 43.
PLL1 Loop Filter
The PLL1 loop filter requires the connection of an external
capacitor from LF1_EXT_CAP (Pin 7) to ground. The value of the
external capacitor depends on the use of an external VCXO, as
well as such configuration parameters as input clock rate and
desired bandwidth. Normally, a 0.3 µF capacitor allows the loop
bandwidth to range from 10 Hz to 100 Hz and ensures loop
stability over the intended operating parameters of the device
(see Table 44 for R
ZERO
values).
R
ZERO
C
POLE1
R
POLE2
C
POLE2
CHARGE
PUMP
LF1_EXT_CAP
LDO_PLL1
BUFFER
1kΩ
0.3µF
OSC_CTRL
TO VCXO
V
TUNE
INPUT
08439-022
AD9523
Figure 23. PLL1 Loop Filter
Table 20. PLL1 Loop Filter Programmable Values
R
ZERO
(kΩ)
C
POLE1
(nF)
R
POLE2
(kΩ)
C
POLE2
(nF)
LF1_EXT_CAP
1
(µF)
883 1.5 fixed 165 fixed 0.337 fixed 0.3
677
341
135
10
External
1
External loop filter capacitor.
An external R-C low-pass filter should be used at the OSC_CTRL
output. The values shown in Figure 23 add an additional low-pass
pole at ~530 Hz. This R-C network filters the noise associated
with the OSC_CTRL buffer to achieve the best noise performance
at the 1 kHz offset region.
R
ZERO
C
POLE1
R
POLE2
LF1_EXT_CAP
SWITCH-
OVER
CONTROL
REFA
REFB
REFA
REFB
REF_SEL
REF_TEST
DIVIDE-BY-
1, 2, ...1023
CHARGE
PUMP
7 BITS,
0.5µA LSB
VDD3_PLL LDO_PLL1
1.8V LDO
3.3V CMOS
OR 1.8V
DIFFERENTIAL
OSC_CTRL
OSC_IN
DIVIDE-BY-
1, 2, ...1023
DIVIDE-BY-
1, 2, ...1023
DIVIDE-BY-
1, 2, ...63
P
F
D
VCXO
C
POLE2
AD9523
08439-021
Figure 24. Input PLL (PLL1) Block Diagram