Datasheet

AD9523 Data Sheet
Rev. C | Page 22 of 60
PLL1 Input Dividers
Each reference input feeds a dedicated reference divider block.
The input dividers provide division of the reference frequency
in integer steps from 1 to 1023. They provide the bulk of the
frequency prescaling that is necessary to reduce the reference
frequency to accommodate the bandwidth that is typically
desired for PLL1.
PLL1 Reference Switchover
The reference monitor verifies the presence/absence of the
prescaled REFA and REFB signals (that is, after division by the
input dividers). The status of the reference monitor guides the
activity of the switchover control logic. The AD9523 supports
automatic and manual PLL reference clock switching between
REFA (the REFA and
REFA
pins) and REFB (the REFB and
REFB
pins). This feature supports networking and infrastructure
applications that require redundant references.
There are several configurable modes of reference switchover.
The manual switchover is achieved either via a programming
register setting or by using the REF_SEL pin. The automatic
switchover occurs when REFA disappears and there is a reference
on REFB.
The reference automatic switchover can be set to work as follows:
 Nonrevertive: stay on REFB. Switch from REFA to REFB
when REFA disappears, but do not switch back to REFA
if it reappears. If REFB disappears, then go back to REFA.
 Revert to REFA. Switch from REFA to REFB when REFA
disappears. Return to REFA from REFB when REFA returns.
See Table 43 for the PLL1 miscellaneous control register bit
settings.
PLL1 Holdover
In the absence of both input references, the device enters
holdover mode. Holdover is a secondary function that is
provided by PLL1. Because PLL1 has an external VCXO
available as a frequency source, it continues to operate in the
absence of the input reference signals. When the device
switches to holdover, the charge pump tristates. The device
continues operating in this mode until a reference signal
becomes available. Then the device exits holdover mode,
and PLL1 resynchronizes with the active reference. In addition
to tristate, the charge pump can be forced to VCC/2 during
holdover (see Table 43, Bit 6 in Register 0x01C).
COMPONENT BLOCKS—OUTPUT PLL (PLL2)
PLL2 General Description
The output PLL (referred to as PLL2) consists of an optional
input reference doubler, phase-frequency detector (PFD),
a partially integrated analog loop filter (see Figure 25), an
integrated voltage-controlled oscillator (VCO), and a feedback
divider. The VCO produces a nominal 3.8 GHz signal with an
output divider that is capable of division ratios of 4 to 11.
The PFD of the output PLL drives a charge pump that increases,
decreases, or holds constant the charge stored on the loop filter
capacitors (both internal and external). The stored charge
results in a voltage that sets the output frequency of the VCO.
The feedback loop of the PLL causes the VCO control voltage to
vary in a way that phase locks the PFD input signals. The gain
of PLL2 is proportional to the current delivered by the charge
pump. The loop filter bandwidth is chosen to reduce noise
contributions from PLL sources that could degrade phase noise
requirements.
The output PLL has a VCO with multiple bands spanning a
range of 3.6 GHz to 4.0 GHz. However, the actual operating
frequency within a particular band depends on the control
voltage that appears on the loop filter capacitor. The control
voltage causes the VCO output frequency to vary linearly within
the selected band. This frequency variability allows the control
loop of the output PLL to synchronize the VCO output signal
with the reference signal applied to the PFD. Typically, the
device automatically selects the appropriate band as part of its
calibration process (invoked via the VCO control register at
Address 0x0F3).
N DIVIDER
TO DIST/
RESYNC
×2
PLL1_OUT
LDO
LDO
PLL_1.8V
LDO_PLL2VDD3_PLL2
LDO_VCO
DIVIDE BY
1, 2, 4, 8, 16
DIVIDE BY
4, 5, 6, ...11
DIVIDE-BY-4
PRESCALER
A/B
COUNTERS
CHARGE PUMP
8 BITS, 3.5µA LSB
PFD
R
ZERO
R
POLE2
C
POLE1
C
POLE2
LF2_EXT_CAP
08439-023
AD9523
Figure 25. Output PLL (PLL2) Block Diagram