AD9523BCPZ Datasheet, PDF (23/60 Pages) Analog Devices – Jitter Cleaner and Clock Generator with 14 Differential or 29 LVCMOS Outputs

Data Sheet
Input 2× Frequency Multiplier
The 2× frequency multiplier provides the option to double
the frequency at the PLL2 input. This allows the user to take
advantage of a higher frequency at the input to the PLL (PFD)
and, thus, allows for reduced in-band phase noise and greater
separation between the frequency generated by the PLL and the
modulation spur associated with PFD. However, increased
reference spur separation results in harmonic spurs introduced
by the frequency multiplier that increase as the duty cycle
deviates from 50% at the OSC_IN inputs. As such, beneficial
use of the frequency multiplier is application-specific. Typically,
a VCXO with proper interfacing has a duty cycle that is
approximately 50% at the OSC_IN inputs. Note that the
maximum output frequency of the 2× frequency multipliers
must not exceed the maximum PFD rate that is specified in
Table 12.
PLL2 Feedback Divider
PLL2 has a feedback divider (N divider) that enables it to provide
integer frequency up-conversion. The PLL2 N divider is a com-
bination of a prescaler (P) and two counters, A and B. The total
divider value is
N = (P × B) + A
where P = 4.
The feedback divider is a dual modulus prescaler architecture, with
a nonprogrammable P that is equal to 4. The value of the B counter
can be from 4 to 63, and the value of the A counter can be from 0 to 3.
However, due to the architecture of the divider, there are constraints,
as listed in Table 46.
PLL2 Loop Filter
The PLL2 loop filter requires the connection of an external
capacitor from LF2_EXT_CAP (Pin 11) to LDO_VCO (Pin 14),
as illustrated in Figure 25. The value of the external capacitor
depends on the operating mode and the desired phase noise
performance. For example, a loop bandwidth of approximately
500 kHz produces the lowest integrated jitter. A lower bandwidth
produces lower phase noise at 1 MHz but increases the total
integrated jitter.
Table 21. PLL2 Loop Filter Programmable Values
RZERO
(Ω)
CPOLE1
(pF)
RPOLE2
(Ω)
CPOLE2
(pF)
LF2_EXT_CAP¹
(pF)
3250
48
900
Fixed at 16 Typical at 1000
3000
40
450
2750
32
300
2500
24
225
2250
16
2100
8
2000
0
1850
1 _{External loop filter capacitor.}
AD9523
VCO Divider
The VCO divider provides frequency division between the
internal VCO and the clock distribution. The VCO divider can
be set to divide by 4, 5, 6, 7, 8, 9, 10, or 11.
VCO Calibration
The AD9523 on-chip VCO must be manually calibrated to ensure
proper operation over process and temperature. This is accom-
plished by setting the calibrate VCO bit (Register 0x0F3, Bit 1) to 1.
(This bit is not self-clearing.) The setting can be performed
as part of the initial setup before executing the IO_Update bit
(Register 0x234, Bit 0 = 1). A readback bit, VCO calibration in
progress (Register 0x22D, Bit 0), indicates when a VCO
calibration is in progress by returning a logic true (that is, Bit 0 = 1).
If the EEPROM is in use, setting the calibrate VCO bit to 1 before
saving the register settings to the EEPROM ensures that the VCO
calibrates automatically after the EEPROM has loaded. After
calibration, it is recommended that a sync be initiated (see the
Clock Distribution Synchronization section).
Note that the calibrate VCO bit defaults to 0. This bit must
change from 0 to 1 to initiate a calibration sequence. Therefore,
any subsequent calibrations require the following sequence:
1. Register 0x0F3, Bit 1 (calibrate VCO bit) = 0
2. Register 0x234, Bit 0 (IO_Update bit) = 1
3. Register 0x0F3, Bit 1 (calibrate VCO bit) = 1
4. Register 0x234, Bit 0 (IO_Update bit) = 1
VCO calibration is controlled by a calibration controller that
runs off the VCXO input clock. The calibration requires that
PLL2 be set up properly to lock the PLL2 loop and that the
VCXO clock be present.
During power-up or reset, the distribution section is automatically
held in sync until the first VCO calibration is finished. Therefore,
no outputs can occur until VCO calibration is complete and PLL2
is locked.
Initiate a VCO calibration under the following conditions:
• After changing any of the PLL2 B counter and A counter
settings or after a change in the PLL2 reference clock
frequency. This means that a VCO calibration should be
initiated any time that a PLL2 register or reference clock
changes such that a different VCO frequency is the result.
• Whenever system calibration is desired. The VCO is designed
to operate properly over extremes of temperature even
when it is first calibrated at the opposite extreme. However,
a VCO calibration can be initiated at any time, if desired.
Rev. C | Page 23 of 60