AD9524SLASHPCBZ datasheet(22/56 Pages) AD

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AD9524

Data Sheet

Rev. E | Page 22 of 56

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 9) to LDO_VCO (Pin 12),

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

900

Fixed at 16

Typical at 1000

3000

450

2750

300

2500

225

2250

2100

2000

1850

External loop filter capacitor.

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 AD9524 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

(Register 0x0F3, Bit 1) 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 (for more information, 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.

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