ADF4351 datasheet(19/28 Pages) AD

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Data Sheet

ADF4351

Rev. 0 | Page 19 of 28

Reference Doubler

Setting the DB25 bit to 0 disables the doubler and feeds the REFIN

signal directly into the 10-bit R counter. Setting this bit to 1 multi-

plies the REFIN frequency by a factor of 2 before feeding it into

the 10-bit R counter. When the doubler is disabled, the REFIN

falling edge is the active edge at the PFD input to the fractional

synthesizer. When the doubler is enabled, both the rising and

falling edges of REFIN become active edges at the PFD input.

When the doubler is enabled and the low spur mode is selected,

the in-band phase noise performance is sensitive to the REFIN duty

cycle. The phase noise degradation can be as much as 5 dB for

REFIN duty cycles outside a 45% to 55% range. The phase noise

is insensitive to the REFIN duty cycle in the low noise mode and

when the doubler is disabled.

The maximum allowable REFIN frequency when the doubler is

enabled is 30 MHz.

RDIV2

Setting the DB24 bit to 1 inserts a divide-by-2 toggle flip-flop

between the R counter and the PFD, which extends the maximum

REFIN input rate. This function allows a 50% duty cycle signal to

appear at the PFD input, which is necessary for cycle slip reduction.

10-Bit R Counter

The 10-bit R counter (Bits[DB23:DB14]) allows the input reference

frequency (REFIN) to be divided down to produce the reference

clock to the PFD. Division ratios from 1 to 1023 are allowed.

Double Buffer

The DB13 bit enables or disables double buffering of

Bits[DB22:DB20] in Register 4. For information about how

double buffering works, see the Program Modes section.

Charge Pump Current Setting

Bits[DB12:DB9] set the charge pump current. This value should

be set to the charge pump current that the loop filter is designed

with (see Figure 26).

Lock Detect Function (LDF)

The DB8 bit configures the lock detect function (LDF). The LDF

controls the number of PFD cycles monitored by the lock detect

circuit to ascertain whether lock has been achieved. When DB8 is

set to 0, the number of PFD cycles monitored is 40. When DB8

is set to 1, the number of PFD cycles monitored is 5. It is recom-

mended that the DB8 bit be set to 0 for fractional-N mode and

to 1 for integer-N mode.

Lock Detect Precision (LDP)

The lock detect precision bit (Bit DB7) sets the comparison

window in the lock detect circuit. When DB7 is set to 0, the

comparison window is 10 ns; when DB7 is set to 1, the window

is 6 ns. The lock detect circuit goes high when n consecutive

PFD cycles are less than the comparison window value; n is set

by the LDF bit (DB8). For example, with DB8 = 0 and DB7 = 0,

40 consecutive PFD cycles of 10 ns or less must occur before

digital lock detect goes high.

For fractional-N applications, the recommended setting for

Bits[DB8:DB7] is 00; for integer-N applications, the recom-

mended setting for Bits[DB8:DB7] is 11.

Phase Detector Polarity

The DB6 bit sets the phase detector polarity. When a passive

loop filter or a noninverting active loop filter is used, this bit

should be set to 1. If an active filter with an inverting charac-

teristic is used, this bit should be set to 0.

Power-Down (PD)

The DB5 bit provides the programmable power-down mode.

Setting this bit to 1 performs a power-down. Setting this bit to 0

returns the synthesizer to normal operation. In software power-

down mode, the part retains all information in its registers. The

When power-down is activated, the following events occur:

•

Synthesizer counters are forced to their load state conditions.

•

VCO is powered down.

•

Charge pump is forced into three-state mode.

•

Digital lock detect circuitry is reset.

•

RFOUT buffers are disabled.

•

Input registers remain active and capable of loading and

latching data.

Charge Pump Three-State

Setting the DB4 bit to 1 puts the charge pump into three-state

mode. This bit should be set to 0 for normal operation.

Counter Reset

The DB3 bit is the reset bit for the R counter and the N counter

of the ADF4351. When this bit is set to 1, the RF synthesizer

N counter and R counter are held in reset. For normal opera-

tion, this bit should be set to 0.

Control Bits

When Bits[C3:C1] are set to 011, Register 3 is programmed.

Figure 27 shows the input data format for programming this

Band Select Clock Mode

Setting the DB23 bit to 1 selects a faster logic sequence of band

selection, which is suitable for high PFD frequencies and is

necessary for fast lock applications. Setting the DB23 bit to 0 is

recommended for low PFD (<125 kHz) values. For the faster

band select logic modes (DB23 set to 1), the value of the band

select clock divider must be less than or equal to 254.

Antibacklash Pulse Width (ABP)

Bit DB22 sets the PFD antibacklash pulse width. When Bit DB22

is set to 0, the PFD antibacklash pulse width is 6 ns. This setting is

recommended for fractional-N use. When Bit DB22 is set to 1,

the PFD antibacklash pulse width is 3 ns, which results in phase

noise and spur improvements in integer-N operation. For

fractional-N operation, the 3 ns setting is not recommended.

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