ADF5610 datasheet(24/48 Pages) AD

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ADF5610

Preliminary Technical Data

Rev. PrE | Page 24 of 48

Figure 26. Charge Pump Gain and Offset Control

Charge Pump Phase Offset

In integer mode, the phase detector operates with zero offset.

The divided reference signal and the divided VCO signal arrive

at the phase detector inputs at the same time. Integer mode

does not require any CP offset current. When operating in

integer mode, disable the CP offset in both directions (up and

down) by writing Register 0x09[22:21] = 00b, and set the CP

offset magnitude to zero by writing Register 0x09[20:14] = 0.

In fractional mode, CP linearity is of paramount importance.

Any nonlinearity degrades phase noise and spurious perfor-

mance. These nonlinearities are eliminated by operating the PD

with an average phase offset, either positive or negative (either

the reference or the VCO edge always leads, that is, arrives first

at the PD).

A programmable CP offset current source adds dc current to

the loop filter and creates the desired phase offset. Positive

current causes the VCO to lead, whereas negative current

causes the reference to lead.

The CP offset is controlled via Register 0x09. Increasing the offset

current causes the phase offset to scale from 0° to 360°.

The specific level of charge pump offset current (Register 0x09,

Bits[20:14]) is calculated using Equation 9 and shown in Figure 27.

Required CP Offset = min ((4.3 × 10−9 × fPD × ICP), 0.25 × ICP)

(9)

where:

fPD is the comparison frequency of the phase detector (Hz).

ICP is the full-scale current setting (A) of the switching charge

pump (set in Register 0x09[6:0] and Register 0x09[13:7]).

Figure 27. Recommended CP Offset Current vs. Phase Detector Frequency for

Typical CP Gain Currents, Calculated Using Equation 9

Do not allow the required CP offset current to exceed 25%

of the programmed CP current. It is recommended to

enable the up offset and disable the down offset by writing

Operation with CP offset influences the required configuration

of the lock detect function. See the description of the lock

detect function in the Lock Detect section.

Phase Detector Functions

special phase detector features.

Setting Register 0x0B[5] = 0 masks the PD up output, which

prevents the charge pump from pumping up.

Setting Register 0x0B[6] = 0 masks the PD down output, which

prevents the charge pump from pumping down.

Clearing both Register 0x0B[5] and Register 0x0B[6] tri-states

the charge pump while leaving all other functions operating

internally.

REF PATH

VCO PATH

LOOP

FILTER

UP GAIN

REG 0x09[13:7]

UP OFFSET REG 0x09[21]

0µA TO 635µA

5µA STEP

REG 0x09[20:14]

DOWN OFFSET REG 0x09[22]

0µA TO 635µA

5µA STEP

REG 0x09[20:14]

0mA TO 2.54mA

20µA STEP

0mA TO 2.54mA

20µA STEP

DOWN GAIN

REG 0x09[6:0]

100

200

300

400

500

600

700

20406080

100

PHASE DETECTOR FREQUENCY (MHz)

CP CURRENT = 2.5mA

CP CURRENT = 2mA

CP CURRENT = 1mA

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