ADMV4420ACPZ-R2 datasheet(39/61 Pages) AD

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Data Sheet

ADMV4420

Rev. A | Page 39 of 61

CLR2

DOWN

HIGH

CPOUT

–IN

+IN

CHARGE

PUMP

DELAY

CLR1

Figure 123. PFD and CP Simplified Schematic

LOOP FILTER

Defining a loop filter for a PLL is dependent on several dynamics,

such as the PFD frequency, the N counter value, the tuning

sensitivity characteristics (kVCO) of the VCO, and the selected

CP current. A higher fPFD has the advantage of lowering inband

phase noise performance at the expense of integer boundary spur

levels when operating in fractional-N mode. Consequently, a

lower fPFD can allow the PLL to operate in integer-N mode,

which can eliminate integer boundary spurs at the expense of

higher inband phase noise performance. Given the trade-offs,

care must be taken with frequency planning and fPFD selection

to ensure the appropriate inband phase noise performance is

met with acceptable spur levels for the end application.

The loop filter, as implemented in the ADMV4420-EVALZ

evaluation board, is a third-order passive filter, as shown in

Figure 124. The filter is designed with the following simulation

input parameters: fPFD = 50 MHz, kVCO = 80 MHz/V, fVCO = 9.4 GHz

and ICP = 1.25 mA. The resulting loop filter bandwidth and

phase margin are 60 kHz and 45°, respectively.

For additional guidance with loop filter simulations on the

ADMV4420, contact Analog Devices, Inc., for technical

support.

CPOUT

VTUNE

470pF

6.8nF

680Ω

1.5kΩ

220pF

DNI

0Ω

Figure 124. Recommended Loop Filter Schematic

CP CURRENT SETUP

For a specifically designed loop filter, the CP current (ICP) must

be set by adjusting the CP_CURRENT value in Register 0x22E.

The CP current follows the equation,

ICP = (CP_CURRENT + 1) × 312.5 μA

(7)

where CP_CURRENT is an integer value (0 to 15).

Note that the default value of CP_CURRENT is 3, which yields

a current of 1.25 mA. The applicable range is 312.5 µA to 5 mA,

with 312.5 µA steps.

To change the fPFD, if no change has been made to the existing

loop filter components, it is recommended to scale the ICP using

the following equation:

()

1.25 mA 50 MHz

CP DEFAULT

PFD DEFAULT

CP NEW

PFD NEW

(8)

where:

ICP(NEW) is the new desired ICP.

ICP(DEFAULT) is the default ICP.

fPFD(DEFAULT) is the default fPFD.

fPFD(NEW) is the new desired fPFD.

When ICP(NEW) is obtained, the CP_CURRENT value in

()

ROUND

312.5 μA

CP NEW

CP CURRENT



−





(9)

where ROUND is the mathematical round function.

BLEED CURRENT (CP_BLEED) SETUP

The charge pump includes a binary scaled bleed current (IBLEED),

which is set by using the CP_BLEED value in Register 0x22F.

The bleed current introduces a slight phase offset in the PFD to

improve integer boundary spurs when operating in fractional-N

mode.

Generally, the bleed current follows Equation 10 and provides a

value that can be applicable for most applications, but there can

be additional spur level improvement by empirically

determining the appropriate bleed current value from actual

measurements for the intended application. The applicable

range is 0 µA to 956.25 µA, with 3.75 µA steps.

3.75 μA

BLEED

CP BLEED

(10)

where CP_BLEED is an integer value (0 to 255).

When IBLEED is obtained, the CP_BLEED value in Register 0x22F

can be updated using the round function,

ROUND

3.75 μA

BLEED

CP BLEED

















(11)

where IBLEED is the desired charge pump bleed current.

MUXOUT

The on-chip multiplexer output (MUXOUT) allows access to

various internal signals, in addition to providing a digital lock

detect function. A representative diagram is shown in

Figure 125. The state of the MUXOUT pin is determined from

the PLL_MUX_SEL value in Register 0x213. See Table 31 for

full details.

ADMV4420ACPZ-R2 数据表(PDF) 39 Page - Analog Devices

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