AD9577BCPZ-R7 datasheet(32/44 Pages) AD

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AD9577

Data Sheet

Rev. A | Page 32 of 44

PLL1 PHASE FREQUENCY DETECTOR (PFD) AND

CHARGE PUMP

The PFD determines the phase difference error between the

reference divider output and the feedback divider output clock

edges. The outputs of this circuit are pulse-width modulated up

and down signal pulses. These pulses drive the charge pump

circuit. The amount of charge delivered from the charge pump to

the loop filter is determined by the instantaneous phase error. The

action of the closed loop is to drive the frequency and phase error

at the input of the PFD toward zero. Figure 42 shows a block

diagram of the PFD/CP circuitry.

CLR1

REFCLK

HIGH

CLR2

HIGH

DOWN

CHARGE

PUMP

3.3V

GND

FEEDBACK

DIVIDER

Figure 42. PFD Circuit Showing Simplified Charge Pump

PLL1 VCO

PLL1 incorporates a low phase noise LC-tank VCO. This VCO

has 32 frequency bands spanning from 2.15 GHz to 2.55 GHz.

At power-up, a VCO calibration cycle begins and the correct band

is selected based on the feedback divider setting (Na). Whenever a

new feedback divider setting is called for, the VCO calibration

process must run by writing 1 followed by 0 to the NewAcq bit,

PLL1 FEEDBACK DIVIDER

The feedback divider ratio, Na, is used to set the PLL1 VCO

frequency according to Equation 3. Note that the Na value is set

by adding the offset value of 80 to the value programmed to

The maximum Na value is 131. For example, to set Na to 85, the

AF0[5:0] register is set to 5.

SETTING THE OUTPUT FREQUENCY OF PLL1

For example, set the output frequency (fOUT0) on Port 0 to

156.25 MHz, the output frequency (fOUT1) on Port 1 to 100 MHz,

and both the reference frequency (fREF) and the PFD frequency

(fPFD) to 25 MHz.

The frequency fOUT0 presented to OUT0 can be set according to

Equation 4.

The frequency fOUT1 presented to OUT1 can be set according to

Equation 5.

To determine if both 156.25 MHz and 100 MHz can be derived

from a common fVCO1 frequency in the 2.15 GHz to 2.55 GHz

range, use the lowest common multiple (LCM) of 156.25 MHz

and 100 MHz to determine the lowest VCO frequency that can

be divided down to provide both of these frequencies.

LCM(156.25 MHz, 100 MHz) = 2.5 GHz

(6)

Therefore, set the VCO frequency to 2.5 GHz. With fPFD =

25 MHz, from Equation 3, Na must be set to 100.

For 156.25 MHz on Port 0, set

V0 × D0 = 16

(7)

This can be achieved by setting V0 to 4 and D0 to 4. For

100 MHz on Port 1, set

V1 × D1 = 25

(8)

This can be achieved by setting V1 to 5 and D1 to 5. With a

reference frequency of 25 MHz, the reference divider value, R,

must be set to 1 by setting Register G0[1] to 0. Table 22

summarizes the register settings for this configuration.

Table 22. Register Settings for Example PLL1 Configuration

Parameter

Divide Value

I2C Register

RegisterValue

100

AF0[5:0]

010100

ADV0[7:5]

100

ADV0[4:0]

00100

ADV1[7:5]

101

ADV1[4:0]

00101

G0[1]

PLL2 INTEGER/FRACTIONAL-N PLL

The lower PLL in Figure 32, PLL2, is a fractional-N PLL. The

input frequency to the PLL from the reference circuit is fPFD.

The VCO frequency, fVCO2, is programmed by setting the values

for Nb, FRAC, and MOD according to

)

(

MOD

FRAC

PFD

VCO

(9)

where Nb is programmable in the 80 to 131 range. To provide

the greatest flexibility and accuracy, both the FRAC and MOD

values can be programmed to a resolution of 12 bits, where

FRAC < MOD. The VCO output frequency can tune over the

2.15 GHz to 2.55 GHz range to fractional multiples of the PFD

input frequency.

By setting each of the VCO divider (V2 and V3) and output

divider (D2 and D3) values, the VCO frequency can be divided

down to the required output frequency, independently, for each

of the output ports, OUT2 and OUT3. The fOUT2frequency

presented to OUT2 can be set according to

MOD

FRAC

PFD

OUT2

)

(

(10)

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