STB5701 datasheet(18/35 Pages) STMICROELECTRONICS

18 / 35 page

Functional description

STB5701

18/35

4.2

Programmable Phase Locked Loop synthesizer (PLL)

Figure 3.

PLL block diagram

The synthesized programmable local oscillator is a Phase Locked Loop (PLL) using a

'parallel-resonant' quartz crystal as frequency reference. The PLL block contains a phase

detector, charge pump, VCO, Programmable N and R Divider and a Crystal Oscillator. The

synthesized frequency (fVCO) is set by programming the 'N' Divider through the I2C

interface (PINs 12, 13).

fR = fXTAL / R

fVCO = N

. f

N = N

fR = (N / R)

fXTAL

4.2.1

Divider (a-counter, m-counter and prescaler)

The main divider (N) of the PLL contains a 3-bit A-counter, a 10-bit M-counter and an 8/9

prescaler. The divider ratio of the prescaler is controlled by the program counter and the

swallow counter. During one cycle, the prescaler divides by 9 until the swallow A-counter

reaches its terminal count. Afterwards the prescaler divider by 8 until the program counter

reaches its terminal count. Therefore the overall feedback divider ratio can be expressed as:

N = 9 . A + 8

. (M - A)

VCO

PLL

Synthetizer

PFD

Pierce

Crystal

Oscillator

1 R

1 N

: 2

Clk

I2C

(PLL_A, PLL_M)

I2C

(PLL_R)

I2C

(CLKOUT_ENABLE)

(5) XTAL2

(6) XTAL1

(4) VCC_XTAL

(3) VCC_Div

(8) LF

CK_OUT

(7)

EN_Xtal

(11)

VCC_VCO

(9)

Image Rejection Mixer

2 f

VCO

f XTAL

TANK

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