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SA8027W

2.5 GHz low voltage, low power RF fractional-N/IF integer frequency synthesizer

Philips Electronics 

Philips Semiconductors

2.5 GHz low voltage, low power

RF fractional-N/IF integer frequency synthesizer

Product data

SA8027

1.5 Main Output Charge Pumps and Fractional

Compensation Currents (see Figure 8)

The main charge pumps on pins PHP and PHI are driven by the

main phase detector and the charge pump current values are

determined by the current at pin RSET in conjunction with bits CP0,

CP1 in the C-word (see Table 1). The main charge pumps will enter

speed up mode after the A-word is set and strobe goes High. When

strobe goes Low, charge pump will exit speed up mode. The

fractional compensation is derived from the current at RSET, the

contents of the fractional accumulator (FRD) and by the program

value of the FDAC. The timing for the fractional compensation is

derived from the main divider.

1.6 Principle of Fractional Compensation

The fractional compensation is designed into the circuit as a means

of reducing or eliminating fractional spurs that are caused by the

fractional phase ripple of the main divider. If ICOMP is the

compensation current and IPUMP is the pump current, then for each

charge pump:

IPUMP_TOTAL = IPUMP + ICOMP.

The compensation is done by sourcing a small current, ICOMP, see

Figure 9, that is proportional to the fractional error phase. For proper

fractional compensation, the area of the fractional compensation

current pulse must be equal to the area of the fractional charge

pump ripple. The width of the fractional compensation pulse is fixed

to 128 VCO cycles, the amplitude is proportional to the fractional

accumulator value and is adjusted by FDAC values (bits FC7–0 in

the B-word). The fractional compensation current is derived from the

main charge pump in that it follows all the current scaling through

external resistor setting, RSET, programming or speed-up operation.

For a given charge pump,

ICOMP = ( IPUMP / 128 ) * ( FDAC / 5*128) * FRD

FRD is the fractional accumulator value and is automatically

updated.

The theoretical values for FDAC are: 128 for FMOD = 1 (modulo 5)

and 80 for FMOD = 0 (modulo 8).

REFERENCE R

MAIN M DIVIDE RATIO

N

N

N+1

N

N+1

CHARGE PUMP OUTPUT

ACCUMULATOR VALUE (FRD)

FRACTIONAL COMPENSATION

CURRENT (ICOMP)

2

4

1

3

0

PULSE

WIDTH

MODULATION

mA

IPUMP–TOTAL

µA

PULSE LEVEL

MODULATION

GRAPHS NOT TO SCALE.

NOTE: For a proper fractional compensation, the area of the fractional compensation current pulse must be equal to the area of the charge pump output.

Figure 8. Waveforms for NF = 2 Modulo 5 → fraction = 2/5

SR02359

fRF

MAIN DIVIDER

FRACTIONAL

ACCUMULATOR

ICOMP

fREF

Σ IPUMP

LOOP FILTER

& VCO

Figure 9. Current Injection Concept

SR01800

2001 Aug 21

10

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