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SA7026DK

1.3GHz low voltage fractional-N dual frequency synthesizer

Philips Electronics 

Philips Semiconductors

1.3GHz low voltage fractional-N dual synthesizer

Objective specification

SA7026

Main Output Charge Pumps and Fractional

Compensation Currents.

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. 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. See table of charge pump ratios.

Principle of Fractional Compensation

The fractional compensation is designed into the circuit as a means

of reducing or eliminating fractional spurs that predominate when the

accumulator rolls over and the main divider counts one extra RF

input cycle (+1, swallows a cycle). Since ICOMP is the compensation

current and IPUMP is the pump current, for each charge pump,

IPUMP_TOTAL = IPUMP + ICOMP.

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

and 80 for FMOD = 0 (modulo 8). Fractional division will cause the

pump to output a charge that is compensated for in order to reduce

fractional spurs. This compensation is done by sourcing a small

current, iA, see Figure 7, that is proportional to the fractional error

phase. Figure 6 shows that for proper fractional compensation, the

area of the fractional compensation current pulse must be equal to

the area of the charge pump ripple.

This means IPUMP*Q=ICOMP*128, where Q equals fractional-N

modulus e.g., 2/5 for NF = 2 and FMOD = 1. The fractional

compensation current is derived from the main charge pump in that

it follows all the current scaling through external resistor setting, RN,

programming or speed-up operation. For a given pump,

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

FRD is the fractional accumulator value.

REFERENCE R

MAIN M

DIVIDE RATIO

N

N

N+1

N

N+1

DETECTOR

OUTPUT

2

ACCUMULATOR

FRACTIONAL

COMPENSATION

CURRENT

OUTPUT ON

PUMP

PULSE

WIDTH

MODULATION

PULSE LEVEL

MODULATION

4

1

3

0

mA

µA

Figure 6. Waveforms for NF = 2, Fraction = 0.4

Fig 6. shows that for a proper fractional compensation, the area of the fractional

compensation current pulse must be equal to the area of the charge pump ripple output.

fRF

1930.140 MHz

MAIN DIVIDER

N = 8042

240.016 kHz

fREF

240 kHz

FRACTIONAL

ACCUMULATOR

iA

iC

LOOP FILTER

& VCO

Figure 7. Current Injection Concept

SR01682

1998 Oct 13

9

SR01416

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