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Philips Semiconductors
1.3GHz low voltage fractional-N synthesizer
Main Output Charge Pumps and Fractional
Compensation Currents (see Figure 6)
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 R
CP1 in the B-word (see table of charge pump ratios). The fractional
compensation is derived from the current at R
the fractional accumulator FRD and by the program value of the
FDAC. The timing for the fractional compensation is derived from
the main divider. 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.
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 I
compensation current and I
PUMP
charge pump:
I
= I
PUMP_TOTAL
REFERENCE R
MAIN M
DIVIDE RATIO
DETECTOR
OUTPUT
ACCUMULATOR
FRACTIONAL
COMPENSATION
CURRENT
OUTPUT ON
PUMP
NOTE: 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.
1999 Nov 04
in conjunction with bits CP0,
SET
, the contents of
SET
is the
COMP
is the pump current, then for each
+ I
.
PUMP
COMP
N
N
2
PULSE
WIDTH
MODULATION
PULSE LEVEL
MODULATION
Figure 6. Waveforms for NF = 2 Modulo 5
f
MAIN DIVIDER
RF
1930.140 MHz
N = 8042
240.016 kHz
f
REF
240 kHz
Figure 7. Current Injection Concept
The compensation is done by sourcing a small current, I
Figure 7, 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, R
SET
For a given charge pump,
I
= ( I
COMP
FRD is the fractional accumulator value.
The target values for FDAC are: 128 for FMOD = 1 (modulo 5) and
80 for FMOD = 0 (modulo 8).
N+1
N
4
1
3
2
fraction =
/
5
FMOD
FRACTIONAL
ACCUMULATOR
NF
I
COMP
I
PUMP
LOOP FILTER
& VCO
9
Product specification
SA7016
COMP
, programming or speed-up operation.
/ 128 ) * ( FDAC / 5*128) * FRD
PUMP
N+1
0
mA
A
SR01416
SR01682
, see
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