sa8027 NXP Semiconductors, sa8027 Datasheet - Page 10

• Manufacturer Part Number: sa8027
• Description: 2.5 Ghz Low Voltage, Low Power Rf Fractional-n/if Integer Frequency Synthesizer
• Manufacturer: NXP Semiconductors

Philips Semiconductors
1.5 Main Output Charge Pumps and Fractional
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 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 R
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 I
compensation current and I
charge 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 output.
2001 Aug 21
GRAPHS NOT TO SCALE.
2.5 GHz low voltage, low power
RF fractional-N/IF integer frequency synthesizer
FRACTIONAL COMPENSATION
ACCUMULATOR VALUE (FRD)
Compensation Currents (see Figure 8)
CHARGE PUMP OUTPUT
MAIN M DIVIDE RATIO
CURRENT (I
REFERENCE R
I
PUMP_TOTAL
I
PUMP–TOTAL
COMP
PUMP
)
= I
is the pump current, then for each
PUMP
SET
PULSE LEVEL
MODULATION
PULSE
WIDTH
MODULATION
in conjunction with bits CP0,
f
RF
+ I
Figure 8. Waveforms for NF = 2 Modulo 5
N
COMP
COMP
.
is the
MAIN DIVIDER
Figure 9. Current Injection Concept
f
REF
N
2
SET
, the
N+1
10
4
I
PUMP
Figure 9, that is proportional to the fractional error phase. For proper
the B-word). The fractional compensation current is derived from the
ACCUMULATOR
The compensation is done by sourcing a small current, I
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
main charge pump in that it follows all the current scaling through
external resistor setting, R
For a given charge pump,
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).
FRACTIONAL
I
COMP
N
1
fraction =
I
COMP
LOOP FILTER
& VCO
SR01800
= ( I
2
SET
/
3
5
PUMP
N+1
, programming or speed-up operation.
/ 128 ) * ( FDAC / 5*128) * FRD
0
mA
A
SR02359
SA8027
Product data
COMP
, see