SA7025 Datasheet, PDF (13/22 Pages) NXP Semiconductors – Low-voltage 1GHz fractional-N synthesizer

Philips Semiconductors
1GHz low-voltage Fractional-N synthesizer
Product specification
SA7025
Table 3. PR Modulus
PR Modulus Prescaler
01
2
10
3
NM1
12
12
Bit Capacity
NM2
8
4
NM3
–
4
When the serial input A word is loaded, the output circuits are in the
“speed-up mode” as long as the STROBE is H, else the “normal
mode” is active. In the “normal mode” the current output PHP is:
where:
I_{PHP_N} + I_PHP ) I_{PHP_comp}
|I_PHP|
+
CN @ I_RN
32
:charge pump current
|I_{PHP_comp}|
+
FRD
@
I_RF
128
:fractional comp.
current
The current in PHI is zero in “normal mode”.
In “speed-up mode” the current in output PHP is:
I_{PHP_S} + I_PHP ) I_{PHP_comp}
ǒ Ǔ |I_PHP| +
CN @ I_RN
32
(2^CL)1 ) 1)
ǒ Ǔ |I_{PHP_comp}| +
FRD @ I_RF
128
(2^CL)1 ) 1)
In “speed-up mode” the current in output PHI is:
I_{PHI_S} + I_PHI ) I_{PHI_comp}
where:
ǒ Ǔ |I_PHI| +
I_RNCN
32
(2^CL)1) CK
ǒ Ǔ |I_{PHI_comp}| +
I_RF FRD
128
(2^CL)1) CK
Figure 9 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 output. This means that the current
setting on the input RN, RF is approximately:
I_RN
I_RF
+
(Q @ f_VCO)
(3 @ CN @ F_INR)
where:
Q=
f_VCO = f_INM × N,
F_INR =
fractional-N modulus
input frequency of the prescaler
input frequency of the reference divider
PHI pump is meant for switching only. Current and compensation
are not as accurate as PHP.
1996 Aug 6
REF_IN
L
“1” _D
Q
REFERENCE
DIVIDER
C
R
R
V_DDA
P
P-TYPE
CHARGE PUMP
AUX/MAIN
DIVIDER
“1” _D R
C
X
Q
PH
N-TYPE
CHARGE PUMP
N
REF_IN
L
R
X
P
N
I_P
H
V_SSA
Figure 8. Phase Detector Structure with Timing
13
SR00607