U2894B Datasheet, PDF (10/16 Pages) TEMIC Semiconductors – Modulation PLL for GSM, DCS and PCS Systems

U2894B
Application Hints
Interfacing
For some of the baseband ICs it may be necessary to
reduce the I/Q voltage swing so that it can be handled by
the U2894B. In those cases, the following circuitry can be
used.
R₁
I
I
NI
Baseband IC
Q
R₁
R₂
R₁
R₂
NI
U2894B
Q
NQ
NQ
R₁
14562
Figure 13. Interfacing the U2894B to I/Q baseband circuits
Due to a possible current offset in the differential base-
band inputs of the U2894B the best values for the carrier
suppression of the I/Q modulator can be achieved with
v W ^{voltage driven I/NI-, and Q/NQ-inputs. A value of}
R_source = R2/2*R_S 1.5 k should be realized. R_S is the
sum of R1 (above drawing) and the output resistance of
the baseband IC.
Charge-Pump Current Programming
Mode Control
U2894B
VS2
U2894B
VS2
R_Mode
MC
R_Mode1 R_Mode2
MC
a) any single mode b) any 2 modes
U2894B
VS2
U2894B
VS2
R_Mode
MC
R_Mode
MC
36 kΩ or
10 kΩ
c) any mode
& mode 5
d) mode 5 & mode 3 or mode 4
14563
Figure 15. Application examples for programming
different modes
GND
CPC
1 nF
R_CPC1
R_CPC1 = 2.2 kΩ
R_CPC2 = 1 kΩ (incl. rds_on of FET)
R_CPC2
‘H’ _{|ICPO | = 4 mA}
‘L’ ^|ICPO ^{| = 2 mA}
12497
Figure 14. Programming the charge-pump current
10 (16)
Rev. A4, 30-Sep-98