NEC DB2000, Service Manual

Page: 22 / 199

Fig.6: The UHF Mixer / Amplifier (IC101 / IC102)
(IC203)VCO
GSM VCO = (1160-1230MHz)
PCN VCO = (1530-1610MHz)
(IC202)VCO
PMB2333
PMB2333
V_VCO1
V_VCO2
3V_P_TX
GSM_PRE_ON
16
16
2
2
BPF
BPF
2
2
4
4
9
9
6
6
To IC103
To FL103
15
15
Loop Filter
RF-PLL
3
3
3
5
TCXO
1
8
180Mhz - W2013
11
270Mhz - W2013
11
The UHF Mixer Amplifier
The PM2333 IC’s (IC101/102) converts the IF modulated signal to RF and
incorporates an active double-balanced mixer.
The modulated TX IF signal GSM(270Mhz) or PCN(180Mhz) is mixed onto the
GSM(890Mhz-915Mhz) or PCN(1710Mhz-1785Mhz) TX RF frequency.
This is achieved by mixing the TX IF signal with the TX UHF VCO output from IC202
for GSM and IC203 for PCN.
Hence:-
GSM TX = UHF VCO – 270 ie, (1160 – 270 = 890Mhz)
PCN TX = UHF VCO +180 ie, (1530 + 180 = 1710 Mhz)
The modulated RF signal is then output on #6 to a BPF and then re-enters the
PMB2333 (IC101/102) at #2. The RF modulated signal is then amplified by the
pre-driver within the PMB333 IC .GSM_PRE_ON and 3V_P_TX are control lines for
the pre-driver amplifier.
The RF signal is then output on #15 of the PMB2333 and passed through
another stage of filtering (BPF:FL103) and pre-amplified via IC103/104 before the
power amplifier stage (See
Fig.8: The P.A. Circuit and its Control ).