Motorola 38C V100 Service Manual page 16

Circuit Description
12
the TX VCO, the resulting burst would not meet the world standards for GSM with
respect to bandwidth as shown in Figure 5.
Chan ne l 24
Figure 4. CP_TX Overshoot
Acceptable
Figure 5. GSM Bandwidth Standard
The loop filter acts as a very large capacitor and resistor to introduce a long R-C
time for smoothing. It uses a small capacitor and the very high input impedance of
an op amp buffer. During the TX_EN (Whitecap IC Pin C1) period when the
transmitter is preparing to operate, the capacitor charges. On receipt of DM_CS
(Whitecap IC Pin E2), when the Transmitter actually fires, the capacitor discharges
through the op amp giving a smooth TX VCO tuning voltage. The support voltage
for the loop filter is V1_FILT (V2 from GCAP II through Q913 which creates V1_SW.
V1_SW, in turn, creates V1_FILT).
The TX VCO IC produces the required output frequency and is controlled by:
• TX_DCS (TX275 + DCS_SEL through (Q1101)
• TX275_GSM (GSM_SEL + RX275) (Q110)
• TX275_DPCS (GSM_SEL + RX275) (Q2102)
These signals configure the VCO for correct mode of operation (GSM 900 / 1800 /
1900), support voltage being SF_OUT (Magic IC Pin C1).
The TX VCO signal is then fed through buffer amplifier Q330, which is supported
by TX275. The TX VCO signal is split, a sample of the output frequency being
directed back to the Magic IC Pin A3 for use within the TIC part of the Magic IC
to implement the TX phase-locked loop.
Isolation diode CR330 prevents unstabilized TX VCO output from being amplified
and transmitted. CR330 is biased on by the exciter voltage from the PAC IC (power
amplifier control IC) U390 Pin 7 which allows the TX output frequency through to
MOTOROLA CONFIDENTIAL - PROPRIETARY
November 20, 2000
Channel 56
Unacceptable
3dB
Bandwidth
Bandwidth
Product Family 38C
Overshoot
000716-O
3dB
000717-O
6881036B20