Circuit Description; Overview - Motorola 38C V100 Service Manual

Level 3 Service Manual

Circuit Description

Overview

6881036B20
3
38C
6881036B20
The PF38C Personal Communicator's hardware is capable of operating in the GSM,
DCS, and PCS bands. One or more bands may be disabled in the software depending
on the country of sale.
Component count has been reduced by use of custom LSI ICs Magic, Whitecap, and
G-Cap II. The Magic IC provides the interface between logic and RF sections. During
receive it drives the RX VCO, demodulates the IF signal and produces base-band
digital data for processing by the logic section. In transmit, it modulates the
transmit data using an internal look up ROM and selects the correct band as well
as controlling the transmit frequency and power level requirements. Whitecap
contains the CPU and interfaces for memory, keypad, display, battery charger,
alarms, and SIM (subscriber identity module) card. The G-Cap II IC provides
voltage regulation, audio, real-time clock, and logic control functionality.
Receive
The RF signal passes from the antenna to an RF switch. The switch is operated by
control voltages that route the signal, depending on whether the receive signal is
GSM or DCS / PCS.
If the signal is GSM, it is routed to a bandpass filter that allows only EGSM signals
through. The filtered input signal is routed to a low noise amplifier having a fixed
gain. The amplified signal is then passed through a second filter that provides
additional selectivity before being fed to the Mixer.
A received PCS signal initially follows the same path as GSM. The output from the
RF switch is filtered to allow only PCS signals to pass, then fed to the PCS switch.
The switch routes the PCS signal to a low noise amplifier. A second provides
additional selectivity before passing to the mixer.
The dual transistor mixer is split between GSM and PCS. Output from the RX VCO
is mixed with the incoming receive signal, resulting in a mixer output consisting of
the sum frequency, difference frequency, and harmonics of the input signal and the
RX VCO. The mixer output passes through a SAW filter which passes only the 400
MHz difference signal.
The RX VCO is a single module Colpitts oscillator capable of operating over three
bands. It is tuned by the charge pump output from the Magic IC. The charge pump
is a steering voltage used to make the oscillator operate at the required frequency.
An isolation amplifier is finely tuned to 400 MHz to amplify the IF and filter
spurious signals from the mixer that passed through the SAW filter. The IF signal
is then passed into the Magic IC for demodulation and generation of the digital
base-band signal.
The 800 MHz local oscillator is the same for all three bands and is controlled by the
Magic IC. It is divided by 2 internally in the Magic IC and used to generate baseband
I and Q signals. These baseband signals are filtered and amplified to provide RxI
and Q. The RxI and Q signals are converted into digital outputs and sent over a
serial bus. The chip will provide for AGC control through the SPI bus.
MOTOROLA CONFIDENTIAL - PROPRIETARY
November 20, 2000
Circuit Description
7