Samsung SGH-E800 Service Manual page 5

2. SGH-E800 Circuit Description
1. SGH-E800 RF Circuit Description
1) RX PART
1. ASM(U100) → Switching Tx, Rx path for GSM900, DCS1800 by logic controlling.
2. ASM Control Logic (Q100, Q102) → Truth Table
GSM Tx Mode
DCS Tx Mode
GSM Rx Mode
DCS Rx Mode
3. FILTER
Converting electromagnetic field wave to acoustic wave and then pass the specific frequency band.
- GSM FILTER (F101) → For filtering the frequency band between 925 and 960 MHz
- DCS FILTER (F102) → For filtering the frequency band between 1805 and 1880 MHz.
4. VC-TCXO (OSC101)
This module generates the 26MHz reference clock to drive the logic and RF. After additional process, the reference clock
applies to the U102 Rx IQ demodulator and Tx IQ modulator. And then, the oscillator is controlled by serial data to
select channel and use fast lock mode for GPRS high class operation.
5. Transceiver (U102)
The receiver front-end which amplifies the GSM, DCS aerial signal, converts the chosen channel down to a low IF signal
of 100 kHz. The first stage is composed of symmetrical low noise amplifiers (LNAs). They are matched to 50 ohm. The
LNAs are followed by an IQ down mixer. It consists of two mixers in parallel but driven by quadrature out of phase LO
signals. The In phase(I) and Quadrature phase(Q) IF signals are low pass filtered to provide protection from high
frequency offset interferes. The low IF I and Q signals are then fed into the channel filter. The front-end low IF I and Q
outputs enter the integrated bandpass channel filter with provision for five 8 dB gain steps in front of the filter.
2) TX PART
Baseband IQ signal fed into offset PLL, this function is included inside of U102 chip. OSC100 chip generates modulator
signal which power level is about 6.5dBm and fed into Power Amplifier(U101). The PA output power and power ramping
are well controlled by Auto Power Control circuit. We use offset PLL below table.
Modulation Spectrum
VC1
H
L
L
L
200kHz offset
30 kHz bandwidth
400kHz offset
30 kHz bandwidth
600kHz ~ 1.8MHz offset
30 kHz bandwidth
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VC2
L
H
L
L
GSM
DCS
GSM
DCS
GSM
DCS
2-1
-36dBm
-36dBm
-36dBm
-36dBm
-51dBm
-56dBm