Samsung STH-N271 Service Manual page 67

AMPS (Limiting and Discrimination)
The downconverted signal from the output of F301 enters to the limiter and discriminator of U304. For the
better suppression of the unwanted interferers, discrete LC filter is used in the limiter section. The signal
through the limiter goes into the discriminator (or quadrature detector) of U304 with incorporation of the
quadrature tank circuit (LC resonator) placed outside of U304, and thus the audio signal comes out from the
pin 16 of U304. This demodulated signal is connected to U101.
TDMA (Rx AGC and I, Q Demodulator)
The downconverted signal from the output of F301 enters to the Rx AGC and I, Q demodulator of U304. The
Rx AGC has the performance of 80-dB dynamic range, 10 dB NF, and -85 dBm IIP3. In the process of I, Q
demodulation, 450 kHz local signal, made by dividing 14.4 MHz reference clock, is used.
2-4.Transmitter
In the AMPS mode, Mod signal from U101 (interface IC) is applied to the tuning voltage terminal of
OSC303 (175.05 MHz IF VCO) with the internal IF PLL of U304. In the TDMA mode, Tx I, Q signals
coming from U101 are applied to the I,Q modulator.
The modulated signal goes into the Tx AGC of U304. The Tx AGC has the 65-dB dynamic range. The
output of the Tx AGC enters the upconverter of U304. For the upconversion, the local frequency source
from OSC301 (dual RF VCO) is used, as well. The typical output power of the upconverter is -10 dBm at
MAC: 0000. The ACPR of the upconverter is 32 dB at adjacent channel and 60 dB at alternate channel.
The upconverted signal passes through F310, U307, and F309. F309 and F310 are same RF Tx SAW filters.
U307 (Tx driver) performs the amplification of the input signal by 27 dB, in order to provide enough input
power into U305 (power amplifier). The ACPR of U307 is 33 dB at adjacent channel and 61 dB at alternate
channel.
U305 has 30-dB linear gain, and maximum output power of 31 dBm (AMPS) and 30 dBm (TDMA). The
ACPR of U305 is 29 dB at adjacent channel and 49 dB at alternate channel. This satisfies the TIA/EIA-136
specification. The output power of U305 is transmitted to F313 (Cellular band coupler) in order to help the
Tx power control by using the power detector diode. In result, the Tx power is transmitted to the air
interface via an antenna.
SAMSUNG Proprietary-Contents may change without notice
11-7
STH-N271
Circuit Description