Samsung STH-N271 Service Manual page 66

STH-N271
Circuit Description
2. RF Section
2-1.Introduction
From the starting point of developing the STH-N271, the cellular and the PCS band operations have been
intended. Therefore, the STH-N271 contains many dual-band components and rooms for PCS band
components, in order to widen the operating band for the next product. For playing essential role in the
radio part, The STH-N271 includes many active components. They are U301 (dual-band LNA), U305
(Cellular band power amplifier), U304 (transceiver including up/down converter, RF and IF PLLs, Rx and
Tx AGCs, IQ modulator and demodulator, limiter, and discriminator), OSC301 (dual-band RF VCO), and
OSC303 (dual-band IF VCO). In order to obtain good selectivity and sensitivity simultaneously, STH-N271
has dual IF conversion system. The 1st IF and 2nd IF are 130.05 MHz and 450 kHz, respectively.
2-2.Front-End part
RF signal from the air interface passes F305 (diplexer) and F306 (duplexer). The purpose of F305 is the
band selection in conjunction with U303 (SPDT switch) according to the band control signal from the
baseband chipset. F306 plays the role of Rx and Tx filtering and the isolation between Rx and Tx in the
cellular band (AMPS and Digital TDMA).
2-3.Receiver
The Rx signal from F306 goes into U301. U301 amplifies the very weak Rx signal with adding the
negligible thermal noise. U301 features 20 dB gain, 1.3 dB NF, and 7 mA current consumption. For the
wider dynamic Rx AGC (automatic gain controller) range, the gain of U301 is controlled by PREAMP_G
signal coming from U203, resulting in around 30-dB step gain change. By checking the RSSI, U203 decides
to whether turn on U301 or not. The amplified signal goes into F304 (RF SAW filter) in order to suppress
the unwanted band data.
The signal coming out from F304 inflows to the pin 45 (1st Rx mixer input) of U304, and it is
downconverted to the IF of 130.05 MHz. This 1st mixer has around 7 dB conversion gain, 6dB NF, and 5
dBm IIP3. For the downconversion, the local frequency source from OSC301 (dual RF VCO) is used. As for
the local frequency, the Rx frequency plus 1st IF (that is, high injection type local frequency) is used, and
this frequency is controlled by the RF PLL in U304 in conjunction with U203 (baseband IC).
The 1st mixer output is connected to F302 (130.05 MHz 1st IF SAW filter). The main role of F302 is
suppression of undesirable interferers. For the 2nd IF conversion, the output of F302 is connected to the
2nd Rx mixer of U304. The 2nd mixer has the typical characteristics of 13.5 dB gain, 12 dB NF, and -20 dBm
IIP3. The local frequency source for making 2nd downconversion is supplied from U304 internally by 9
times multiplying the fundamental frequency of OSC302 (14.4 MHz VC-TCXO). The downconverted signal
passes F301 (450 kHz Ceramic filter) in order to suppress the unwanted interferers and the image
frequency signals.
Until now, the Rx signal path is common for both of AMPS and Digital TDMA. From the filtered 450 kHz
signal output, AMPS path and TDMA path become separate for proper treating of the Rx signal. To begin
with, AMPS path will be described.
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