S1M8656 Samsung Semiconductor, Inc., S1M8656 Datasheet - Page 13

• Manufacturer Part Number: S1M8656
• Description: RX if / Bba With Agc
• Manufacturer: Samsung Semiconductor, Inc.

RX IF/BBA WITH AGC
S1M8656A
FUNCTIONAL DESCRIPTION
S1M8656A is a CDMA/AMPS receive-only baseband analog IC, located between the RF mid-frequency
processing terminal and baseband processing terminal. The RF analog mid-frequency signal terminal(IF SAW
filter output), directly connected to the S1M8656A mid-frequency input pin, converts and processes the baseband
signal and sends the corresponding digital signal to the modem IC. Baseband analog processing uses QPSK
modulation, LPF, and A-D converter and the modem IC performs digital CDMA/AMPS baseband modulation on
the digitalized analog baseband signal it receives.
S1M8656A uses a 0.5um BiCMOS, equipped with high-frequency bipolar and low power standardized CMOS
logic, to operate safely in the low power range, consisting of power voltage between 2.7V – 3.6V and operating
temperature between -30 C – +85 C.
CDMA Receive Signal Path
S1M8656A is composed of a receive circuit, installed with TCXO/N, CHIPx8 like clock generator, mode
conversion switch and serial I/F apparatus. The receive circuit has the Rx AGC, an automatic gain controller, and
baseband LPF and output terminal with the A-D converter, and VCO and mixer etc. The input signal is received
as a differential signal, which is modulated to 1.23 MHz spread-spectrum for CDMA. The mid-frequency is
220.38MHz for Korea-PCS, 1.23MHz for US-PCS, and 85.38MHz for cellular; they are set based on the time
constants of the components involved with the external VCO and external Rx PLL. Rx AGC , connected to both
the IF SAW filter and matching component in the RF-IF converter output located in the RF block, amplifies or
reduces according to the signal size. It takes its orders from the modem chip when it sets the appropriate receive
level as required by the CDMA system. Gain is controlled by applying a DC voltage to the RAGC_CONT pin. The
applied DC is produced when the PDM signal, generated as a control signal in the modem, passes through the R-
C filter. The control band of this AGC is approx. 90dB. The QPSK Baseband modulator separates and modulates
the IF signal sent by the AGC using I(In-phase) and Q(Quad-phase) baseband signal. Essentially, two signals, I-
LO and Q-LO (Local oscillator), are mixed with AGC's IF output signals, respectively. The LO(local oscillator)
signal is generated by the internal oscillating components, externally connected tank coil, and Varactor, and the
externally independent PLL device is used to generate its exact oscillation mid-frequency.
T=0
Q-CH
I-CH
Figure 5. Received I/Q Phase in S1M8656A
Defining of the I-Phase and Q-Phase receive path is very important to its design. The polarities of these paths
are also important to digital baseband modulation. Therefore, the output of the QPSK baseband modulation
determines the I and Q phases; I-phase is defined as the phase leading the Q-phase by exactly 90 , but it simpler
to think of I as Cosin and Q as Sin. The figure related to this is shown in Figure 5. This definition is valid only
when the QPSK IF input signal is higher than the IF mid-frequency. The baseband signal, output by the QPSK
modulator, includes various other unnecessary surrounding band noises, which are removed by the use of the
LPF(Low-Pass-Filter).
13