cxa3003r Sony Electronics, cxa3003r Datasheet - Page 23

• Manufacturer Part Number: cxa3003r
• Description: Baseband Analog Processing Ic For Dual-mode Cdma/fm Cellular Phone
• Manufacturer: Sony Electronics

3. CDMA Low-Pass Filtering
4. CDMA Analog-to-Digital Conversion
5. FM Receive Signal Path
RXFMSTROBE
After mixing, the receive signal path splits into CDMA and FM sections. For CDMA, the baseband signal
extends from 1 kHz to 630 kHz. Frequency components above 750 kHz are out-of-band for CDMA
operation. The mixers and the subsequent CDMA low-pass filters combine to form the down-converter
which outputs the CDMA baseband signals. The passband, transition band, and rejection band
characteristics of these low-pass filters, in conjunction with external IF bandpass filtering, contribute to the
ability of the receiver to select the desired baseband signals from the jamming effects of unwanted
signals.
The need to control the offset at the inputs of the ADCs is critical to the receive signal path and the digital
processing section. The offset control inputs : IOFFSET and QOFFSET, are provided for this purpose.
Analog I and Q baseband components are converted to digital signals by the two identical
4-bit flash (parallel) ADCs. The CDMA ADCs output a new digital value on each falling edge of the ADC
clock signal, CHIPx8.
The CHIPx8 ADC clock frequency of 9.8304 MHz is synthesized from the system crystal oscillator
frequency of 19.68 MHz. The system crystal oscillator frequency is applied to the TCXO input of this IC.
The receive signal path for FM operation is similar to that for CDMA operation. Differences lie in the
characteristics of the I and Q low-pass filters and the ADCs. The IF frequency is the same as in CDMA
(85.38 MHz), but the modulation can only extend ±15 kHz from the IF center frequency, forming a 30 kHz
wide channel. The low-pass filters for FM operation have a much lower bandwidth than those used in
CDMA. The offset of the FM low-pass filters is controlled just like the CDMA low-pass filters by the
IOFFSET and QOFFSET input pins.
The lower bandwidth of the FM baseband signal gives rise to the use of very low power 8-bit successive-
approximation ADCs. The FM I and Q analog baseband signals are sampled and held during the analog
to digital (A/D) conversion process. The A/D conversion is initiated with a external strobe signal. A serial
data stream is output beginning with the most significant bit (MSB) of the result.
RXVCO T1
RXVCO T2
QOFFSET
IOFFSET
FMCLK
RXIFB
TCXO
RXIF
VCO
Fig. 7 Receive Section Block Diagram of CXA3003R
1/2
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CDMA Q
RX LPF
FM Q
RX LPF
FM I
RX LPF
CDMA I
RX LPF
CHIP
X8
1/4
CDMA Q
ADC
CDMA I
ADC
FM Q
RSSI
ADC
ADC
FM I
RSSI
TCXO4
RXQD3
RXQD2
RXQD1
RXQD0
RXID3
RXID2
RXID1
RXID0
CHIPX8
RXQFMDATA
RXIFMDATA
CXA3003R