08-16-2017, 10:21 PM
CDMA LSI Chip Set
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CDMA Portable/Mobile Phone Systems
First, consider the signal flow during
reception. Figure 1 shows a block
diagram of a CDMA portable phone.
The very weak receive signal from a
base station is down-converted to the IF
frequency band by the RF receiver, and
amplified by the RX AGC unit. The IF
signal is down-converted to an analog
baseband signal by the analog baseband
processor, and then the analog signal is
converted to a digital signal. This digital
signal is input to the mobile station
modem where it undergoes CDMA
demodulation, and is then output to the
speaker via the CODEC.
CDMA Signal Processing
In the receiver, the CDMA spread spectrum
modulated IF signal input to the
CXA3003R via the RF front-end block
is multiplied in the mixer block by an
orthogonal local oscillator signal generated
inside the CXA3003R. The signals
pass through the CDMA low-pass
filters and are converted to analog
baseband signals, and are then converted
into two 4-bit digital signals, I
and Q, in the respective A/D converters,
before being output.
FM Signal Processing
In the receiver, the signal path is the
same as for CDMA as far as the mixer
block. After this, the signals pass
through the FM low-pass filters and are
converted to FM baseband signals, and
are then converted into serial 8-bit digital
signals before being output.
In the FM transmitter, the FM digital
baseband input signal input from the
digital processor passes through the D/
A converter and FM low-pass filter, and
is converted to an FM analog baseband
signal. This signal modulates the transmit
VCO frequency (130.38 MHz) and
the result is output as an FM modulated
signal.
Excellent Analog Filter Technology
The current service frequency band for
CDMA is 800 MHz. Since this frequency
band contains powerful waves
generated by the AMPS analog FM
mobile phone system, very stringent
filter characteristic specifications are
used to prevent interference from these
waves. The provision of an on-chip
automatic control filter configuration
which determines the gm of the filters
in the IC according to the input reference
frequency, as shown in figure 3,
results in a good frequency characteristic
which is unaffected by either
temperature fluctuations or element
variations .
Wide Dynamic Range and Low Noise
As a basic principle, CDMA communication
requires level adjustment of both
receive and transmit signals using AGC.
Reception AGC requires a wide input
dynamic range, and transmission AGC,
a wide output dynamic range. The gain
control range in both cases is 80 dB or
more, and this must also be achieved
with a low noise level.