–
(SAW) filter) to remove unuseful signals in corresponding frequency band. After passing through the RF
band-pass filter and LNA (Q6102), the RF signals together with the first LO signal are sent to the mixer
for the first frequency conversion, to generate the first IF signal of 73.35 MHz. Meanwhile, The LO signal
(12–13 dBm) from the VCO feeds to Q6103 to get a gain (8 dB). The first LO signal (73.35 MHz) passes
through the IF filter (Z6100) to filter the out-of-band spurious signals, and then is sent to the two-stage IF
amplifier circuit (composed of Q6104 and Q6105) for amplification. Finally the amplified signals go to the
IF IC (U701) for digital processing.
Receiver Back-end
Σ-ΔADC
DECIMATION
FILTER
FORMATTING/SSI
CONTROL LOGIC
DAC AGC
SPI
VOLTAGE
REFERENCE
CLK SYN
LO
SYN
L
N
A
LO VCO AND LOOP
FILTER
LOOP FILTER
16dB
IOUTL
LOP LON IOUTC CLKP CLKN
VREFP
VCM VREFN PC PD PE SYNCB
DOUTA
DOUTB
FS
CLKOUT
MXOP MXON
IF2P IF2N
GCP GCN
IFIN
FREF
SSI TO OMAP
The first IF signal (73.35 MHz) output by the IF amplifier goes into IF IC via Pin 47, where the signal is
converted to the second IF signal (2.25 MHz). Then the un-demodulated digital I/Q signal output from
the SSI is sent to U302 for demodulation. IF IC employs a reference frequency of 19.2 MHz and shares
the crystal with U302. The second LO VCO consists of the external transistor, varactor and relevant
components, to provide the second LO signal. The clock frequency is 18 MHz, which is generated by the
external LC (D702, L715 and C745) resonance loop.
DSP (U302) acquires the audio signal from IF IC. Then this signal goes through U302 to output data
signal, which is sent to U231 and PIN44 (SSI_DI) for digital-to-analog conversion. Finally, the output
analog audio signal will be subjected to gain control, and then fed to the speaker.
5.3.3 FGU Circuit
The PLL synthesizer that consists of the crystal oscillator (X100), PLL IC (U100) and VCO, is used to
supply excitation signal source to the transmitter, and LO signal to the receiver.
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