Operating instructions
Frequency Generation Circuitry 6-5
The output of the second bandpass filter is passed to a mixer. The mixer is a passive silicon
monolithic double balanced mixer (U401). After mixing with the first LO signal from the voltage control
oscillator (VCO) using high side injection (for 800MHz) or low side injection (for 900MHz), the RF
signal is down converted to the 73.35 MHz IF signal. The 50 Ohm matching for the mixer at the LO
input port is provided by C431 and L414.
The IF signal coming out of the mixer is applied to the crystal filter (FL400) through a resistor pad and
a diplexer (L411 and C427). Matching to the input of the crystal filter is provided by L412 and C428.
The crystal filter provides the necessary adjacent channel selectivity and inter-modulation protection.
The output of the crystal filter (FL400) is matched to the input of the IF amplifier transistor by C432,
C433, L415 and L416. Voltage supply to the IF amplifier is 5V_RX, taken from RX VCO. The IF
amplifier provides a gain of 28 dB. The IF amplifier output provides a 50 Ohm matched at the IF
amplifier output to ABACUS (U600) pin 47 via L418 and C446.
2.2 Receiver Back-End
ABACUS (U600) is tuned via software at radio start-up. The IF signal applied to pin 47 of U600 is
down-converted to a second IF of 2.25 MHz. The second IF of 2.25 MHz will then be fed into an ADC,
generating the Is and Qs data. This data is in the form of SSI (Serial Synchronous Interface) at pin 29
of U600, which will then be sent to the DSP (OMAP1710) for further processing.
The second VCO is a Colpitts oscillator built around transistor Q602. The VCO has a varactor diode,
D600 to adjust the VCO frequency. The second LO frequency could either be 71.1 MHz or 75.6 MHz,
depending on the system requirement. The control signal is derived from a loop filter consisting of
C617, C618, C619, R605 and R606. The RSSI circuitry is achieved via software programming.
3.0 Frequency Generation Circuitry
Figure 6-4: Frequency Generating Unit Block Diagram
800MHz
VCO
900MHz
VCO
RX INJ
TX INJ
TX_SEL
900M_VCO_EN
800M_VCO_EN
BUFFER_EN
Loop
Filter
16.8MHz
GPS
ABACUS
OMAP SQUARER
RX_LNA
TX_CLK, TX_DATA,
TX_FSYNC
SPI_MOSI, SPI_MISO,
SPI_CLK, THK_CE
DMCS
LOCK DETECT
DEEPSLEEP
GP02
GP04
GP03
GP01
MN_CP
MN_ADAPT
HP_MOD_OUT
REF_IN
AUX_DAC
TX SSI
SPI
DMCS
TEST1
DEEPSLEEP
TRIDENT IC
SF_OUT
MN_CP_VCC
REF_OSC_OUT
LNA_EN
LNA_AGC
LNA_ATTN
VMULT1
VMULT2
PRESCALER
TCXO
Voltage
Multiplier