21 © 2002 Motorola, Inc.
T720
Circuit Description &
Theory of Operation
IFR IC to MSM for decoding etc.
TRANSMITTER SECTION
Digital signal is applied in 8 bit to RFT3100
IC, the MSM output I and Q signals one by
one and each of these signals passes the digi-
tal LPF and is sent to the quadrature modula-
tor. From complex signal within the
baseband the quadrature modulator makes
the real spectrum of double-side band having
228.6Mhz (263.6Mhz for PCS) having
obtained by dividing VCO frequency
457.2Mhz (527.2 Mhz for PCS) into half as
carrier(Tx IF).
MSM5100 count from the received signal
strength level (Open loop power) and gener-
ates PDM signals according to TX_AGC
level appropriate for the number of power
control bit not inconsistent with the total sum
of Eb/No defined by the base station(Close
Loop Power Control) and the calculated base
station power. The dynamic range is approxi-
mately 90 dB.
The Tx IF signal of 228.6Mhz (263.6Mhz
for PCS) is up-converted to UHF signal. The
signal converted into transmit frequency is
filtered and finally amplified by the power
amplifier (PA), which then pass through the
isolator, duplexer and diplexer and transmit-
ted to the antenna.
The transmitter section occupies a frequency
band of 824 Mhz to 849 Mhz in cellular band
and a frequency band of 1850Mhz to
1910Mhz in PCS band.
FREQUENCY SYNTHESIZER SECTION
The frequency synthesizer uses VC-TCXO-
19.2Mhz as reference frequency. 19.2 Mhz is
input to the frequency synthesizer as refer-
ence input of VCO/PLL MODULE IC and
then divided to frequency appropriate for the
channel spacing by the reference divider in
the IC.
This IC generates LO signal for the conver-
sion of Transmit/Receive carrier frequency.
One is used LO signal to down convert at the
1
st
mixer in the receiver and the other is used
as local signal for transmitter up converting
mixer. Inside the IC the prescaler and the
variable divider divides the input fre-
quency(feed back) according to the channel
number and frequency and then the phase
detector compares it with the reference fre-
quency and any correction is communicated
through the LPF to the VCO.
The digital data for the determination of
channels for the frequency synthesizer is
sent from the MSM and consists of
SYNTH_CLK, SYNTH_DATA, and
SYNTH_LATCH in serial interface.
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