Service Manual
85
L-C network.
9.1.3 Driver Stage PA
OS power tube (Q402). It can further amplify the RF signals from Q401 to 38dBm,
9.1.5 Harmonic Suppression filter
9.1.6 Directional Coupler
The role of directional coupler is to detect forward and reverse power, so as to monitor and diagnose the
operating status of transmitter. The forward power passes through a ʌ-type attenuator, and then goes
into the logarithmic amplifier, where it can automatically limit the output power of the transmitter to a
relatively constant value (50W) according to the preset power level “Vset”. After going to the
analog-to-digital converter (ADC) on the baseband board via the diode D402, the forward power is used
to calculate the current TX power of the transmitter, which will be displayed in the diagnosis software in a
real-time way. The directional coupler locates behind the LPF and is near the antenna output end. It is
used to detect the reverse power. After going to the ADC on the baseboard via the diode D401, the
reverse power is used to calculate the VSWR of antenna connector. An alarm will be generated when
the VSWR is larger than the set threshold. Both forward and reverse detection voltages are calculated
via a differential amplifier circuit (U407A). The output value is used to control the power level “Vset”.
When the VSWR is too high, the TX power will be reduced to avoid damage to the PA.
9.1.7 Power Control
The transmitter power is controlled by power control circuit composed of logarithmic amplifier and
. After the transmitter power passes through the directional coupler, the detected RF
a constant output
30dBm, which then will be sent to driver-stage PA via
This PA unit is a LDM
which is then sent to final-stage PA.
9.1.4 Final-stage PA
This PA unit consists of two LDMOS power tubes (Q403 and Q404). It can reduce the Q value of power
input/output network and expand the bandwidth of PA circuit, ensuring PA circuit performance.
The harmonic filter of transmitter is a four-order LPF filter comprising C455, L410, C456, L412, C459,
C460, L413, C461, C462, L414 and C465. It can decrease the harmonic component by increasing the
out-of-band rejection capability.
directional coupler
signal goes to RF_IN of logarithmic amplifier via the Pi-type attenuator. Then the detected RF signal will
adjust the output “Vapc” of logarithmic amplifier in a real time way based on the current power level
“Vset”, so as to control the bias voltage at the gates of Q401 and Q402 and to ensure
power.
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