TRANSCEIVER 400 MHz
MTP700 Portable Radio / Detailed Service Manual
4.1 - 15
Wideband RF stability of the PA is optimised by insertion of RF losses (R4008, R4006,
R4004) in the gate circuit of Q4001.
The U4001 amplifier gain is set in the factory using the VBLIN signal (from WPIC). VBLIN
is amplified by DC amplifier U4002. The voltage output from U4002 sets the quiescent
current and hence the gain of the LDMOS driver IC U4001.
The gain of Q4001 is set in factory through serial SPI commands (from RedCap2) to the
E-pot IC U4102. The output voltage of the E-pot is dependent on the internal resistance
divider ratio and amplified by DC amplifier U4100. During transmission, the 2.775V sup-
ply to the E-pot is enabled using the signal PA_BIAS (from RedCap2). When this line sig-
nal is 2.775V (HIGH), the voltage to bias the final stage device is enabled.
Coupler and Feedback Path
The output signal from the RF PA passes through directional coupler U5001 to the Isola-
tor FL5001. The sampled signal from the coupler provides feedback for linearization and
power control. The sampled signal is routed via attenuator R5001, R5002, R5003 to the
LNODCT (pins 37, RFIN; 36, RFINB). The feedback signal is routed back to LNODCT
and mixed down to baseband in a quadrature mixer, amplified and summed with the
baseband input signal.
Isolator
The signal in the forward path is fed to pin 4 of the isolator FL5001. The isolator is placed
at the PA output to decrease the influence of antenna impedance variation on the PA per-
formance. The reflected power from the antenna is absorbed in a 50-Ohm resistor inside
the isolator. The isolator also protects the Cartesian loop from sudden VSWR variations
that could lead to loop instability.
Antenna Switch
PIN diodes D5001, D5002 and D5003 are used as antenna switches. These PIN diodes
are turned on during the transmission time slot. The RF signal from the Isolator is applied
to the PIN diodes D5001 and D5002. The RF signal is then routed through the harmonic
filter (C5011, L5008, C5015 and C5016) to the antenna. D5003 provides the needed RX
isolation during Transmission mode.
In the Receiving mode the PIN diodes are turned off, the input RF signal is directed to the
receiver path via
λ
/4 quarterwave equivalent LC-circuit (C5011, L5007 and C5020) and a
further low pass filter (C5020, L5011, C5022 and C5021).
The condition of the PIN diodes is controlled by the voltage switch Q5001 by applying the
ANT_EN signal (from RedCap2) to the switch. The VCNTO signal (from WPIC) applied to
the voltage switch Q5001 is set LOW during the Receiving and Transmission modes. The
resistors R5006 and R5008 determine the DC current through the PIN diodes.
The RF output signal is disabled during the CLCH (Training) mode by setting the VCNTO
signal HIGH thus providing low output power to the antenna during the training slot.
Harmonic Filter
From the antenna switch the signal is routed to a single-section LC filter, consisting of
L5008, C5011, C5015 and C5016. The filter attenuates the harmonics of the transmitter
signal and reduces leakage of the Rx LO signal.
Resistor R5014 provides a discharge path for any build-up of static electricity at the
antenna.
Содержание IMPRES Tetra MTP700
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