2-20 Theory of Operation
2.5.1
Power Controlled Stage
The first stage (U3401) is a 20 dB gain integrated circuit containing two LOMOS FET amplifier
stages.
It
amplifies the RF signal from the VCO (TXINJ). The output power
of
stage U3401
is
controlled
by
a de voltage applied to pin 1 from the power control circuit (U3501 pin
4,
with U3402
providing current gain and level-shifting). The control voltage simultaneously varies the bias of two
FET stages within U340 1. This biasing point determines the overall gain
of
U340 1 and therefore its
output drive level to 03421 , which
in
turn controls the output power
of
the
PA.
In
receive mode the voltage control line is at ground level which switches off the biasing voltage to
U3401.
2.5.2 Pre-Driver Stage
The next stage is an LOMOS device
(03421)
providing a gain
of
+13 dB. This device requires a
positive gate bias and a quiescent current
flow for proper operation. The voltage
of
the line
PCIC_MOSBIAS_1 is set during transmit mode by the PCIC pin 24, and fed to the gate
of
03421 via
the resistive network R3415-16. The bias voltage is factory tuned.
2.5.3 Final Stage
The following stage is an enhancement-mode N-Channel MOSFET device
(03441)
providing a gain
of
10 dB. This device also requires a positive gate bias and a quiescent current flow for proper
operation. The
voltage
of
the line MOSBIAS_2 is set
in
transmit mode by the ASFIC and
fed
to the
gate
of
03441 via the resistive network R3404, R3406, and R3431-5. This bias voltage
is
also tuned
in
the factory. If the transistor
is
replaced, the bias voltage must be tuned using the Customer
Programming Software (CPS). Care must be taken not to damage the device by exceeding the
maximum
allowed bias voltage. The device's drain current is drawn directly from the radio's de
supply voltage input, PASUPVLTG, via L3437.
A matching network consisting
of
C3441-2, C3445-6, and L3443 and
two
striplines, transforms the
impedance to approximately 50 ohms and feeds the directional coupler.
2.5.4 Directional Coupler
The directional coupler is a microstrip printed circuit, which couples a small amount
of
the forward
and reflected power delivered by 03441. The coupled signals are rectified by 03451. The resulting
de voltage is proportional to
RF
output power and feeds the RFIN port
of
the PCIC (U3501, pin 1
).
The PCIC controls the gain of stage U3401 as necessary to hold this voltage constant, thus
ensuring the forward power out
of
the radio to be held to a constant value.
An abnormally high reflected power level, such as may be caused by a damaged antenna, also
causes the
de
voltage applied to the PCIC to increase, and this will cause a reduction
in
the gain
of
U3401, reducing transmitter output power to prevent damage to the final device due to an improper
load.
2.5.5 Antenna Switch
The antenna switch consists
of
two PIN diodes, 03471 and 03472.
In
the receive mode, both diodes
are off.
Signals applied at the antenna jack J3401 are routed, via the harmonic filter, through
network L3472, C3474 and C3475, to the receiver input. In the transmit mode, the keyed A+ turns
on 03471 which enables current sink
03472
, set to
96
rnA by R3473 and VR3471. This completes
a de path from PASUPVLTG, through L3437, L3443, 03471, L3472, 03472, L3471 , R3474 and the
current sink, to ground. Both diodes are forward biased into conduction. The transmitter
RF
from the
directional coupler is routed via 03471 to the harmonic filter and antenna jack.
03472
also
conducts, shunting RF power and preventing it from reaching the receiver port (RXIN). L3472 is
selected to appear as a broadband quarter-wave transmission
line, making the short circuit
presented
by
03472
appear as an open circuit at the junction
of
03472
and the receiver path.
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