Motorola pro7100 Manuel de service Page 43

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Theory of Operation 2-29
2.8.3 Driver Stage
The following stage is an enhancement-mode N-Channel MOSFET device (Q3431) 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 Q3431 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 devices drain current is drawn directly from the radio’s dc supply
voltage input, PASUPVLTG, via L3431 and L3432.
2.8.4 Final Stage
The final stage uses bipolar device Q3441. The device’s collector current is also drawn from the
radio’s dc supply voltage input. To maintain class C operation, the base is dc-grounded by a series
inductor (L3441) and a bead (L3442). A matching network consisting of C3446-52, C3467, L3444-
5, and two striplines, transforms the impedance to approximately 50 ohms and feeds the directional
coupler.
2.8.5 Directional Coupler
The directional coupler is a microstrip printed circuit, which couples a small amount of the forward
and reflected power delivered by Q3441. The coupled signals are rectified by D3451-2 and
combined by R3463-4. The resulting dc 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 dc 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.
2.8.6 Antenna Switch
The antenna switch consists of two PIN diodes, D3471 and D3472. 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 9 volts turns on
Q3471 which enables current sink Q3472, set to 96 mA by R3473 and VR3471. This completes a dc
path from PASUPVLTG, through L3473, D3471, L3477, L3472, D3472, 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 D3471 to the harmonic filter and antenna jack. D3472 also conducts,
shunting RF power and preventing it from reaching the receiver port (RXIN). L3472 is selected to
appear as a broadband guarter-wave transmission line, making the short circuit presented by D3472
appear as an open circuit at the junction of D3472 and the receiver path.
2.8.7 Harmonic Filter
Components L3491-L3494 and C3490-C3498 form a nine-pole Chebychev low-pass filter to
attenuate harmonic energy of the transmitter. R3490 is used to drain electrostatic charge that might
otherwise build up on the antenna. The harmonic filter also prevents high level RF signals above the
receiver passband from reaching the receiver circuits, improving spurious response rejection.
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