Driver Stage; Final Stage; Antenna Switch; Harmonic Filter - Motorola CDM1550 LS+ Detailed Service Manual

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Theory of Operation
In the receive mode the DC voltage control line is at ground level which switches off the biasing
voltage to U3401 via U3503.
2.8.2 Driver Stage
2-27
The next stage is an 11 dB gain LDMOS device (Q3420) which requires a positive gate bias and a
quiescent current flow for proper operation. The voltage of the line PCIC_MOSBIAS_1 is set in
transmit mode by PCIC pin 24 and fed to the gate of Q3420 via the resistive network R3525-6. The
bias voltage is tuned in the factory.
2.8.3 Final Stage
The final stage is an enhancement-mode N-Channel LDMOS device (Q3440) providing a gain of
17 dB. This device also requires a positive gate bias and a quiescent current flow for proper
operation. The voltage of the line Bias_2_700_PA_1 is set in transmit mode by the ASFIC and fed to
the gate of Q3440 via the resistive network R3527 -8. 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, A+,
via L3442. A matching network consisting of C3448-C3452 and L3442 transforms the impedance to
50Q and feeds the antenna switch and harmonic filter.
2.8.4 Antenna Switch
The antenna switch utilizes the existing de feed (A+) to the last stage device (Q3440). Both PIN
diodes 03470 and 03471 are turned on during key-up by forward biasing them. Forward biasing is
achieved pulling down the voltage at the cathode end of 03471 to around 11.8V (0.7V drop across
each diode). Q3471 is configured as a current source that maintains the current through the
antenna switch diodes at 90 mA, as set by VR3470 and R3472. The current source is enabled only
during transmit by Q3470 and U3501 pin 32.
2.8.5 Harmonic Filter
Inductors L3474-6, capacitors C3481-2 and two microstrip stubs form a low-pass filter to attenuate
harmonic energy from the transmitter. R3475 is used to drain any electrostatic charges 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. L3479
and C3492-3 form a trap that improves 2nd harmonic rejection.
2.8.6 Bi-Directional Coupler
The Bi-directional coupler is a microstrip printed circuit which couples a small amount of the forward
and reverse power of the RF power from Q3440. The coupled signal is rectified to an output power
proportional de voltage by the diodes 03472 and 03473 and sent to the RFIN of the PCIC. The
PCIC controls the gain of stage U3401 as necessary to hold this voltage constant. This ensures the
forward power out of the radio is held to a constant value.
2.8. 7 Power Control
The transmitter uses the Power ControiiC (PCIC, U3501) to control the power output of the radio. A
portion of the forward RF power from the transmitter is sampled by the bi-directional coupler and
rectified, to provide a de voltage to the RFIN port of the PCIC (pin 1) which is proportional to the
sampled RF power.
The PCIC has internal digital to analog converters (DACs) which provide the reference voltage of
the control loop. The reference voltage level is programmable through the SPI line of the PC I C. This
reference voltage is proportional to the desired power setting of the transmitter, and is factory