First Power Controlled Stage; Power Controlled Driver Stage; Final Stage; Directional Coupler - Motorola CDM Series Detailed Service Manual

Section 13: 2-4
3.1

First Power Controlled Stage

The first stage (U5401) is a 20 dB gain integrated circuit containing two LDMOS FET amplifier
stages. It amplifies the RF signal from the VCO (TXINJ). The output power of stage U5401 is
controlled by a DC voltage applied to pin 1 from Q5473. The control voltage simultaneously varies
the bias of two FET stages within U5401. This biasing point determines the overall gain of U5401
and therefore its output drive level to Q5421, which in turn controls the output power of the PA.
Switch S5440 is a pressure pad with a conductive strip which connects two conductive areas on the
board when the radio's cover is properly screwed to the chassis. When the cover is removed, S5440
opens and cuts the supply voltage to Q5473 thus disabling control voltage to U5401. This prevents
transmitter key up while the devices do not have proper thermal contact to the chassis.
3.2

Power Controlled Driver Stage

The next stage is an LDMOS device (Q5421) providing a gain of 12 dB. This device requires a
positive gate bias and a quiescent current flow for proper operation. The bias is set during transmit
mode by U5501-24 and fed to the gate of Q5421 via the resistive network R5480, R5408, R5415
and R5416.
3.3

Final Stage

The final stage is an LDMOS MRF1570 device Q5441. It provides a gain of 11 dB. It integrates two
MRF1535 die inside. These two die are being run in parallel. The output of each of the device (die)
inside are combined to constitute the total output power. This device also requires a positive gate
bias and a quiescent current flow for proper operation. The voltage of the line MOSBIAS_2 and
MOSBIAS_3 are set in transmit mode by the ASFIC and fed to the gate of Q5441 via two resistive
networks R5631, R5634, R5485, R5486 and R5632, R5635, R5481, R5489. These bias voltages
are tuned in the factory. If the transistor is replaced, the bias voltage must be tuned using the Global
Tuner. 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 DC supply voltage input,
PASUPVLTG, via L5421, L5437 and L5436, L5438. A matching network consisting of C5440 – 45,
C5466 – 67, C5476 – 77, C5487, C5491, C5455 – 56, C5449 – 50, C5465, C5468 and striplines
transforms the impedance to 50 ohms and feeds the directional coupler.
3.4

Directional Coupler

The directional coupler is a microstrip printed circuit, which couples a small amount of the forward
power delivered by Q5441. The coupled signal is rectified by D5451. The DC voltage is proportional
to the RF output power and feeds the RFIN port of the PCIC (U5501 pin 1). The PCIC controls the
gain of stage U5401 as necessary to hold this voltage constant, thus ensuring the forward power out
of the radio to be held to a constant value.
6881091C63-F
UHF Band 2 (450–512/520 MHz) 25–40 W LDMOS Transmitter Power Amplifier (PA) 40 W