Motorola Astro Digital Mobile Radio Xtl5000 Detailled 6881096c74 B Manual

6881096C74-BMay 25, 2005
Theory of Operation: Receiver Back-End3-25
3.5.4.1  Intermediate Frequency (IF) Filter
The XTL 5000 radio uses two leadless, surface-mount, two-pole, third-overtone, quartz crystal filters 
(B6350, B6351) separated by a 20 dB gain IF amplifier. The filter is centered at 73.35 MHz. This 
narrow-bandpass filter gives the radio its adjacent-channel and alternate-channel rejection 
performance. Components L6350, L6351, L6352, L6353, C6351, C6352, C6353, C6355, C6356, 
and C6357 are...

May 25, 20056881096C74-B
3-26Theory of Operation: Transmitter
3.5.4.2.1  Second Local Oscillator (LO)
The ABACUS III IC local oscillator (LO) synthesizer controls the second LO. Signal FREF is the 
16.8 MHz reference from the frequency generation unit (FGU). The second LO frequency is 
75.6 MHz by default, or 71.1 MHz in special cases as necessary to avoid radio self-quieters. The 
second LO signal mixes with IFIN to produce a 2.25 MHz final IF. The external VCO consists of 
transistor Q6000, together...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-27
First Stage
The RFPA first stage provides gain that is determined by the control voltage, RFPA_CNTRL. This 
control voltage is generated in the power control section and is a function of the final-stage output 
power, temperature, and current, as well as the control and A+ voltage levels. See “3.6.1.4. Power 
Control (for 50W and 100W Transmitter)” on page 3-30 for a detailed explanation of the power 
control section.
The 2 mW TX_INJ signal is...

May 25, 20056881096C74-B
3-28Theory of Operation: Transmitter
3.6.1.2  100-Watt Transmitter
The following text discusses the 100-W transmitter.
3.6.1.2.1  RF Power Amplifier (RFPA)
The RFPA consists of three gain stages, which are shown in Figure 3-22.
Figure 3-22.  100-Watt RF Power Amplifier (RFPA) Gain Stages (VHF)
First Stage
The RFPA first stage provides gain that is determined by the control voltage, RFPA_CNTRL. This 
control voltage is generated in the power control section and is a function of...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-29
Dual Final Stage
C3540-1, C3543-4 and transmission lines form a bandpass. R3541-8 provide stability for Q3540. 
R3550-7 provide stability for Q3541. R3540 and R3568 supply the VGBIAS1 gate bias to Q3540. 
R3569 and R3558 supply the VGBIAS2 gate bias to Q3541. L3540-4 and C3560-1 form the A+ drain 
bias circuit. C3548-55 and transmission lines form a low-pass. Q3540-1 operate Class AB.
R3560-1, C3561, and U3561 comprise the final-stage,...

May 25, 20056881096C74-B
3-30Theory of Operation: Transmitter
3.6.1.4  Power Control (for 50W and 100W Transmitter)
The power control section is comprised of a control loop to level forward power and protection 
mechanisms to reduce power to a safe level for the given environmental conditions 
(see Figure 3-24).
Figure 3-24.  Power Control Components (VHF)
Power Control Loop
VFORWARD from the ON is buffered via the non-inverting, variable-gain stage U0956-2 whose gain 
is set by EPOT U0952. The proper...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-31
Protection Mechanisms
Final-stage temperature is sensed in the RFPA resulting in VTEMP, which is proportional to 
temperature. VTEMP is compared against a reference voltage TEMP_1 (U0959, pin 9) via U0957-1. 
When VTEMP exceeds TEMP_1, the U0957-1, pin 1, voltage increases and forward biases one of 
the D0951 diodes, which cuts back power. Power continues to cut back with rising temperature until 
the voltage level at the junction of R0978 and...

May 25, 20056881096C74-B
3-32Theory of Operation: Transmitter
First Stage
The RFPA first stage provides gain that is determined by the control voltage, RFPA_CNTRL. This 
control voltage is generated in the power control section and is a function of the final-stage output 
power, temperature, and current, as well as the control and A+ voltage levels. See “3.6.2.4. Power 
Control (for 40W and 100W Transmitter)” on page 3-35 for a detailed explanation of the power 
control section.
The 0.5 mW TX_INJ signal...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-33
3.6.2.2  100-Watt Transmitter
The following text discusses the 100-W transmitter.
3.6.2.2.1  RF Power Amplifier (RFPA)
The RFPA consists of three gain stages, which are shown in Figure 3-26.
Figure 3-26.  100-Watt RF Power Amplifier (RFPA) Gain Stages (UHF Range 1)
First Stage
The RFPA first stage provides gain that is determined by the control voltage, RFPA_CNTRL. This 
control voltage is generated in the power control section and is a...

May 25, 20056881096C74-B
3-34Theory of Operation: Transmitter
Dual Final Stage
C5530, C5531, C5532, C5533, C5536, C5537-41 and transmission lines form a low pass, splitter 
match that transfers power to the LDMOS dual final-stage transistors Q5541 and Q5540. Q5540 and 
Q5541 contain two transistors in a single package, each with its own gate and drain lead. R5542-5, 
R5548-51 provide stability for Q5540. R5552-5, R5557-60 provide stability for Q5541. R5546, C5602 
and C5542 supply the VGBIAS1 gate bias...