Motorola Astro Digital Mobile Radio Xtl5000 Detailled 6881096c74 B Manual

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-35
Power Detector
The power detector consists of a main microstrip transmission line that transfers power from the 
harmonic filter to the antenna and two parallel lines that are used to detect the forward and reverse 
power. The forward power detection line is terminated via R5707 and R5708. RF energy coupled 
onto this line is rectified and filtered via D5704 and C5715 to provide a DC voltage to the power 
control circuitry that is proportional...

May 25, 20056881096C74-B
3-36Theory of Operation: Transmitter
Figure 3-28.  Power Control Components (UHF Range 1)
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 gain is determined during power-detection calibration tuning. 
Buffered VFORWARD (U0956-2, Pin 7) is added to PWR_SET via R0971 and R0972 and then 
compared to a reference determined by R0974 and R0975. PWR_SET is supplied by the digital-to-...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-37
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
3-38Theory of Operation: Transmitter
The 0.5 mW TX_INJ signal is routed to the U5501 first stage device (Pin 16, RFIN) via C5508 to an 
integrated, wide-band input match. U5501 is a two-stage LDMOS device with a bandpass interstage 
match consisting of L5503, C5507, and C5509 routed between VD1 (pin 14) and G2 (pin 11). L5502 
and L5505 provide the K9.1V drain bias voltage for the first and second stages to VD1 (pin 14) and 
RFOUT1/2 (pins 6 and 7), respectively. The RFPA_CNTRL...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-39
Antenna Switch
The antenna switch functions in two modes determined by the presence of K9.1V. The K9.1V switch 
bias is applied via L5701and C5702. When K9.1V is present, the switch is in TX mode. D5701, 
D5702 and D5703 are forward biased forming a low-loss path from the RFPA final stage to the 
harmonic filter and a 20 dB isolation path between the RFPA final stage and the RX front-end. 
Isolation is achieved via a quarter-wave transmission...

May 25, 20056881096C74-B
3-40Theory of Operation: Transmitter
Figure 3-31.  Power Control Components (UHF Range 2)
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 gain is determined during power-detection calibration tuning. 
Buffered VFORWARD (U0956-2, Pin 7) is added to PWR_SET via R0971 and R0972 and then 
compared to a reference determined by R0974 and R0975. PWR_SET is supplied by the digital-to-...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-41
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
3-42Theory 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 “3.6.4.1.3. Power 
Control” on page 3-43 for a detailed explanation of the power control section.
The 2 mW TX_INJ signal is routed to the U6500...

6881096C74-BMay 25, 2005
Theory of Operation: Transmitter3-43
Antenna Switch
The antenna switch functions in two modes, which are determined by the presence of K9.1V. The 
K9.1V switch bias is applied via L6700, L6702, and C6700. When K9.1V is present, the switch is in 
TX mode. D6701 and D6702 are forward biased forming a low-loss path from the RFPA final stage to 
the harmonic filter and a 20 dB isolation path between the RFPA final stage and the RX front-end. 
Isolation is achieved via a quarter-wave...

May 25, 20056881096C74-B
3-44Theory of Operation: Transmitter
Figure 3-34.  Power Control Components (700–800 MHz)
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 gain is determined during power-detection calibration tuning. 
Buffered VFORWARD (U0956-2, Pin 7) is added to PWR_SET via R0971 and R0972 and then 
compared to a reference determined by R0974 and R0975. PWR_SET is supplied by the digital-to-...