Motorola Astro Digital Mobile Radio Xtl5000 Detailled 6881096c74 B Manual

6881096C74-BMay 25, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-45
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...

May 25, 20056881096C74-B
3-46Theory of Operation: Frequency Generation Unit (FGU)
Figure 3-35.  Frequency Generation Unit Block Diagram (VHF)
3.7.1.1  Reference Oscillator
The radios frequency stability and accuracy is derived from the 16.8 MHz reference oscillator 
(Y3750). The 16.8 MHz reference oscillator circuitry provides a 16.8 MHz reference to the LV Frac-N 
(U3751), receiver backend IC (U3000), and the controller section of the XTL 5000 radio. The 
reference oscillator circuitry consists of the...

6881096C74-BMay 25, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-47
U3751 is a mixed-mode IC containing digital and analog circuits. Separate power supply inputs are 
used for the various functional blocks on the IC. Inductors L3757 and L3756 provide isolation 
between supply pin 20 (AVDD - analog supply input) and pin 36 (DVDD - digital supply input) 
connected to F3.0v. This 3.0 V DC supply is provided by U3750, a 3-V regulator IC.
All programmable variables on the synthesizer IC, such...

May 25, 20056881096C74-B
3-48Theory of Operation: Frequency Generation Unit (FGU)
3.7.1.6  Charge Pump Bias
External circuitry connected to pin 39 (Bias 2) and pin 40 (Bias 1) of U3751 determine the current 
that is applied to the charge-pump circuitry. During receive mode, resistors R3778, R3763, and 
R3758 set the current supplied to pin 40 (Bias 1). Transistor Q3755 and resistors R3779, R3756, 
and capacitor C3808 form a circuit that momentarily increases the current to pin 40 (Bias 1) during...

6881096C74-BMay 25, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-49
3.7.1.10 Receiver Injection
The receiver (RX) injection string is a three-stage amplifier that supplies the RX feedback signal to 
the FGU synthesizer loop and supplies the first local oscillator (LO) signal to the RX front-end mixer. 
The RX VCO output is attenuated 3 dB via resistors R3825 through R3827 to increase isolation. This 
buffered signal is amplified by the first-stage amplifier Q3761, which is supplied by the...

May 25, 20056881096C74-B
3-50Theory of Operation: Frequency Generation Unit (FGU)
Figure 3-37.  Frequency Generation Unit Block Diagram (UHF Range 1)
3.7.2.1  Reference Oscillator
The radios frequency stability and accuracy is derived from the 16.8 MHz reference oscillator 
(Y5750). The 16.8 MHz reference oscillator circuitry provides a 16.8 MHz reference to the LV Frac-N 
(U5752), receiver back-end IC (U5002), and the controller section of the XTL 5000 radio. The 
reference oscillator circuitry consists...

6881096C74-BMay 25, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-51
All programmable variables on the synthesizer IC, such as the synthesizer frequency; reference- 
oscillator warping; adapt-timer duration; modulation-attenuator setting; and auxiliary-control outputs, 
which select one of five voltage-controlled oscillators, can be programmed through a serial 
peripheral interface (SPI). The SPI is connected to the controller microcomputer via three 
programming lines, namely the data (pin...

May 25, 20056881096C74-B
3-52Theory of Operation: Frequency Generation Unit (FGU)
3.7.2.6  Charge Pump Bias
External circuitry connected to pin 39 (Bias 2) and pin 40 (Bias 1) of U5752 determine the current 
that is applied to the charge-pump circuitry. During receive mode, resistors R5754, R5759, and 
R5765 set the current supplied to pin 40 (Bias 1). Transistor Q5750 and resistors R5752, R5753, 
and capacitor C5759 form a circuit that momentarily increases the current to pin 40 (Bias 1) during...

6881096C74-BMay 25, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-53
3.7.2.11 Transmitter VCOs
Transmitter frequencies are generated from two VCOs, Q5825 and Q5826.
• Q5825 supplies frequencies in the range 380 MHz up to (but not including) 425 MHz.
• Q5826 supplies frequencies in the range from 425 MHz to 470 MHz.
3.7.2.12 Receiver VCOs
Receiver first local-oscillator frequencies are generated from three VCOs, Q5901, Q5903 and 
Q5905. 
• Q5901 supplies frequencies in the range 489.65 MHz...

May 25, 20056881096C74-B
3-54Theory of Operation: Frequency Generation Unit (FGU)
3.7.3 UHF Range 2 (450–520 MHz) Band
The FGU (Figure 3-39) provides the XTL 5000 radio with a 16.8 MHz reference frequency, receiver 
1st local oscillator, and a modulated transmitter RF carrier that is further amplified by the power 
amplifier section of the radio.
The FGU consists of the following:
• Reference oscillator (Y5750)
• Low-voltage Fractional-N (LV Frac-N) synthesizer (U5752)
• Three receiver voltage-controlled...