Figure 3-19. Vhf Frequency Generation Unit Diagram - Motorola ASTRO XTL 5000 Basic Service Manual

3-28
3.14 VHF Frequency Generation Unit (FGU)
Rx FE
RFPA
The VHF frequency generation unit (FGU)
• Fractional-N synthesizer IC
• 16.8 MHz reference oscillator IC
• Four voltage-controlled oscillator (VCO) circuits (two receive and two transmit VCOs)
• VCO buffer/amplifier circuits
• Associated circuitry
The reference oscillator IC provides a frequency standard to the fractional-N synthesizer IC, the
back-end receiver IC, and the controller section. The synthesizer turns on one of the four VCOs
(determined by mode and frequency of operation) and tunes it to the receiver (RX) local oscillator
(LO) or transmitter (TX) carrier frequency.
The VCOs employ a Colpitts configuration with a bipolar transistor. The LC tank circuit's capacitive
portion consists of varactor diodes and a coupling capacitor. The inductive portion consists of High Q
inductors for TX VCO and High Q inductors for RX VCOs. The varactor changes the oscillator
frequency when the DC voltage of the steering line changes. The output of each oscillator is coupled
to a transistor for impedance buffering, and its output is coupled to respective TX/RX buffer
amplifiers.
In TX mode, the transmitter VCO output is coupled to a buffer before being injected into the power
amplifier (see "3.10 Radio-Frequency Power Amplifier (RF PA) & Output Network (ON)" on page
3-24). In RX mode, the receiver VCO output is buffered and amplified with a transistor circuit before
going to the injection amplifier. The outputs of the last VCO buffer transistors are split into two paths.
One path feeds back to the synthesizer prescaler; the other path is injected into the TX power
amplifier, or RX injection amplifier. The output of the RX injection amplifier provides the proper signal
level for the LO port of the RX front-end mixer (see
October 30, 2006
Basic Theory of Operation: VHF Frequency Generation Unit (FGU)
8.2 V Out
Adapt
Loop Filter
Steering
Line Voltage
-
+
9.3 V
Rx
Buffer
Dual Rx VCOs
- +
9.1 V
Tx
Buffer
Dual Tx VCO

Figure 3-19. VHF Frequency Generation Unit Diagram

- +
9.3 V
Super Filter
LV Frac-N
Dual
Synthesizer
Charge
Pump
Transmit
Modulation
AUX-
PreScaler
Tx/Rx Select IN
(Figure 3-19) consists of the following:
"3.11 VHF Receiver Overview" on page
- +
3 V
SPI
TCXO
16.8 MHz
Buffer
Controller
Rx BE
MAEPF-27588-B
3-25).
6871769L01-A