Uhf Range 1/Uhf Range 2 Radios; Mhz Radios - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

Product Overview: Frequency Generation Unit
• Associated circuitry
The reference oscillator IC provides a frequency standard to the Fractional-N synthesizer IC, the
ABACUS III digital back-end IC, and the controller section. The synthesizer turns on one of the four
VCOs (determined by mode and band of operation) and tunes it to the receiver (RX) local oscillator
(LO) or transmitter (TX) carrier frequency.
All four voltage-controlled oscillators (VCOs) employ a discrete Colpitts configuration with a N-
channel J-FET transistor. The VCOs tank consists of a varactor diode, coupling capacitor, and a
resonator. The varactor changes the oscillator frequency when the DC voltage of the steering line
changes. The output of the VCOs is coupled to the second 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 three-stage buffer before being injected into
the power amplifier. In RX mode, the receiver VCO output is buffered and amplified with a two-
stages. The output of the second-stage transistor is split into two paths. One path feeds back to the
synthesizer prescaler; the other path is injected into the third-stage. The output of the third-stage
provides the proper signal level for the LO port of the RX front-end mixer.
The superfilter supplies the voltage to the first two stages of the TX buffer and to the first two stages
transistor of the RX buffer/amplifier. The voltage for the third stage of the TX buffer is supplied by a
keyed 9.1 V source to conserve current drain while the radio is receiving. The third-stage of the RX
buffer/amplifier is supplied by a 9.3 V regulator.
2.6.2

UHF Range 1/UHF Range 2 Radios

The UHF Range 1 (380–470 MHz)/UHF Range 2 (450–520 MHz) frequency generation unit consists
of the following:
• Low-voltage fractional-N synthesizer IC
• 16.8 MHz reference oscillator IC
• Three receive voltage-controlled oscillators (VCO)
• Two transmit VCOs
• VCO buffer/amplifier circuits
• Associated circuitry
See 2.6.3 700–800 MHz Radios
2.6.3 700–800 MHz Radios
The 700–800 MHz frequency generation unit consists of the following:
• Low-voltage fractional-N synthesizer IC
• 16.8 MHz reference oscillator IC
• Two voltage-controlled oscillator (VCO) modules (receive and transmit, containing two VCOs
each)
• VCO buffer/amplifier circuits
• Associated circuitry
The reference oscillator IC provides a frequency standard to the fractional-N synthesizer IC, the
ABACUS III digital back-end IC, and the controller section. The synthesizer turns on one of the four
VCOs (determined by mode and band of operation) and tunes it to the receiver (RX) local oscillator
(LO) or transmitter (TX) carrier frequency.
6881096C74-B
below for an overview of the FGU sections.
2-11
May 25, 2005