Frequency Generation - Motorola PRO1150 Manual De Uso

Theory of Operation

Frequency Generation

Power Amplifier
The transmit power amplifier is a three-stage design.
The first stage is a MMBR951 transistor with an
input level of 3 dBm (2 mW) and 10 – 15 dB gain.
The second stage utilizes a MRF8372 device (NPN)
with a specified maximum output of 0.5 Watts. The
third, and final stage is a MRF5003 FET with a
maximum output of 2.0 Watts.
Antenna Switch
The antenna switch circuit disables the receiver
during transmission. In the transmit mode, TX B+
shorts out the receiver port, and the P network,
which operates as a quarter-wave transmission line.
This creates a high impedance at the input of the
harmonic filter. When not in transmit mode, a low
attenuation path between the antenna and receiver
ports allows reception.
Harmonic Filter
Attenuation for harmonics is provided by a Low Pass
Filter before, and after, the RF Power Amplifier; as
well as by the Harmonic Filter after the Antenna
Switch. The Low Pass Filter in the RF PA is composed
of C503, C504, and L501 on the input, and L507
and C522 on the output. The Harmonic Filter is a
fifth-order Low Pass Chebyshev, with 0.1 dB of
ripple; it is composed of C101, L101, C103, L102,
C102, and C104.
Matching Network
To optimize the performance of the transmitter and
receiver into the antenna, a network is used to
match the antenna's impedance to the harmonic
filter.
NOTE: To measure the output power of the
transmitter, the antenna must be removed
and replaced with the Antenna Adapter
(HLN8262_).
Power Control Circuit
The power is controlled by the microcontroller, by
way of a discrete D/A converter. This places a voltage
on the base of the final stage of the power amplifier.
Frequency Generation
The carrier frequency is generated using a fractional-
N synthesizer inside the ULTZIF IC (U801). This
4-2
consists of a phased-locked loop circuit with a
voltage-controlled oscillator (VCO). The VCO
output, which runs at twice the carrier frequency, is
fed back to a programmable divider chain. The
divide ratios are determined from information
stored in the microcomputer and are bussed to the
synthesizer via the microcomputer.
Modulation occurs by a combination of directly
coupling the modulation signal to the Low Pass
Filter in the loop, and by processing the digitized
analog modulation signal to alter the divider values.
A temperature-compensated reference oscillator
(TCXO), which is 16.8 MHz, and stable to 2.5 parts
per million over temperature extremes, is used for
the reference. The 16.8 MHz reference is further
divided into one of three reference frequencies,
which is compared to the divided-down VCO
output in a phase detector. The phase detector, in
turn, provides a DC steering voltage back to the
VCO. (Please refer to Figure 4-19.)
Reference
WARP
Oscillator
PRESCALER
ULTZIF
Loop
Filter
CONTROL VOLTAGE
DATA
CLOCK
ENABLE
Test_D
Figure 4-19. Frequency Generator
PRO1150™ Service Manual
VCO
Buffer
Oscillator
x 2fc
mP
Tx RF INJECTION
RF PA
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