E.F. Johnson Company 5100 Series, Руководство по обслуживанию

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CIRCUIT DESCRIPTION
5-7
Analog / Digital Conversion and Processing -
Sigma-delta converters provide I and Q sampling
directly from the second IF frequency. The
resulting digital words are first filtered by internal
programmable FIR filters and then clocked out of
the AD9864 via a serial data bus using a program-
mable data rate.
5.3.2 SYNTHESIZER
PLL IC
A CX72301 sigma-delta modulated PLL (U29)
forms the basis of the main synthesizer that is used for
both receive and transmit modes. This PLL chip
provides good phase noise capabilities to reduce adja-
cent channel interference and quick switching between
the receive and transmit modes.
In receive mode the PLL is programmed for a
local oscillator frequency that is 64.455 MHz away
from the receive frequency. In transmit mode the PLL
is programmed directly for the transmit frequency.
Reference Oscillator
A 16.8 MHz oscillator (Y1) is used as the
frequency reference to the synthesizer and also to the
receiver backend IC. The center frequency of this
oscillator is corrected using a DC tuning voltage from
the digital board during receive and voice and data
modulation during transmit.
Analog Switches and PLL Loop Filters
An analog switch (U17) provides quicker
switching of signals during channel changes by
varying the time constant of the PLL loop filter.
VCOs
Two different VCOs minimize the tuning range of
the VCOs in order to meet phase noise specifications.
Both VCO's (U1 and U5) are used for transmit and
receive modes. Their combined tuning range covers
the entire transmit and receive frequency bands.
Transmit modulation is provided to each oscillator's
modulation port from the digital board.
A RF2361 buffer amplifier (U6) provides the
required level of drive for the receiver mixer's local
oscillator signal as discussed above.
5.3.3 TRANSMITTER
Modulation
A “dual-port” modulation scheme is used in order
to provide DC coupling of the signal required for data
modulation applications. Modulation applied to the
PLL's frequency reference provides low-frequency
modulation, whereas modulation applied to the PLL's
transmit VCO's (U1 and U5) provide high-frequency
modulation. Signals for both modulation ports are
provided from DACs on the digital board.
Power Amplifier
The power amplifier (U3) is a Mitsubishi
RA07M4047M-01 module for the low band version
and a Mitsubishi RA07M4452M-01 module for the
high band version. The PA module provides the
desired RF power output level and is stable over a
wide range of VSWR conditions. The PA is driven by
a SGA-6589 driver (U20) that typically pr21
dBm output power. The PA is turned on and off by
switching the power to this driver via transistor D10.
ALC
To maintain the specified Transmitter output
power level, Automatic Level Control (ALC) is
provided to control the drive level to the PA. The
detected forward power is compared to a reference
level provided by the digital board via op amp U21A.
The resulting error voltage is applied to a power level
control port of the power amplifier module. Trans-
mitter on/off splatter filtering is provided by an RC
network (R76, R109, C267 and C141.)
T/R Switching and Harmonic Filter
The output of the power amplifier is applied to
the transmit/receive RF PIN switch (D12 & D13.) This
is a high dynamic-range switch that is capable of
passing the desired transmit power with minimal
compression. Any harmonics generated by the PA
module and the RF T/R switch are filtered by a
harmonic filter that is between the RF T/R switch and
the antenna jack.
UHF RF BOARD (VERSION C)