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

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CIRCUIT DESCRIPTION
5-10
backend IC. The center frequency of this oscillator is
corrected using a DC tuning voltage from the digital
board during receive and it is modulated with voice or
data during transmit.
Analog Switches and PLL Loop Filters
Analog switches U16, U17, and U18 provide
faster channel switching by changing the time constant
of the PLL loop filters.
VCOs
Three different VCOs are used. VCO U1 is used
strictly in transmit mode. It's tuning range covers the
entire transmit and receive frequency bands. Transmit
modulation is provided to this oscillator's modulation
port from the digital board through R9.
Two receive VCOs (U5 and U6) are required to
meet the phase noise requirements. Each VCO func-
tions for only one of the two receive bands. An
RF2361 buffer amplifier (U40) provides the required
level of drive for the receiver mixer's local oscillator
signal as discussed above.
5.4.3 TRANSMITTER
Modulation
A “dual-port” modulation scheme is used to
provide DC coupling of the signal for data modulation
applications. Modulation applied to the PLL frequency
reference provides low-frequency modulation, and
modulation applied to the PLL transmit VCO (U1)
provides high-frequency modulation. Signals for both
modulation ports are provided from DACs on the
digital board.
Power Amplifier
Power Amplifier U3 is a RA03M8087M module.
It 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 tran-
sistor 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 RC
network R63, R64, C141 and C211.
T/R Switching and Harmonic Filter
The output of the power amplifier is applied to
transmit/receive RF switch U4. 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.
5.5 RF BOARD OVERVIEW (VERSION A/B)
NOTE: The following describes the earlier Version A
and B RF boards described in Section 1.13.1. The RF
Board is not field serviceable. It must be replaced as a
unit with a new board.
The receiver front end consists of a preselector,
an RF amplifier, a second preselector, and a mixer (see
Figure 5-4). With VHF and UHF models, both prese-
lectors are varactor-tuned, two- pole filters controlled
by the control logic. With 800 MHz models, these
filters are fixed-tuned. The RF amplifier is a dual-gate,
gallium-arsenide based IC. The mixer is a double-
balanced, active mixer coupled by transformers. Injec-
tion is provided by the VCO through an injection filter.
Refer to Table 5-1 for local oscillator (LO) and first IF
information.
Table 5-1 LO and First IF Frequencies
VHF UHF 800 MHz
LO Frequency
range 181.15 -
219.15 MHz 329.65 -
446.65 MHz 776.65 -
796.65 MHz
First IF
Frequency 45.15 MHz 73.35 MHz 73.35 MHz
RF BOARD OVERVIEW (VERSION A/B) 700/800 MHz RF BOARD (VERSION C)