Uhf Rf Board (Version C); Receiver - E.F. Johnson Company 5100 Series Service Manual

VHF RF BOARD (VERSION C)
applied to the transmit PLL transmit VCO (U47)
provides high-frequency modulation. Signals for both
modulation ports are provided by DACs on the digital
board.
Power Amplifier
The power amplifier (U3) is a Mitsubishi
RA07M1317M module. This 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 provides +21
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 used
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, R12, C43 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 compres-
sion. Any harmonics generated by the PA module and
the RF T/R switch are filtered by a harmonic filter that
is located between the RF T/R switch and the antenna
jack.

5.3 UHF RF BOARD (VERSION C)

NOTE: The following describes the new revised
Version C RF board described in Section 1.13.1. The
RF Board is not field serviceable. It must be replaced
as a unit with a new board.

5.3.1 RECEIVER

Front End Bandpass Filter
A harmonic filter is followed by a PIN diode
transmit/receive switch. Following the switch, two
UHF RF BOARD (VERSION C)
fixed tuned bandpass filters are used in the front-end
of the receiver. Depending on the desired receive band,
the appropriate filter is selected using RF switches
(U37/U41). This filter provides first-image rejection
with minimal loss in order to provide the desired level
of receiver sensitivity. A variable attenuator, which
follows the filter, increases the dynamic range of the
receiver when receiving high-level signals.
Front End LNA and Bypass Switching
The Low Noise Amplifier (LNA) is critical in
determining the overall noise figure of the receiver
chain. An MGA-71543 amplifier (U39) provides
optimum noise figure, gain, intercept point, and power
consumption.
Post-LNA Bandpass Filters
Additional bandpass filters are positioned after
the LNA. These filters are identical to the front-end
filters previously described. RF switches U42 and U40
are used to select between the two bands. These filters
provide additional image rejection.
Mixer and LO Filter
A double-balanced, low-level ADEX-10L mixer
(MX1) with a LO drive level of +4 dBm is used for the
first conversion. This mixer provides good dynamic
range with a 3 dB lower LO drive than the more tradi-
tional +7 dBm drive mixers. This reduces power
consumption and also the conducted and radiated local
oscillator leakage from the receiver.
For the low band UHF version, a high-side mix is
used for the 380-444 MHz receive band and a low-side
mix is used for the 444-470 MHz receive band. For the
high band UHF version, a high-side mix is used for the
450-455 MHz receive band and a low-side mix is used
for the 455-520 MHz receive band. This band plan
reduces the tuning range requirements for the VCOs.
A LO filter is used prior to LO port of the mixer
to reduce the impact of wideband noise from the LO
synthesizer on the receiver sensitivity.
IF Filter and Amplifier
A two-pole 64.455 MHz crystal filter (U2) is used
to provide the desired level of adjacent channel
5-5
CIRCUIT DESCRIPTION