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

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CIRCUIT DESCRIPTION
5-13
5.6.2 ANTENNA SWITCH
The antenna switch is a current device consisting
of a pair of diodes (CR108/ CR109) that electroni-
cally steer RF between the receiver and the transmitter.
In the transmit mode, RF is routed through transmit
switching diode CR108, and sent to the antenna. In the
receive mode, RF is received from the antenna, routed
through receive switching diode CR109, and applied
to the RF amplifier Q1 (VHF) or U1 (UHF). In
transmit, bias current, sourced from U101, pin 21, is
routed through L105, U104, CR108, and L122 (VHF)
and L105, CR108, and L122 (UHF). Sinking of the
bias current is through the transmit ALC module,
U101, pin 19. In the receive mode, bias current,
sourced from switched B+, is routed through Q107
(pin 3 to pin 2), L123 (UHF), L121, CR109, and L122.
Sinking of the bias current is through the 5-volt regu-
lator, U106, pin 8.
5.6.3 RECEIVER FRONT END
The RF signal is received by the antenna and
coupled through the external RF switch. The UHF
board applies the RF signal to a low-pass filter
consisting of L126, L127, L128, C149, C150, and
C151. The VHF board bypasses the low-pass filter.
The filtered RF signal is passed through the antenna
switch (CR109) and applied to a bandpass filter
consisting of (VHF) L11 - L14, CR1 - CR9, C4, C2,
and C3 or (UHF) L30, L31, L32, L34, L35, CR6 -
CR9, C1, C2, and C3. The bandpass filter is tuned by
applying a control voltage to the varactor diodes in the
filter (CR1 - CR9 VHF and CR6 - CR9 UHF).
The bandpass filter is electronically tuned by the
D/A IC (U102), which is controlled by the microcom-
puter.The D/A output range is extended through the
use of a current mirror consisting of Q108 and R115
and R116. When Q108 is turned on via R115, the D/A
output is reduced due to the voltage drop across R116.
Depending on the carrier frequency, the microcom-
puter will turn Q108 on or off. Wideband operation of
the filter is achieved by retuning the bandpass filter
across the band.
The output of the bandpass filter is applied to
wideband GaAs RF amplifier IC U1 (UHF) or active
device Q1 (VHF). The RF signal is then further
filtered by a second broadband, fixed-tuned, bandpass
filter consisting of C6, C7, C8, C80, C86, C87, C88,
C97, C99, L3, L4, L5, and L30 (VHF) or C4 -C7,
C88 - C94, C99, and L11 - L15 (UHF) to improve the
spurious rejection.
The filtered RF signal is routed through a broad-
band 50-ohm transformer (T1) to the input of a broad-
band mixer/buffer (U2). Mixer U2 uses GaAs FETs in
a double-balanced, Gilbert Cell configuration.
The RF signal is applied to the mixer at U2 pins 1
and 15. An injection signal (1st LO) of about –10 dBm
supplied by the FGU is applied to U2, pin 8. Mixing of
the RF and the 1st LO results in an output signal that is
the first IF frequency. The first IF frequency is 45.15
MHz for the VHF band and 73.35 for the UHF band.
High side injection is used for VHF and low side for
UHF. The first IF signal output at U2, pins 4 and 6 is
routed through transformer T2 and impedance
matching components, and applied to a two-pole
crystal filter (FL1), which is the final stage of the
receiver front end. The two-pole crystal filter removes
unwanted mixer products. Impedance matching
between the output of the transformer (T2) and the
input of the filter (FL1) is accomplished by C605 and
L605 (VHF) or C611, C614, and L605 (UHF).
5.6.4 RECEIVER BACK END
The output of crystal filter FL1 is matched to the
input of IF buffer amplifier transistor Q601 by C610
and L604 (VHF) and C609, C610, and L600 (UHF).
Transistor Q601 is biased by the 5V regulator (U202).
The IF frequency on the collector of Q601 is applied
to a second crystal filter through a matching circuit.
The second crystal filter (FL2) input is matched by
C604, C603, and L601 (VHF) and C604, L601, and
L602 (UHF). The filter supplies further attenuation at
the IF sidebands to increase the radios selectivity. The
output of FL2 routed to pin 32 of U401 through a
matching circuit which consists of L603, L606, and
C608 (VHF) and L603, C606, and C605 (UHF).
In the ABACUS IC (U401), the first IF frequency
is amplified and then down-converted to the second IF
frequency of 450 kHz. At this point, the analog signal
is converted into two digital bit streams by a sigma-
delta A/D converter. The bit streams are then digitally
VHF/UHF RF BOARD (VERSION A/B)