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

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CIRCUIT DESCRIPTION
5-17
capacitor C213. The mixer's LO port is matched to the
radio's PLL by a capacitive tap, C207 and C206.
A balun transformer (T202) is used to couple the
RF signal into the mixer. The primary winding of
T202 is matched to the preceding stage by capacitor
C223, with C227 providing a DC block to ground. The
secondary winding of T202 provides a differential
output, with a 180° phase differential being achieved
by setting the secondary center tap to virtual ground
using bypass capacitors C210, C211, and C212. The
secondary of transformer T202 is connected to pins 1
and 15 of the mixer IC, which drives the source leg of
dual FETs used to toggle the paralleled differential
amplifier configuration within the Gilbert Cell.
The final stage in the receiver front end is a two-
pole crystal filter (FL1). The crystal filter provides
some of the receiver's adjacent channel selectivity. The
input to the crystal filter is matched to the first mixer
using L605, C600, and C614. The output of the crystal
filter is matched to the input of IF buffer amplifier
transistor Q601 by L600, C609, and C610.
5.7.4 RECEIVER BACK END
The IF frequency on the collector of Q601 is
applied to a second crystal filter (FL2) through a
matching circuit consisting of L601, L602, C604, and
C612. The filter supplies further attenuation at the IF
sidebands to increase the radio's selectivity. The output
of FL2 is routed to pin 32 of U401 through a matching
circuit consisting of L603, C603, and C606, and DC
blocking capacitor C613.
In the ABACUS IC (U401), the first IF frequency
is amplified and then down-converted to the second IF
of 450 kHz. The analog signal is then converted into
two digital bit streams by a sigma-delta A/D converter.
The bit streams are then digitally filtered, mixed down
to baseband, and filtered again. The differential output
data stream is then sent to the ADSIC on the logic
board, where it is decoded to produce the recovered
audio.
The ABACUS IC (U401) is electronically
programmable. The amount of filtering is dependent
on the radio channel spacing and signal type, and is
controlled by the microcomputer. Additional filtering,
which used to be provided externally by a conven-
tional ceramic filter, is replaced by internal digital
filters in the ABACUS IC. The ABACUS IC contains
a feedback AGC circuit to expand the dynamic range
of the sigma-delta converter. The differential output
data contains the quadrature (I and Q) information in
16-bit words, the AGC information in a 9-bit word,
imbedded word sync information, and fill bits which
are dependent on sampling speed. A fractional N
synthesizer is also incorporated on the ABACUS IC
for 2nd LO generation.
The second LO/VCO is a Colpitts oscillator built
around transistor Q1. The VCO has a varactor diode
(VR401), which is used to adjust the VCO frequency.
The control signal for the varactor is derived from a
loop filter consisting of C426, C428, and R413.
5.7.5 TRANSMITTER
The 800 MHz RF power amplifier (PA) is a five-
stage amplifier (U502). The RF power amplifier has a
nominal input and output impedance of 50 ohms.
An RF input drive level of approxi3
dBm, supplied from the VCO buffer IC (U303), is
applied to pin 1 of U502. The DC bias for the internal
stages of U502 is applied to pins 3 and 4 of the
module. Pin 3 is switched through Q502 and pin 4 is
unswitched B+ to the final amplifier stage. Power
control is achieved by varying of the DC bias to pin 2,
the third and fourth amplifier stages of the module.
The amplified RF signal leaves the PA module at pin 5
and is applied to the directional coupler (U501).
The purpose of U501 is to sample both the
forward power and the reverse power. Reverse power
is present when a load other than 50 ohms exists at the
antenna port. The sampling is achieved by coupling
some of the reflected power, forward and/or reverse, to
a coupled leg on the coupler. The sampled RF signals
are applied to diode CR501 for rectification and
summing. The resultant DC signal is applied to the
ALC IC (U504, pin 2) as RFDET, to be used as an
indicator of the strength of the RF signal being passed
through the directional coupler (U501).
The transmit ALC IC (U504) is the main part of
the power control loop. The REF V line (U504 pin 7),
a DC signal supplied from the D/A IC (U503), and the
RF DET signal described earlier, are compared inter-
nally in the ALC IC to determine the amount of C
BIAS, pin 4, to be applied to the base of transistor
800 MHz RF BOARD (VERSION A/B)