Transmitter - E.F. Johnson Company 5100 Series Service Manual

filtered, mixed down to baseband, and filtered again.
The differential output data stream is then sent to the
logic board where it is decoded to produce the recov-
ered audio.
The ABACUS IC (U401) is electronically
programmable. The amount of filtering, which is
dependent on the radio channel spacing and signal
type, is controlled by the microcontroller. Additional
filtering, which used to be provided externally by a
conventional 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 dependent on sampling speed. A fractional N
synthesizer is also incorporated on the ABACUS IC
for 2nd LO generation.
The 2nd LO/VCO is a Colpitts oscillator built
around transistor Q401 (VHF) or Q1 (UHF). The VCO
has a varactor diode, VR401 (VHF) or CR5 (UHF) to
adjust the VCO frequency. The control signal for the
varactor is derived from a loop filter consisting of
C426, C428, and R413.

5.6.5 TRANSMITTER

The transmitter consists of three major sections:
•
Harmonic Filter
•
RF Power Amplifier Module
•
ALC Circuits
Harmonic Filter
With VHF versions, RF from PA module U105 is
routed through coupler U104 and passed through the
harmonic filtering network to antenna switch CR108.
With UHF versions, RF from the PA module U105 is
routed through coupler U104 and passed through
transmit antenna switch CR108 and applied to a
harmonic filtering network. The harmonic filtering
circuit is composed of (VHF) L126, L127, L128,
C149, C150, and C151 or (UHF) L126, L127, L128,
C129, C130, C149, C150, and C151. Resistor R117
(VHF) or R117 (UHF) provides a current-limited 5V
to J2.
VHF/UHF RF BOARD (VERSION A/B)
RF Power Amplifier Module
three-stage (VHF) or four-stage (UHF) amplifier.
Nominal input and output impedance of U105 is 50
ohms. The DC bias for U105 is on pins 2, 4, 5. In the
transmit mode, the voltage on U105, pins 2 and 4
(close to the B+ level) is obtained via switching tran-
sistor Q101. Transistor Q101 receives its control base
signal as follows:
•
•
•
both received, the Tx ALC IC (pin 13) sends a control
signal to turn on transistor Q101.
ALC Circuits
reverse power of the PA output voltage. Reverse
power is present when there is other than 50 ohms
impedance at the antenna port. Sampling is achieved
by coupling some of the forward and/or reverse power,
and applying it to CR102 (VHF) or CR101 (UHF) and
CR103 for rectification and summing. The resultant
DC signal is then applied to the Tx ALC IC (U101, pin
2) as RFDET to be used as an RF strength indicator.
the heart of the power control loop. Circuits in the Tx
ALC module compare the signals at U101, pins 2
and 7. The resultant signal, C BIAS, at U101, pin 4 is
applied to the base of transistor Q110. In response to
the base drive, transistor Q110 varies the DC control
voltages applied to the RF PA at U105, pin 3, thus
controlling the RF power of module (U105).
Tx ALC IC. If an abnormal operating condition exists
that causes the PA temperature to rise to an unaccept-
able level, the thermistor forces the ALC to reduce the
set power.
5-14
RF power amplifier module U105 is a wide-band,
The microcomputer keys the D/A IC to produce a
ready signal at U 102 pin 3,
the ready signal at U102 pin 3 is applied to the Tx
ALC IC at U101 pin 14 (5V), and
the synthesizer sends a LOC signal to the Tx ALC
IC (U204 pin 40 to U101 pin 16).
When the LOC signal and the ready signal are
Coupler module U104 samples the forward and
The transmit ALC circuit, built around U101, is
Thermistor RT101 senses the temperature of the
CIRCUIT DESCRIPTION