CIRCUIT DESCRIPTION
5-9
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.
RF Power Amplifier Module
RF power amplifier module U105 is a wide-band,
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:
• 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
both received, the Tx ALC IC (pin 13) sends a control
signal to turn on transistor Q101.
ALC Circuits
Coupler module U104 samples the forward and
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 transmit ALC circuit, built around U101, is
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).
Thermistor RT101 senses the temperature of the
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.
5.5 800 MHz RF BOARD (VERSION A/B)
NOTE: The following describes the earlier Version A
and B RF boards described in Section 1.12.1. The RF
Board is not field serviceable and must be replaced as
a unit if it is defective.
5.5.1 FREQUENCY SYNTHESIS
The complete synthesizer subsystem consists of
the reference oscillator (U304), the voltage-controlled
oscillator (VCO U307), a buffer IC (U303), and the
synthesizer (U302).
The reference oscillator contains a temperature-
compensated 16.8 MHz crystal. This oscillator is digi-
tally tuned and contains a temperature-referenced,
five-bit, analog-to-digital (A/D) converter. The output
of the oscillator (pin 10 on U304) is applied to pin 14
(XTAL1) on U302 through capacitor C309 and
resistor R306.
Voltage-controlled oscillator module U307 is
varactor tuned. Therefore, as the voltage being applied
to pins 1 and 7 of the VCO varies (2-11V), so does the
varactor's capacitance which changes the VCO output
frequency. The 800 MHz VCO is a dual-range oscil-
lator that covers the 806-825 MHz and the 851-870
MHz frequency bands.
The low-band VCO (777-825 MHz) provides the
first LO injection frequencies (777-797 MHz) that are
73.35 MHz below the carrier frequency. In addition, in
the transmit mode when the radio is operated through
a repeater, the low-band VCO generates the transmit
800 MHz RF BOARD (VERSION A/B)VHF/UHF RF BOARD (VERSION A/B)
To Next Page