User Manual

SECTION 1: THEORY OF OPERATION
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Injection Synthesizer
The synthesizer chip (U3) is the major contributor of the injection synthesizer. This device contains the
key components of a phase locked loop (PLL), including a 1.1 GHz prescaler, programmable divider, and
phase detector. The selected frequencies are loaded into U3 as a clocked serial bit stream via the PLL
DATA and PLL CLOCK inputs. The microcontroller provides the serial data.
A 10 MHz reference frequency is provided by voltage controlled, temperature compensated crystal
oscillator module (VCTCXO)(Y3). This device has an input (REFMOD) that accepts transmit modulation
and voltage from a RX FREQ ADJUST pot RV3. The pot allows the receiver to be fine-tuned to the exact
operating frequency.
Diode (D1), capacitor (C34), and inverter (U44E) form a lock indication circuit. For troubleshooting, a
green light emitting diode (LED) D5 glows and indicates the synthesizer is locked and working properly.
The UHF signal is generated by a wide-range voltage controlled oscillator (VCO) module VCO1. A
voltage on the VT input determines the VCO frequency. The voltage is generated by the phase detector
output (PDOUT) of U3 driving a loop filter consisting of R4, C33, C23, R3, and C32. It integrates the
pulses that normally appear on PDOUT into a smooth DC control signal for the VCO. During transmit, the
analog signal from the modem and transmit processing circuitry is applied to VCO1 via the VCOMOD
input.
The output of VCO1 RFOUT goes to a two-way power divider (U30). One port of the U30 passes through
a 3V attenuator (AT1) and provides the transmit injection (TXINJ) signal for the transmitter circuit while
the other port drives another two-way power divider (U31). The first port of U30 provides the receive
injection (RXINJ2) signal for Receiver 2, while the second port output is boosted by wide-band amplifier
(U32). The amplifier output provides the receive injection (RXINJ1) signal for Receiver 1.
Transmitter
The transmitter section consists of an exciter, power amplifier, and power control circuitry. The exciter is
built around an RF power amplifier chip (U28). To transmit, 5-volt power is applied to the 5VKEY line.
This causes the U28 to power up and amplify the TXINJ signal. A gain control circuit inside of the U28
maintains the output power level to a constant value throughout the UHF spectrum. Simultaneously, the
12VKEY line is powered up. This causes power amplifier (U1) to boost the RF power to the desired level.
Up to 40 watts are available from the transmitter.
The power amplifier modules are self-contained hybrid devices that contain both active and passive
circuitry. There are five (5) modules available and each covers a 20-to-30 MHz portion of the UHF band.
Should replacement of U1 be required, the exact replacement part must be used.
Receiver 1 Front-End
This section consists of the components that form Receiver 1 Front-End. These components include a
T/R switch, bandpass filters, RF amplifiers, and a mixer circuit.
SW1 is a hybrid monolithic device that serves as the T/R switch and protects the Receiver from RF
damage by isolating it from the transmitter output. SW1 is normally in the receive position. At the start of
a transmission, a control voltage is applied to the TRSWCNTL input and SW1 transfers the antenna to
the transmitter power amplifier.