User's Manual

SECTION 1: THEORY OF OPERATION
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The lower op amp (U9B) amplifies the signal from the low pass filter and applies it to the VCO via the
VCOMOD output. Pot RV2 and RV5 are used to adjust maximum deviation.
Injection Synthesizer
Two dual fractional synthesizer chips (U5 and U6) are the major contributor of the receiver and transmitter
injection oscillators. This device contains the key components of a phase locked loop (PLL), including a
prescaler, programmable divider, and phase detector. The selected frequencies are loaded into U38 as a
clocked serial bit stream via the PLL_DATA, PLL_CLOCK, and PLL_ENABLE signals.
Frequency stability is determined by a temperature-compensated crystal oscillator module (VCTCXO)
(Y5) at a frequency stability of 1 PPM from –30C to +60C. This device has an input (REFMOD) that
accepts transmit modulation and voltage from a RX FREQ ADJUST pot. The pot allows the receiver to
be fine-tuned to the exact operating frequency.
Two (2) voltage control oscillators (VCO) are formed by integrated low-noise oscillators with buffered
outputs (VCO1 and U40) and associated circuitry. The VCO’s generate receiver and transmit injection
signals. The output of U40 is split by a two-way power divider (U41) leading to outputs RXINJ1 and
RXINJ2. A second output of U40 is returned to the synthesizer FIN input via RXFB. This completes the
loop signal path.
Transmitter/TR Switch
The transmitter section consists of a driver amplifier (U11) and a final power amplifier (U35). To transmit,
PA12V line is powered up. This causes power amplifier (U35) to boost the RF power to the desired level.
Up to 60 watts are available from the transmitter. Harmonic suppression is provided by C82, C83, and
L11.
Power and Analog Ground
These sections consist of the power supplies and transmit control circuitry. Power from the vehicle’s
battery appears at VBATT. Diode D1 protects the voltage regulators by clamping any transient spikes on
the supply line. Such spikes typically occur while the engine is started. The supply line powers a series
of voltage regulators and the transmitter control circuitry, as follows:
 Voltage regulator U46 provides 8-volt power for most other sections in the radio.
 Voltage regulator U21 powers the transmit driver and T/R switch diodes as controlled by the
microcontroller.
 Voltage regulators VR1 and VR2 provide low noise 3.3-volt and 5.0-volt sources for the radio
electronics.
In the transmit control circuitry, to transmit, the microcontroller makes TXKEYOUT* high. Forcing the P-
channel device to conduct, applying 12-volts via PA12V to the transmitter power amplifier bias pins.