User's Manual

F32700N25.doc Page 9
The lower op amp (U23C) amplifies the signal from the low pass filter and applies it to
the VCO via the VCOMOD output. Pots RV5 and RV5 are used to adjust maximum
deviation.
Injection Synthesizer
The fractional synthesizer chip (U17) is the major contributor of the receiver and
transmitter injection oscillator. 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 U17 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) (TCXO1) at a frequency stability of 1 PPM from –30C to +60C.This
device has an input (REFMOD) that accepts transmit modulation and voltage from the
scaled down output of the DAC (U21). At the factory the frequency adjusted to the
nearest frequency of than 0.05 ppm accuracy and maintains the accuracy when is
locked to the base station through the correction values determined by the base station.
a voltage control oscillator (VCO1) is formed by integrated low-noise oscillators The
VCO’s generate receiver and transmit injection signals. The output of VCO1 is split by
two two-way power dividers (U16, U18) leading to outputs RXINJ1 and RXINJ2. One of
the outputs of a third two-way divider (U18) is returned to the synthesizer RFIN input.
This completes the loop signal path. The other one is the input to the driver amp of the
transmitter.
Transmitter/TR Switch
The transmitter section consists of an amplifier (U10). The output of the driver is applied
to the power amplifier (U11) to boost the power to 5 watts Max. Output A of DAC (U21)
through an Op amp. (U22A) is applied to the gain control of U11 to adjust the output
power. VBATT is applied to the Drain of U11. Harmonic suppression is provided by
C11, L10, C14, C12 and C15. Control signal TRSW is applied to pin diodes CR1, CR2
and CR3 and associated circuits to switch radio between receive and transmit mode.
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 VR3 provides 8-volt power for most other sections in the radio.