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Technical Description
Version 4 4-7 04/02/04
in the VCO. In this way, the output voltage from the phase detector
controls the output frequency of the VCO.
DC-Offset Amplifier
The DC-offset amplifier offsets the output voltage from the phase
detector. The amplifier is realized with an integrated circuit,
containing two operational amplifiers. The DC-offset amplifier is fed
with an analog voltage. The first amplifier is configured as a DC
voltage follower. The second amplifier is configured as a
differential amplifier.
After the DC-offset amplification, the DC voltage from the phase
detector will be approximately 8-9V. This voltage is applied to a
capacitance diode in the VCO and controls the VCO frequency.
Voltage-Controlled Oscillator (VCO)
The VCO generates the output frequency from the exciter (RF
output). The frequency range for the output frequency of the VCO
is 150 to 174 MHz. The output frequency of the VCO will be
voltage-controlled from either the phase detector or the frequency
synthesizer.
The phase detector will control the output frequency of the VCO,
except when the channel is to be changed. The phase detector
controls the frequency generated by the VCO so that this frequency
will exactly follow the modulated signal 10.7 MHz ± deviation, from
the modulator.
When the phase detector controls the VCO, an output voltage from
phase detector is applied to the varactor, via the DC-offset
amplifier. The transistor, the varactor, the inductor and the
associated capacitors form an oscillating circuit. The signal is
amplified in the following transistor. The output signal of the VCO
is fed to an amplifier.
The output voltage from the phase detector is not sufficient to
perform a channel shift. During a channel shift the output
frequency of the VCO is controlled by the frequency synthesizer.
The frequency synthesizer works as a digital phase detector.
Amplifier (following VCO)
The amplifier outside the loop is used to amplify the signal from the
VCO. The amplified RF signal is fed to the low-pass filter.