Page 50 of 101
DWG ID: 181-0101-00A
Date: 2003-09-26
filter the output such that only the third harmonic remains for use as the 2
nd
local
oscillator.
12.2.1.6
VCO and Synthesizer
The synthesizer is responsible for generating the carrier in transmit and the first local
oscillator in receive. A voltage-controlled oscillator (VCO) is an oscillator whose
frequency can be controlled by an external signal. The synthesizer, almost wholly
contained within IC108, divides the VCO frequency by digital dividers and compares the
result with an accurate reference. An error signal, proportional to the frequency error is
created, which is routed to the frequency control input of the VCO. This action locks the
VCO to a frequency, which is equal to the reference frequency multiplied by the divider
number. To set the VCO frequency, different divider numbers can be programmed into
the synthesizer. In most synthesizer designs, the divider must be an integer, programmed
into the synthesizer. In most synthesizer designs, the divider must be an integer, which
forces the reference frequency to be equal to the synthesizer step size. The synthesizer IC
used in this radio, however, allows the use of non-integer values for the divider, which in
turn allows the reference frequency to be much higher than normal. This creates a
synthesizer whose output has lower noise, lower spurious levels, and higher switching
speeds. The reference frequency is derived by digitally dividing the frequency of the
14.4-MHz master oscillator. When locked, the VCO attains the same relative frequency
stability as that of the master oscillator.
The VCO itself is a voltage-follower Colpitts oscillator formed around Q108. One of the
elements in the resonant circuit is a varactor diode, CR106, whose capacitance, when
reverse-biased, varies as a function of the applied voltage. Since the oscillator frequency
is controlled by the resonant circuit, varying the voltage on the varactor diode effects a
change in frequency. To serve as a local oscillator for the first frequency converter, the
VCO operates at a frequency 43.65 MHz below that of the desired receive frequency. In
transmit, the VCO’s oscillating frequency range is shifted upward by about 44 MHz by
switching C190 and L115 into the resonant circuit. The VCO has a tuning range of about
30 MHz when its tuning voltage is varied between 1 and 5 V. To frequency modulate the
VCO for transmit, another varactor diode, CR105, is lightly coupled into the resonant
circuit.
Q107 and Q106 amplify the output of the VCO to a level of about 0 dBm. Q111, R172,
and C196 act as a very low-noise power supply filter for the VCO.
12.2.2
Transmitter
12.2.2.1
PA Driver Stages
The output of the last VCO buffer drives Q105 through R151. The signal level at this
point is about –10 dBm. Q105 amplifies this signal to about +5 dBm. Q104 further
amplifies the signal to +17 dBm, the level required by the PA module. The supply
voltage to these two stages is switched on in transmit by Q113.
