Page 47 of 101
DWG ID: 181-0101-00A
Date: 2003-09-26
12.1.1.6
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 IC109, 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, 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 Hartley oscillator formed around Q107. One of the
elements in the resonant circuit is a varactor diode, CR105, 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 above that of the desired receive frequency. In
receive, the VCO’s oscillating frequency range is shifted upward by about 44 MHz by
switching C191, C192, and L122 into the resonant circuit. The VCO has a tuning range
of about 40 MHz when its tuning voltage is varied between 1 and 5 V. To frequency
modulate the VCO for transmit, another varactor diode, CR106, is lightly coupled into
the resonant circuit.
The output of the VCO is amplified to a level of about 0 dBm by Q106. Q110, R171, and
C196 act as a very low-noise power supply filter for the VCO.
12.1.2
Transmitter
12.1.2.1
PA Driver Stages
The output of the VCO buffer drives Q105 through R152. The signal level at this point is
about –10 dBm. Q105 amplifies this signal to about +5 dBm. Q104 further amplifies the
signal to +13 dBm, the level required by the PA module. The supply voltage to these two
stages is switched on in transmit by Q113.
12.1.2.2
PA Module, Low-pass Filter, and T/R Switch