SG-2000 MANUAL
130
The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
©1995, SGC, Inc.
TEL: (206) 746-6310 FAX: (206) 746-6384
21.7
MicroProcessor Assembly
(
Schematic J30100930)
This board is the primary controller for all functions of the SG-2000. The
frequency and mode information is sent by the controller head processor to the
radio processor. The radio processor then takes this information and converts it
to digital signals used by the VCOs and other function switches. The digital
control signal is sent out serially through the communication port on the
MicroProcessor board to serially programmable devices on the exciter and the
LPA. The signals control the VCO frequencies, the receive and transmit control
voltages, carrier insertion levels, high and low power settings, tone frequency,
and audio filter select.
21.8
VCO's
(Schematic J30100920)
The VCOs (Voltage Control Oscillators) take an analog input voltage and
produce a corresponding frequency depending on the LC resonance of the
circuit. In VCO #1, one of the two inductors, L401 or L402 is switched in via
D401 or D402 to oscillate with the series capacitance of VD401 and VD402. The
series capacitance depends on the potential applied to the diodes VD401 and
VD402. The oscillator is then amplified and buffered for the output. The same
principle applies to VCO #2 where the determinate factors for the frequency are
the capacitance of VD403 and L410. VCO #3 uses a 10.7 MHz crystal to produce
the required frequency. The crystal is “pulled” up or down depending on the
capacitance of VD404.
21.9
PLL's
(Schematic J30100920 )
The PLL (
Phase Lock Loop)
circuits take the reference frequency, 11.94 MHz, and
the VCO output frequency and divides each one by a number determined by the
microprocessor assembly. The circuit then determines the phase difference
between the two results and outputs a high or low voltage, (
0 or 8 volts)
depending on the angle of phase difference. The voltage is then filtered and sent
to the corresponding VCO. When the VCOs are locked, or in other words, when
the two divided frequencies are the same and there is no phase difference, then
pin 7 is set too high, The unit will not transmit unless all the PLLs are locked on
the required frequency. One of the four PLLs is used in the tone generation, and
does not require an output frequency feedback. (
See Tone Generator.)