Lexicon RV-8 Service Manual page 122

RV-8 Service Manual
D16 prevents the control voltage from going much below 2V, ensuring that the VCO is never driven to a
non-oscillating state. D21 prevents the duty cycle integrator from being driven to the wrong polarity when
the loop is out of lock.
VCO B (Sheet 16)
Sheet 16 replicates the VCO and PLL circuitry on sheet 15 for Zone 2. Further enhancement of details
will not be made here.
Power Supply (Sheet 17)
Power Entry Connector (J31)
Power is brought onto the PCB via J31, a Molex style connector rated for high current. C167-C169
provide de-coupling of the +5VD supply at a wider range of frequencies than could be achieved with a
single high value capacitor. All of the logic supply voltages are derived from +5VD. The +/-15V rails are
de-coupled by C171-C176 in the same manner as the +5VD rail. These supply voltages are used to
derive power for both VCO circuits, output triggers, and environment fans. Chassis Ground is picked up
by LUG1, which provides mechanical contact with the chassis via the rear panel.
2.5V Regulator (U12)
This regulator provides a supply voltage for the AVRX FPGA core and for the Decoder DSP core. It is
derived from +5VD via an LM2937. This device is a fixed output voltage regulator specified at 2.5V. It is
rated for 500mA current, which is far less than the required current. C54 provides local bulk capacitance
to the regulator input while C53 provides it for the output.
3.3V Regulator (U38)
U41 is a fixed output voltage regulator that provides a 3.3V supply to most of the logic on this board. It is
derived from +5VD. C158 provides local high frequency de-coupling to the input while C122 provides
local bulk capacitance to the output. L1 and C123 form a low-pass filter to provide a cleaner supply to the
3.3V. It has been determined that this precaution is not needed in this design, and so these components
are not used. W6 provides a zero ohm path from the 5VD supply to the input of U38. Diode D48 ensures
that the output voltage never assumes a level higher than +5VD, which could cause destruction to U38.
Fan Control Circuitry
U47 inverts the pulses supplied by one of the PWM timer circuits inside the Host CPU. R179 acts as a
default pull-down for Q1 in the event that there is no signal source connected to U47 and the output
achieves an indeterminate state as a result. This prevents the fans from turning on in the absence of a
control voltage. R177 provides current limiting to the base of Q1. When positive current flows into the
base of Q1, the transistor saturates, effectively pulling the gate of Q2 low. When a logic zero is applied to
the base of Q1, the transistor appears as an open circuit to the gate of Q2, causing it to conduct from the
drain to the source. The drain of Q2 follows the input signal FAN_DRV. D49, L2, and C178 act as a full
wave rectifier and filter that produces a DC voltage proportional to the duty cycle of FAN_DRV. This
voltage will vary between 3V and 11V. The environment fans are driven from the +15V supply with this
control voltage present on the other side of the fan windings. Therefore, the fans will run on voltages from
4V to 12V, dependent upon the duty cycle of FAN_DRV. Connectors J21, J23, J24, and J32 are
connected in parallel so that all four fans are controlled by the same signal. L3 is a provision to keep
brush noise from the fan from getting into the +15V supply. It has been determined that this protection is
unnecessary, and so it is not installed. R182-R186 Provide a total resistance between +15V and the fans
of approximately 10 Ohms capable of dissipating 1.25W. This resistance suffices to suppress back EMF
from the fans. C177 provides local bulk capacitance for the circuit.
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