Second Crystal Check; Reset Circuit; Power Supply - Decatur Electronics SI-2 Technical Manual

Second Crystal Check

U4, a 74HC4060 counter, generates the second frequency reference that the second crystal check needs. Y2
with resistors R12, R13 and capacitors C4, C5 set the oscillation frequency to 3.6864MHz. The output
frequency of U4 at pin 4 is approximately 57,600Hz. This second crystal also provides the clock reference for
the external serial port baud rates.

Reset Circuit

U1B and U1C with resistors R1, R2 and capacitor C1 form a reset circuit for providing the power-on reset for
the DSP chip.

Power Supply

Power supplied externally to the SI-2 enters the DSP board at JP5 and is routed to two regulators, U6 and U7.
U6, the main power regulator, is configured to supply +5VDC to all logic and analog circuits.
Resistors R17 and R18 program the low-voltage detect circuitry to trip at approximately 5.5VDC. Resistor
R20 maintains the low-voltage output at logic high until a low-voltage threshold is met. Resistors R14, R16 and
transistor Q1 form the power- on enable for the regulator. When the SI-2 is powered, current flows through
the base of transistor Q1 and resistor R14 and forces the transistor to switch on. Collector current from
transistor Q1 flows through resistor R16 increasing the voltage at the regulator enable pin 5. When the voltage
at pin 5 exceeds a threshold, the regulator turns on and begins to supply power to the unit. After the unit
powers up, the digital signal processor (DSP) maintains power to the unit by pulling pin 5 of U6 high within
about 100ms.
Diode D2 protects the DSP chip from reverse currents from the regulator section.
U7 provides power to the Gunn oscillator, which generates the microwave energy. Transistor Q2 and resistors
R15 and R19 form the enable circuitry for the Gunn oscillator power supply. The DSP turns on the regulator
by bringing ANT_ENA to logic high, as long as power is applied to the SI-2 it will transmit. Resistors R21,
R22, and R23 set the voltage on the Gunn oscillator. The nominal voltage level from the regulator is
+5.51VDC.
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