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B.4 Front panel board
The line‑powered AC to DC power supply provides DC power to the
digital board through J100. This powers the primary side of shielded
transformer T100. T100 is chopped by U100, an LM5030 push‑pull
controller, to provide the DC voltages for the amplifier. The chopping
frequency is dithered by the microcontroller through the PWR_SYNC,
and is enabled from standby with the POWER_EN signal. U101 provides
"always on" +3.3 VDC to power the on/standby circuitry for the front‑
panel power button.
The front panel display board powers the 3‑digit seven segment LED
display for gain, as well as discrete LED indicators and the user button
inputs. Displays are all driven statically through shift register daisy‑
chain U9, U6, U4, U7, and U8. Button inputs are latched by parallel‑
loading shift register U11.
The power/standby button is debounced by Schmitt trigger U10 and
latched by flip‑flop U5, all of which are powered by the VAO +3.3V
"always on" power supply.
SR446 400 MHz Programmable Gain Preamplifier
Appendix B Circuit Description
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