DATASONICS PSA-916, Руководство пользователя, Страница: 21 / 26

PSA-916 Sonar Altimeter 4-2
Theory of Operation Board level description
4.2.1 Power Supply
The input power can range from 6 to 24 volts, and is diode protected by CR1
to prevent reverse polarity. The power supply regulates the input power to
5vdc and 12vdc. VR3 regulates the 5 VDC and U6 and VR2 produce the 12
VDC. Whenever the input voltage is below 13 vdc, U6 goes into a switching
mode and boosts the voltage up to 13 volts. This voltage is then regulated to
12 volts by VR2.
4.2.2 Transmitter
The microprocessor (U8) generates a key pulse. The Key pulse puts the
Ultrasonic transceiver(U1) into the transmit mode for the duration of the
pulse. In the transmit mode the L2-C4 tank circuit is switched to oscillator
mode, and a train of 1
µ
s pulses is output from U1 pin 7 at the oscillator
frequency of 200khz. The pulse train is then input to the monostable
multivibrator(U2), which stretches the pulses to 1.8
µ
S. The output of U2 is
sent to the power MOSFET driver(U3). U3 inverts the pulses and drives
MOSFET Q1. Q1 in turn drives transformer T1, Which steps up the voltage
and drives the transducer. The inductance of the output of T1 and the
capacitance of the transducer create a tank circuit, which resonates at
200Khz. Thus the output to the transducer is a sinusoidal 200Khz burst which
lasts for the duration of the key pulse.
4.2.3 Receiver
The transmit pulse and any received echoes are input to the receiver through
a transmit/receive network consisting of R28, C13, CR7 & CR8. The receive
network is capacitively coupled to U1 through C9. JFET Q3 attenuates the
received signal during transmit and during the first few milliseconds of the
receive cycle, when the returned echoes are likely to be the strongest. Once
U1 receives the received signal, it goes through a high gain amplifier within
U1. The receiver uses the same L2-C4 tank circuit as the transmitter, so the
receiver and transmitter are always tuned to the same frequency. U1 also
detects the received echoes. When receiving an echo or its own transmit
signal U1 pulls pin 14 low.