GE Corometrics 126, Руководство по обслуживанию

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4-46 120 Series Maternal/Fetal Monitor Revision B
2015590-001
Theory of Operation: Dual Ultrasound Board
Dual Ultrasound Board
Overview
The Ultrasound Board is a dual channel pulsed Doppler ultrasound system with
Channel A and Channel B. Each channel generates a 1.151 MHz (center frequency)
pulsed carrier signal. This carrier signal causes the crystals in the transducer to emit
ultrasonic sound waves. When these sound waves enter the maternal abdomen, they
create echoes when encountering an interface between tissues of differing acoustic
impedance. If the interface is moving either toward or away from the ultrasound
transducer, the frequency of the reflected sound differs from the frequency sent from
the transducer.
The Ultrasound Transducer
When the reflected sound wave is received by the transducer crystals, it is converted
to an electrical signal. After being amplified, detected, and filtered, this signal is
split into two paths: ultrasound audio and ultrasound envelope. The audio signal is
sent to the Main Motherboard to be amplified for driving the speaker, while the
ultrasound envelope is sent to the DSP Board for heart rate processing.
The center frequency of the transmitted carrier is 1.151 MHz with a pulse repetition
frequency of 4 kHz (single channel use) or 2 kHz per channel (dual channel use).
The total transmit and receive process is 250 µs which is broken down as follows:
‹ 90 µs: transmit pulse-wave Doppler
‹
40 µs: wait state
‹ 90 µs: receive reflected signals
‹
30 µs: idle state
When both ultrasound channels are being used, Channel A will complete a 250 µs
cycle while Channel B is muted; then Channel B will complete a 250 µs cycle while
Channel A is muted.
Ultrasound Oscillator
The main clock used to derive the ultrasound carrier frequency, as well as the
transmit and receive timing is the ultrasound oscillator. The ultrasound oscillator is
formed by the 4.604 MHz crystal X1, three inverters in U18, the timing components
C124, C125, and C149, biasing resistors R153, R155 and R156, coupling capacitor
C126, and two low-pass filters consisting of R154/C123, and R178/C150. The
frequency of the oscillator is precisely set by the adjustment of capacitor C125. The
output of the oscillator (U18, pin 6) provides the clock for counters U19, U21, and
U22, which generate the timing signals used by the remainder of the digital control
section of the board.