5-26 170 Series Monitor Revision C
2000947-004
Theory of Operation: Main Board Theory of Operation
Ultrasound Module
Model 171 and Model 173 Monitors contain a single ultrasound channel board.
Model 172 and Model 174 Monitors contain a dual ultrasound channel board.
The theory contained in this section references the dual ultrasound board but is
applicable to the single ultrasound board as well.
Overview
The ultrasound circuitry is a dual channel pulsed Doppler system with Channel 1
and Channel 2. Each channel generates a 1.151 MHz (center frequency) pulsed
carrier signal. This signal carrier causes the crystals in the transducer to emit
ultrasonic 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.
Isolation Transformer
Model 174 Monitors have an isolation transformer for US Channel 1 signals
since the connector is shared with the FECG Channel.
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 amplified for driving the speaker, while the ultrasound envelope is
processed for heart rate data.
The center frequency of the transmitted carrier is 1.151 MHz with a pulse
repetition frequency of 2 kHz (single or dual use).
When both ultrasound channels are being used, Channel 1 completes a transmit/
receive cycle while Channel 2 is muted; then Channel 2 completes a transmit/
receive cycle while Channel 1 is muted.
Programmable Array Logic (PAL)
All functions of the pulsed Doppler ultrasound circuit are controlled by a PAL
U50. Integrated circuit U48 latches the transmit and demodulator signals to
prevent race conditions and phase jitter caused by the PAL U50.
Table 5-19 provides a summary of PAL outputs. Figure 5-17 provides a block
diagram of the ultrasound module.
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