Philips Avalon FM20, Service Manual

Page: 44 / 260

4 Theory of Operation
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Communication Transceiver (CAN Bus Driver)
The communications transceiver (CAN bus driver) communicates directly with the transducer CPU,
and allows the transducer to communicate with the Bus Master Board via the CAN bus.
EEPROM
The serial EEPROM stores all non-volatile data required to operate the transducer (for example,
calibration and correction factors for front-end gains and offsets, country-specific information, serial
numbers, and error logs).
Toco Transducer Front-end
Uterine activity is measured by evaluating the hardness of the mother’s abdomen with a pressure
sensitive resistor bridge (strain gauge sensor element). The strain gauge sensor element requires an
excitation voltage and its differential output signal is proportional to the pressure applied to it. A DC
excitation voltage is used, and the resulting output signal is fed directly to an A/D signal converter
before being sent to the processor.
Ultrasound Transducer Front-end
The ultrasound front-end is a pulsed Doppler system with a 1.0 MHz ultrasound frequency, and a
pulse repetition rate of 3 kHz. Seven ultrasound crystals are used as transmitter and receiver.
Toco+ Transducer Front-ends
Several parameter front-ends are combined on one board. In addition to the Toco front-end,
additional supported parameters are DECG, MECG, and IUP.
A seven-pin ‘D-type’ socket carries all parameter related inputs and outputs. An external mode resistor,
connected to one of the pins, automatically detects which mode to set when an adapter cable is
plugged in (whether it is DECG, MECG, or IUP).
IUP Front-end
Intrauterine pressure (IUP) is measured via a piezo resistive bridge with AC excitation connected to
the RA/LA input pins of the ECG amplifier. A/D conversion of the IUP signal is done by the 16-bit
A/D converter.
ECG Front-end
The ECG front-end measures both DECG and MECG, using a 3-lead system (RA, LA, and reference
electrode). The ECG mode is automatically detected when an adapter cable is attached. Input lines are
ESD protected.