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Front End Board
54 Chapter 6 Theory of Operation
Frontend Board
for M1353A
(M1350-66517)
Used in monitors with serial number prefix ≤ 3720G.
Figure 6-1 Frontend Board
The Frontend board contains the analog circuitry for measuring all the possible fetal
parameters of the M1353A. The FOCUS ASIC on the board provide the US timing and
window control, and controls the ECG.
Each type of transducer has a specific mode resistor which is recognized by the M1353A
when the transducer is connected to an input socket. In this way, different types of
transducer can be used without having to recalibrate the system.
ECG The FOCUS IC divides the 4 MHz system clock to generate a 181 kHz clock. A power
stage generates the power clock for the ECG transducers. The ECG signal modulates the
supply current of the transducer. The modulated current is detected, amplified and
bandpass filtered (1 - 250 Hz). Block DECG/MECG1 is unused.
US The US transducer transmits 998.4 kHz ultrasound bursts which are generated by the
FOCUS IC. The burst widths are controlled by software. The transmitter amplifier supplies
5 V
pp
at the Cardio sockets. The repetition rate is 3.2 kHz.
The received 998 kHz signal is amplified by a high frequency amplifier with a gain of 120
and then split to provide a reference path
a and a compare path b. The two demodulators a
and
b are independently controlled by software in their receive windows by the FOCUS IC.
The demodulated LF signals are bandpass filtered (100 - 500 Hz) and amplified by a
software controlled gain of 180 to 1860 in eight steps. The
FMP-LF path is independently
demodulated, bandpass filtered and amplified by a factor of 18.
TOCO The pins of the TOCO/IUP socket are electrically isolated, connected by three transformers
(excitation, sense and mode). The excitation voltage is a 2.4 kHz square wave with an
amplitude of 3.5V
RMS
. The frequency is generated in the control chip by dividing the 4
MHz system clock.
The sense input signal is amplified by 93.75 (40
µ;V/V/mmHg) or 750 (5µ;V/V/mmHg),
rectified with a synchronous detector and lowpass filtered with a cutoff frequency of 7.5 Hz.
All LF signals are multiplexed, sampled at either 1600 or 800 Hz, and A/D converted with
12-bit resolution.
The marker cable and the switch board are also connected to the Frontend board.