4-50 120 Series Maternal/Fetal Monitor Revision B
2015590-001
Theory of Operation: Dual Ultrasound Board
U6 drives analog switch U4 through R28. Signals CH1* and CH2* steer the
demodulated output to the appropriate filter channel (Channel A or Channel B)
when U11 is active. Switch U4 is kept inactive when U11 is inactive. Capacitor
C23 holds the demodulated signal amplitude for Channel A between samples; C54
holds the demodulated signal amplitude for Channel B between samples.
When a telemetry system is connected to the monitor, the output from U4 to the
Channel A filters is disconnected by a FET switch in U5. The ultrasound audio from
the telemetry system is instead connected directly into the Channel A filters by a
second switch in U5.
Channel A Filtering
NOTE: The remaining circuitry used on the ultrasound board is non-multiplexed,
but is duplicated for each channel. Therefore; the theory discussed in the
remainder of this document, although referencing only Channel A, shall
apply to Channel B as well.
Main Filters
The main filters for Channel A are band-pass filters consisting of four stages (two,
2-pole low-pass; two, 2-pole high-pass) of U3 and associated components. The 3
dB points are at 100 Hz and 300 Hz with rolloffs of 24 dB/octave for both high and
low frequencies. A variable resistor (R37) is used to set the overall gain between the
detector output and TP1 to 32 dB. The output of the main filter (U3, pin 7) is then
split between the ultrasound audio circuitry and the ultrasound envelope circuitry.
Audio Circuitry
To obtain the Channel A ultrasound audio, the output of the main filter connects to a
full-wave precision rectifier consisting of two sections of U1 and associated
components. The output resulting from the full-wave rectification is a frequency
doubling of the original waveform. The frequency doubled output is then band-pass
filtered by a passive low pass (R57/C44), a passive high-pass filter consisting of
C45, C46, and R59, and an active low-pass using one section of U7, which also
provides a gain of 35. The output is then buffered by a second section of U7 which
provides the US1 AUDIO output to the Main Motherboard.
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