5-30 170 Series Monitor Revision C
2000947-004
Theory of Operation: Main Board Theory of Operation
Filtering
The main filter is a band-pass amplifier with 3 dB points at 120 Hz and 300 Hz,
respectively. The band width slopes are greater than 40 dB/octave. The maximum
gain of the filter is 46 dB with peaking of approximately 48 dB at 250 Hz. The
nominal gain in the system is approximately 36 dB and is controlled by R454. The
Channel 1 main filter is comprised of U51 and associated components.
The output of the track-and-hold circuit of Channel 2 at U46 (pin 1) is connected
directly to the Channel 2 main filter. The main filter is a band-pass amplifier with 3
dB points are 120 Hz and 300 Hz, respectively. The band width slopes are greater
than 40 dB/octave. The maximum gain of the filter is 46 dB with peaking of
approximately 48 dB at 250 Hz. The nominal gain in the system is approximately
36 dB and is controlled by R467. The Channel 2 main filter is comprised of U52
and associated components. Integrated circuit U40 switches the output of either the
Channel 1 demodulator, the Channel 2 demodulator, or telemetry audio to the fetal
movement band-pass filter. This filter has a gain of 3.5 and corner frequencies of
15 Hz. and 39 Hz, respectively. It is comprised of U41 and associated components.
The output of the fetal movement filter is input to the analog-to-digital converter.
The output of the main filters are input to U10 through capacitors C44 and C45. The
input impedance of these inputs is 10 kΩ, thus producing a high-pass filter at 72 Hz.
A tone is also one of the inputs to U10 through a high-pass filter at 15 Hz and a gain
reduction of –21 dB. The components comprising the filter are C47 and R223. U10
is a quad digital potentiometer with 255 step producing a gain range from 0 to
–48 dB. It is controlled by the serial peripheral interface (SPI) system bus.
Ultrasound Audio
Ultrasound audio (Channel 1 and Channel 2) is output on U10 (pin 8) and U10 (pin
4), respectively. These outputs are buffered by U11 (pin 7) and U11 (pin 8),
respectively. The buffered outputs are high-pass filtered at 80 Hz by C57/R236 and
C58/R237. They are then half-wave rectified by U13 (pin 8) and its associated
components. The output of the rectifier is band-pass filtered from 43 to 260 Hz at a
gain of 7.3 by U13 (pin 1) and its associated components. This effectively doubles
the frequency which is necessary because the transducer operates at a low RF
frequency (1.151 MHz); and the frequencies produced from the fetal heart would not
produce acceptable audio quality.
The output of U13 (pin 1) is input to summing amplifier U11 (pin 14). The tone
output of U10 (pin 18) is also summed by U11 (pin 14) which is a low-pass filter at
2600 Hz with a slope of –12dB/octave. The output of U11 (pin 14) is high-pass
filtered at 72 Hz by C52/R231 to the audio power amplifier.
The audio power amplifier U12 is a bridge type configuration with a gain of 2.5 and
capable of delivering 1.5 W of power into 8 Ω. Figure 5-18 provides block diagram
of the audio module.
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