Revision B 120 Series Maternal/Fetal Monitor 4-51
2015590-001
Theory of Operation: Dual Ultrasound Board
Ultrasound Envelope
To generate the Channel A ultrasound envelope, the output of the main filter is
amplitude scaled by resistive divider R180/R48 and then connected to compandor
(U2) which provides at least 40 dB of signal compression. The output from the
compandor is limited by diodes D18–D19 and then precision rectified by a stage of
U1 and its associated components. The gain of the rectifier is adjustable over a range
of 7 to 17 using R183 to accommodate the possible output variations of U2. The
rectified output is then low-pass filtered by a 20 Hz active filter with a gain of 10,
consisting of a stage of U1 and its associated components. The output of the filter is
a unipolar signal which follows the low frequency amplitude variations (or
envelope) of the output from the AGC chip. The filter is biased for an output of –10
V with a no input signal condition. This provides the –10 V to +10 V input required
by the A/D converter on the DSP Board. The US1 ENV output from this filter
connects to the DSP Board for heart rate processing.
Fetal Movement Filters
The input to the Channel A main filter also connects to the Channel A fetal
movement filters. These filters consists of a unity gain non-inverting buffer
amplifier from the detector output, two unity gain active 2-pole high-pass filters,
two unity gain active 2-pole low-pass filters, and an inverting amplifier from a stage
of U12 with an adjustable gain of 2.7 to 14. Input potentiometer R126 sets the
amplifier gain. The 3 dB points are at 15 Hz and 39 Hz with rolloffs of 24 dB/octave
for both high and low frequencies. The output at U12, pin 1 (FMD1) connects to the
DSP Board for movement detection processing.
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