1188
Technical Description
Two zero crossing detectors convert the Doppler signals to corresponding pulse trains. The
pulse trains are then fed to the microcontroller which converts them to pulse width modulated
signals which are then filtered to produce the waveform output voltages.
Zero Crossing Detector (SD2-P, MD2-P, RD2)
The split rail generator is necessary to bias the amplifiers correctly to allow linear operation. The
split rail generator supplies a +2.0V dc reference so that the maximum output swing from the
amplifiers can be achieved.
Split Rail Generator
The data control IC is an RS232 interface device enabling the microcontroller to communicate
with the Dopplex Printa or a computer. The data/waveform socket presents the data from the
microcontroller which represents either bi-directional flow waveforms or fetal heart rate,
depending on operating mode.
Data Output
The All Pass network is used to apply 90° phase shift to one quadrature probe output signal
with respect to the other.
All Pass Network
The two phase shifted signals are added and subtracted to produce the forward and reverse
flow channels.
Flow Separation
The audio power amplifier provides stereo outputs to the headphone socket. These are
combined in the internal loudspeaker.
Stereo Audio Power Amplifier (SD2-P, MD2-P, RD2)
The FHR signal conditioning circuit produces an amplitude envelope from which the
microcontroller calculates the fetal heart rate when using an obstetric probe.
FHR Signal Conditioning
The gain of the first amplifier of the FHR signal conditioning circuit is increased or decreased by
the microcontroller, to regulate the signal level avoiding overloading the following stages.
Gain control
Pulses representing the frequency envelopes of the Doppler signals are output from the
microprocessor to two integrating low-pass filters. The two filter outputs are then fed to the
analogue waveform/data output socket.
Analogue Waveform Outputs
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