1155
Technical Description
Resets the microcontroller whenever the unit is switched on.
Power On Reset
The probe identification feature is used to disable the waveform output whenever an obstetric
probe is being used. The probe identification detects the DC level on the output from the probe
thus determining the type of probe fitted.
Probe Identification (D900-P)
The battery monitor circuit enables the microcontroller to assess the battery voltage by
comparing this to a reference voltage.
Battery Monitor
Active noise reduction is a technique used to reduce the amount of noise or "hiss" depending
on signal level.
When a large signal has been detected the cut off point of the filter is moved so that there is no
reduction in high frequency content of the signal. When the signal level is low, or no longer
present the filter is switched in to reduce the high frequency hiss.
Active Noise Reduction (ANR)
The soft limiter reduces the level of any harsh overload signals that may occur, for example,
when the probe is moved.
Soft Limiter
The audio power amplifier drives the loudspeaker or headphones. The input level is set by the
volume control.
The output signal is fed to the loudspeaker, via the switched headphone socket. On start up
there is a delay in the supply of power to the audio amplifier. This allows the circuit to stabilise.
Audio Power Amplifier
A zero crossing detector converts the Doppler signal to a corresponding pulse train. This is
then fed to the microcontroller which outputs a pulse position modulated signal which is low
pass filtered to produce the waveform output voltage.
Zero Crossing Detector (D900-P)
The split rail generator is necessary to bias the amplifiers correctly to allow linear operation. The
split rail generator supplies a +2.5V dc reference so that the maximum output swing from the
amplifiers can be achieved.
Split Rail Generator
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