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Ultrasound Circuit
Overview
The ultrasound circuit is built up of four discrete sections.
These are:
Oscillator and Transmitter amplifier
Receiver and Detector
Audio amplifier
Signal pre-processing.
These operate to produce a continuous wave of 2MHz ultrasound which is passed to the
transmitter crystal in the transducer. The signal is then reflected from moving interfaces
within the body to the receiver crystal in the transducer, amplified and then detected so the
audio Doppler shift of that moving interface can be heard audibly or passed via signal pre-
processing to the A/D converter for rate calculation.
The circuit board has the option of two ultrasound circuits which operated at different
frequencies, the operation of each is identical as so only one is described here. The current
nominal frequencies are 2.1MHz for US1 and 1.8MHz for US2.
Oscillator and Transmitter output.
L4 forms an oscillator with Q4 and its associated capacitors producing a sine wave drive at
the required frequency. This is fed via a high current output driver Q1 and output transformer
L1.
Receiver and Detector
The reflected Ultrasound signal is fed via a resonant transformer L1 to the gate of Q3, the
drain of this FET connects to the source of Q2 to form a cascode amplifier From the drain
of Q2 the amplitude complex of the received signal is detected by passing the signal
through a detector diode D3 . The raw low frequency heart complex is then amplified and
filtered by U1 where its associated components form a bandpass filter amplifier with a
bandwidth of 150Hz to 1KHz. This signal is passed to the audio section and the signal pre-
processing .
Audio Amplifier
The audio signal then passes to a input selector U25 where the user can select via the front
panel which Ultrasound channel will pass to the audio circuits and then to digital gain control
U24 and buffer amplifier U32, where the volume setting is determined by the microprocessor
subject to the user setting. From here the gain controlled signal is fed to the input of a
monolithic power amplifier U27 and from here to the loudspeaker at J11.
©Ultrasound Technologies Ltd
Ultrasound Technologies Ltd
Issue 1 December 2004
Page 14
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