Ultrasound Technologies Fetatrack 310, Руководство по обслуживанию

Страница: 25 / 81

Circuit Description
Fetatrack310 Service Manual Issue3
©Ultrasound Technologies Ltd, Lodge Way, Portskewett, Caldicot, South Wales NP26 5PS, United Kingdom.
T +44 (0) 1291 425425 F: +44 (0) 1291 427093 E: ultratec@doppler.co.uk W:
www.doppler.co.uk
Ultrasound Technologies Ltd
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 (1.5MHz special order) 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 L2.
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 tuned cascode
amplifier with L3. 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.
The power rail for U27 is provided by the unregulated 36V fed from D19.