GE Corometrics 126, Руководство по обслуживанию

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Revision B 120 Series Maternal/Fetal Monitor 4-49
2015590-001
Theory of Operation: Dual Ultrasound Board
Transmission, Channel A or Channel B
Two channels are implemented; however, both channels share part of the same
signal path. The ultrasound transmitter circuitry is formed by transistors Q1, Q2,
Q3, and Q4, and associated components. Transmission is initiated by the signal
TRANS from PAL U23. The gated 1.151 MHz burst causes Q1 to turn on and off at
that frequency. The resulting waveform at the collector of Q1 will vary in amplitude
between 0 V (Q1 full on) and a positive voltage determined by the voltage division
of the series combination of resistors R6–R8, and the setting of potentiometer R5.
R5 is used to adjust the drive amplitude to the ultrasound transducer. This
waveform is further buffered by the non-inverting amplifier circuit comprised of Q2,
Q3, and Q4, and filtered by a series resonate circuit consisting of L3 and C9 which
couples the amplifier output to the transducer.
Reception, Channel A or Channel B
Two channels are implemented; however, both channels share part of the same
signal path. At the end of the TRANS cycle, the common receiver pre-amplifier
used by both channels is activated. The receiver pre-amplifier consists of Q5, Q6,
and Q7 and associated components. L2 and C12 form a resonant L-type matching
network which provides an impedance match between the transducer and the pre-
amplifier. This matching network provides a voltage gain of approximately 38 dB
due to its low to high impedance transformation. Transistors Q5 and Q7 form a
cascade amplifier with a gain of approximately 13 dB. Q7 is a dual FET which is
connected in parallel to improve the signal-to-noise ratio of the pre-amplifier by 3
dB. The noise improvement occurs since the effective transconductance is doubled
but the noise adds in quadrature. The output load of the pre-amplifier, at the
collector of Q5 consists of inductor L1 and capacitor C14 which forms a parallel
resonant tank circuit with an impedance of approximately 10 k
Ω
. Transistor Q6
splits the pre-amplifier’s output providing a 0 degree and 180 degree differential
output to switch U11. Diodes D6 and D7 protect Q7 and clamp the voltage across
C12 during transmission. Diodes D8 and D9 limit the voltage across capacitor C14
and speed overload recovery.
Quad high-speed analog switch U11 is configured as a balanced ring demodulator,
and is used by both channels. The RF input to the demodulator comes from the pre-
amplifier’s differential outputs at the collector and emitter of Q6. Signals DETQ
and DETQ* cause U11 to alternately switch the differential input signals to the two
detector (differential) outputs at the same frequency as the transmitted signal. The
resultant differential output at capacitors C21 and C22 will be proportional to the
input amplitude at a frequency which is the difference between the input frequency
and the frequency of the DETQ and DETQ* signals. Any input signal at the same
frequency of the DETQ and DEQ* signals will produce either 0 V or a DC voltage
between capacitors C21 and C22. Offset frequencies caused by the Doppler shift
will produce an output at a frequency equal to the Doppler shift. The differential
detector output at C21 and C22 is buffered by two non-inverting stages of U6. These
buffered outputs are converted to single-ended output by a difference amplifier
consisting of a stage of op-amp U6, which also provides a gain of 26 dB. Due to the
alternating signals between transmit and receive, the detector output is active only
one-third of the time. During the switch-off time, capacitor C21 is discharged by
R20 and R21 while C22 is discharged by R22 and R23 to avoid crosstalk between
Channel A and Channel B since the signals for both channels are multiplexed over
the same signal path.