GE Medical Systems Corometrics 340, Service Manual, Page: 50 / 82

4-10 Model 340 Telemetry System Revision A
2006920-001
Theory of Operation: Telemetry Receiver Board Circuitry (No. 13856A)
Ultrasound Channel
The ultrasound channel of the receiver consists of the initial 150 Hz
high-pass filter, a three-pole active low-pass filter at 550 Hz, a two-pole
active low-pass filter at 600 Hz with a gain of four, a three-pole active
high-pass filter at 150 Hz, and the last stage of a three-pole active low-
pass filter at 500 Hz. A gain adjustment is inserted between the 550 and
600 Hz low-pass stages. The output of this channel is then connected to a
FET switch which routes the output to either the ultrasound modulator,
or to a frequency halver circuit, depending on whether a continuous-
wave (CW) monitor or pulsed-Doppler monitor is connected.
The modulator stage utilizes a balance modulator with an offset added to
produce amplitude modulation from the audio and the transmitter
carrier supplied by the fetal monitor. The output tank circuit of the
modulator is switched to accommodate the differences in frequency
between the CW and pulsed-Doppler monitors. Detection of the monitor
type is accomplished with a jumper added in the 115 ultrasound
interconnect cable. The frequency halver circuit breaks the ultrasound
audio off into two channels: a frequency channel; and an amplitude
channel. The frequency channel takes the amplified audio off the input
stage, and using a limiter stage, removes most of the amplitude
variations off the audio signal. The limiter consists of an op-amp with
back-to-back diodes in the feedback circuit to limit the output to ±0.6 V.
The output of the limiter is amplified and connected to a zero crossing
detector consisting of an op-amp configured as a comparator. The output
of the zero crossing detector is converted to CMOS levels using a
transistor buffer. The output of the buffer then drives a CMOS D type
flip-flop configured as a divide by 2. The “Q” and “NOT Q” outputs of the
divider are used as the carrier signal in a switching type amplitude
modulator stage. The amplitude channel consists of a precision rectifier
and a three-pole active low-pass filter at 30 Hz which produces the
envelope of the audio signal. The envelope and carrier signals are
combined in a switching type amplitude modulator that consists of four
FET switches and a differential amplifier. The carrier signal alternately
causes the FET switches to route the envelope signal between the
inverting and non-inverting inputs of the differential amplifier. This
produces positive and negative variation at the output of the differential
stage, equal to the envelope amplitude, at a rate of the carrier frequency.
The amplitude modulated signal is then run through three poles of active
low-pass filtering at 500Hz, and three poles of active high-pass filtering
at 100 Hz. The output of the filters are used for the modulator stage
when a pulsed-Doppler ultrasound monitor is connected.
Power Supply Circuitry
The power supply is comprised of a single center tapped secondary
transformer, a bridge rectifier, and filter capacitors to produce an
unregulated positive and negative supply. Three terminal 78 and 79
series regulators are used for the +15,
– 15, +5, and – 5 V supplies. A
three-terminal adjustable regulator is used to 12 V to the RF
receiver.