GE Medical Systems Corometrics 340, Руководство по обслуживанию, Страница: 48 / 82

4-8 Model 340 Telemetry System Revision A
2006920-001
Theory of Operation: Telemetry Receiver Board Circuitry (No. 13856A)
Receiver Encoded Modulation
The encoded modulation from the receiver is divided into several paths:
the first to extract the mode information; the second to extract the data
signals. Mode information is removed from the composite signal through
a three-pole, active, low-pass filter of 100 Hz. After this stage, a gain
adjustment and an additional two poles of active low-pass filtering with a
gain of four are added. The output of the second low-pass filter connects
to four switched-capacitor, band-pass filters. The 80 Hz band-pass filter
is configured to operate with its own internal R/C clock. The output of
this clock also provides the clock input to a CMOS divider. This divider
provides the additional clocks needed for the other band-pass filters of 40
Hz, 20 Hz, and 10Hz. The outputs of the band-pass filters connect to
individual tone decoders which provide an active TTL low state when a
tone at a decoder’s tuned frequency is present at its input. The outputs
of the tone decoders, along with the lock detect outputs of the PLL
circuits used for ECG and TOCO decoding, provide the gate signals
necessary to control the NOR-gate/FET-switch combination that is used
to activate the different modes in the connected fetal monitor. The mode
enable process for ECG requires additional logic to determine if the ECG
information is FECG or MECG. This is determined by the absence or
presence of the FECG enable tone, in conjunction with the lock detect
output of the ECG phase lock loop (PLL). Two of the tones sent by the
transmitter are not used for mode control but provide a battery low
indication, and a remote mark enable. The remote mark enable is taken
directly off the tone decoder output. The battery low output from the
tone decoder turns on a comparator/astable oscillator which, using a
transistor driver, flashes an LED on the front panel.
The data signals are extracted by first high-passing the composite signal
through a three pole 150 Hz active filter. From that point the data signal
is further separated into a ultrasound, ECG, and TOCO channels.