Toco Channel - GE Medical Systems Corometrics 340 Service Manual

TOCO Channel

ECG Channel
Revision A
Theory of Operation: Telemetry Receiver Board Circuitry (No. 13856A)
The TOCO channel includes an additional three poles of active high-pass
filtering at 1.5 kHz, followed by three poles of active low-pass filtering at
2.0 kHz, followed by a 1.75 kHz active two pole band-pass filter with a
gain of four. The output of this filter chain connect to the input of a PLL
circuit consisting of a FSK decoder, a buffer op-amp, and a differential
amplifier. The differential amplifier converts the frequency modulated
TOCO sub-carrier to amplitude variations that correspond to the output
of the TOCO bridge connected to the transmitter. Gain and offset
adjustments are added in the following amplifier stage which, after
passing through a resistor voltage divider, connect to the UA input of the
connected fetal monitor. The PLL also provides a lock detect output
which is used to enable the UA channel on the fetal monitor.
The ECG channel operates in a similar manner to the TOCO channel
described previously. The composite signal taken off the output of the
1.5 kHz high-pass filter of the TOCO channel is additionally high-pass
filtered with a three-pole, active filter set to 2.4 kHz The output of the
high-pass filter goes to a two-pole, active, 2.4 kHz band-pass filter with a
gain of 4. The output of the band-pass filter connects to a PLL circuit of
similar design to the one used in the TOCO channel. However, the
output of the ECG PLL is AC coupled and does not require the
differential amplifier or offset adjustments. The output stage is
attenuated through a resistor voltage divider, and is connected to the
ECG inputs of the connected fetal monitor. The lock detect output of the
PLL in conjunction with the FECG enable tone detector provide the mode
enable signals to the monitor.
Model 340 Telemetry System
2006920-001
4-9