Philips 50 T Series, Service Manual

Page: 81 / 105

Transmitter Functional Blocks
Chapter 9 - Theory of Operation
delivered to the FSK Generator. The detector algorithm is identical to the
algorithm implemented in the Series 50 Fetal Monitors.
FMP is switched off whenever a Series 50 T Telemetry System (plugged into
the Monitor) is switched on. FMP is switched back on again when the
telemetry system is switched off (or unplugged). If you want to monitor FMP
with a Series 50 T Telemetry System (and have the correct software revision
and interface for Telemetry FMP) switch FMP on by using either the Key or
the barcode method.
Modes
This module checks for connected transducers at the Cardio and Toco
connectors. If a valid transducer is detected, the corresponding operating
modes are set in the Transmitter (ECG/US, TOCO/IUP) and the mode
encoding bits are delivered to the FSK signal generator. If an invalid
transducer is detected, these bits are set to an error code and the Transmitter
frontends are disabled.
Toco Driver
This circuitry drives a connected Toco or IUP transducer with a 1.6 kHz, 5V
peak square wave. The 1.6 kHz frequency is synchronised with the 3.2 kHz
repetition rate of the doppler ultrasound circuitries to avoid interference
between the Toco and ultrasound channels.
Toco Receiver
This module contains a differential intput amplifier, a synchronous
demodulator and a low pass filter, to derive a pressure proportional signal
from the TOCO/IUP transducers bridge.
Toco A/D
The TOCO/IUP pressure signal is A to D converted by a sigma-delta type
converter with a resolution of 12 bits and a 2Hz low pass characteristic. The
repetition rate is 4/sec. These 12 bits are encoded in the serial bitstream by
the FSK Generator module.
To ensure a high accuracy of the Toco pressure signal, correction factors are
stored in a EEPROM during the production test individually for every
Transmitter. Before encoding the Toco pressure values into the serial
bitstream, the pressure values are corrected with these factors. This
mechanism allows the usage of circuitries in the Toco frontends which only
have stability over temperature and aging but not an absolute accuracy.
FSK Generator
The Frequency Shift Key generator (FSK), is a sine wave generator (smoothed
square wave by filters) capable of generating either a frequency of 1.6 kHz
(represents a logic zero) or 2.4 kHz (represents a logic one). It encodes the
following signals in a bitstream:
• Coding pattern identification
• Transmitters Serial Number
• 12 Bit Toco pressure value