Philips 50 T Series, Service Manual

Page: 85 / 105

Receiver Functional Blocks
Chapter 9 - Theory of Operation
RF Receiver M1402A
The MAGIC receiver module M1402A performs the complete baseband
signal recovery. The input signal is prefiltered, mixed down and filtered by a
double superhetrodyne circuitry and demodulated to the baseband signal.
An on board microcontroller does the complete automatic frequency control
tracking of the input signal.
US Bandpass
This module performs a bandfiltering for the ultrasound doppler signal. It
consists of a 60 Hz highpass to remove the low frequency noise of the FM
demodulator, a 450 Hz lowpass to remove the higher frequency noise, and
the FSK subcarrier. The subcarrier attenuation must be so high that it is not
audible in the ultrasound signal (>70 dB at 1.6 kHz).
ECG Bandpass
This module performs a bandfiltering for the fetal scalp ECG signal with a 15
Hz highpass, to remove the low frequency noise (low frequency phase noise of
the RF oscillators) and a 80 Hz lowpass, to remove the high frequency noise
and the FSK subcarrier.
The output of the US or ECG bandpass is selected by the decoded mode
information by the FSK Demodulator and fed to the receiver output as
heartrate output. If the input signal quality is too bad (bit error rate increase
over a certain limit), the heartrate and Toco output is switched off.
FSK Bandpass
This bandpass selects the FSK subcarrier out of the composite signal. It
consists of a 1.6 KHz highpass to remove the low frequency noise and the
heart rate signals, and a 2.4 kHz lowpass to eleminate the high frequency
noise.
FSK Demodulator
The FSK Demodulator consists of:
• FSK Comperator
• FSK Demodulator
The Comperator converts the sine shaped FSK subcarrier signal into a square
wave. The Comperator has a hysteresis to improve the performance of noisy
input signals. The Demodulator demodulates the FSK subcarrier and
recovers the encoded bitstream. The synchronisation is done by searching the
encoded 12 bit serial number. The bitstream is also descrambled and the
CRC is checked. If the test is positive, the complete frame is saved and the
information bits are recovered and appropriate actions are taken:
• Set the Mode outputs
• Display the status values on the front panel.
• Switch On/Off the nurse call beeper
• Switch On all output signals
• Set the fetal movement output