Philips 50 T Series, Service Manual

Page: 80 / 105

Transmitter Functional Blocks
Chapter 9 - Theory of Operation
demodulator. This demodulator is driven by the US Gating module with 1
MHz bursts. The demodulated signals are amplified and bandpass filtered.
The overall gain from the transducer to the US Receiver’s output is 70 dB for
the heartrate signal and 56 dB for the fetal movement signal.
ECG Driver
This circuitry powers an ECG transducer with a 5V peak to peak, 250 kHz
square wave. The driver current is sensed by the ECG Receiver.
ECG Receiver
After converting into a voltage, the signal is amplified and bandpass filtered.
The filter consists of a 15 Hz highpass and a 100 Hz lowpass. The overall
gain from the transducer inputs to the ECG Receiver’s output is 56 dB. In
addition, the ECG Receiver outputs an INOP flag if the 50/60 Hz signal or
an INOP signal (250Hz delivered by the ECG transducer if electrodes have
no contact) of a too high amplitude is detected.
Dynamic Compression Circuitry
The Dynamic Compression Circuitry consists of:
• Programmable Gain Amplifier (PGA)
• Automatic Gain Control (AGC)
The PGA is a special amplifier with a programmable gain factor between 0
dB and 40 dB in 255 steps. It adapts the high dynamic range (>60 dB) of the
heartrate signals to the limited dynamic range of the RF link. The PGA is
controlled by the Automatic Gain Control.
The AGC circuitry sets the gain of the PGA to get the best usage of the RF
link dynamic range. The actual heartrate singal amplitude is measured by a
peak hold detector which is the input signal for the controller algorithm. The
algorithms for ECG and ultrasound signals are different. They are adapted to
the special attributes of the signals. This results in different time constants,
amplitude margins etc.
The gain is changed only between two signal complexes where the signal is
approximately zero to avoid destruction of the signal complexes which may
lead to an inaccurate calculated heartrate in the Fetal Monitor (artificial
jitter).
FMP Detector
The FMP Detector consists of two parts:
• The FMP Envelope
• The FMP Detector
The FMP Envelope circuitry builds the envelope signal of the fetal movement
profile doppler signal from the US receiver. It is a full wave rectifier followed
by a 12 Hz lowpass.
The Fetal Movement Profile Detector searches for fetal movement related
signals in the FMP envelope signal. If a movement is detected, a status bit is