Base Station Communication; Power Supply And Battery Charger; Modulator; Rf Transmitter - Philips Avalon CTS M2720A Service Manual

2 Theory of Operation

Base Station Communication

An active base station communicates with docked transducers to check the correct transducer type, serial
number, to program the RF frequency, to initiate test cycles, and to check the current battery status.

Power Supply and Battery Charger

A Lithium-Ion (Li-Ion) battery powers the transducer. Batteries are charged whenever a transducer is
docked in an base station connected to AC power. Charge power is transferred by the spring-loaded, two-
pin charge contacts. Communication between the base station and the transducer processors is also done
using these contacts.
An intelligent Li-Ion charging circuit in the transducer controls the battery management. See page 72 for
battery charging times.

Modulator

The analog heart rate signals (ultrasound Doppler or fetal ECG) are pre-processed and band-limited by
filters. The signals are then fed into a programmable gain amplifier controlled by an automatic gain
control circuit and a digital FSK modulated subcarrier (1.6 kHz or 2.4 kHz) is added to the RF carrier
signal. The subcarrier is responsible for the digital transmission of the Toco data, along with safety and
status information, such as serial number, bed label, fetal movement data, battery information.

RF Transmitter

The RF transmitter block contains:
• a PLL synthesizer that divides the reference frequency down to 12.5 kHz, and can be tuned in steps of
12.5 kHz.
• a voltage controlled ocillator (VCO).
• a power amplifier (antenna driver).

Toco Frontend Hardware

Uterine activity is measured by evaluating the hardness of the mother's abdomen with a pressure sensitive
resistor bridge (DMS element). The DMS element requires an excitation voltage and its differential
output signal is proportional to the pressure applied to the DMS element. An AC excitation voltage is
used, and the resulting AC output signal is amplified and converted to a pressure proportional DC voltage
by a synchronous rectifier followed by a low pass filter.

Ultrasound Frontend Hardware

The ultrasound frontend is a pulsed Doppler system with a 1.0 MHz ultrasound frequency, and a pulse
repetition rate of 3.2 kHz. Seven ultrasound crystals are used as transmitter and receiver.

ECG Frontend Hardware

Several parameter frontends are combined on the ECG frontend board. Currently supported parameters
are DECG and MECG.
20
Toco Frontend Hardware