Oscillator; Power Supply; Service Interface - Philips 50 T Series Service Manual

Transmitter Functional Blocks

Oscillator

Power Supply

Service Interface


• transducers and operating modes
• fetal movement detection bit
• battery status
• external marker information
• Nurse call button
• INOP condition of ECG
• CRC (checksum over one frame)
• Special transmitter ID after power up (Japan only)
This circuitry produces the different clocks needed in the Transmitter. The
oscillator is crystal controlled and starts operation at power supply voltages as
low as 2V. This is necessary because the oscillator also drives the power
supply. To ensure a start up over the complete specified battery voltage range,
the oscillator must start at this low voltage. The clock freqencies are:
• 2 MHz for the mircocontroller
• 1 MHz for the ultrasound drive/receive clocks and the power supply.
• 500 kHz for the Toco sigma-delta A/D converter.
• 250 kHz for the ECG driver and the Toco sigma-delta A/D converter.
The power supply is a switched mode power supply with step up operation.
It works with a 250 kHz clock frequency (delivered by the oscillator) in a
pulse width modulation mode. It is capable of delivering the necessary power
in a fixed 5V output voltage for input voltages between 2.5V and 4.7V. (This
covers the input voltages delivered by three AA type alkaline batteries or three
AA type NiCd/NiHd accumulators).
Other supply voltages are derived from the 5V supply by linear regulators or
by charge pumps runing at 250 kHz.
• 2.5V as virtual ground (linear regulator)
• 8.5V for ultrasound preamplifier and OP-Amps (charge pump as
voltage doubler.)
• -3.5V for OP-Amps (charge pumps as voltage inverter).
The service and production interface uses the marker input as the I/O
connector for settings and internal status readings for service and production
tests.
It is a RS232 similiar (0.5V only levels) serial interface with a specific
protocol. This interface is used for:
• setting the Transmitter's serial number
• reading some internal values for prodution tests
• writing correction factors for the Toco frontend into the EEPROM.
Chapter 9 - Theory of Operation