GE Corometrics 126, Service Manual

Page: 105 / 440

Revision B 120 Series Maternal/Fetal Monitor 4-47
2015590-001
Theory of Operation: Dual Ultrasound Board
Digital Control Section
The digital control section of the Ultrasound Board is common to both Channel A
and Channel B and is composed of ultrasound oscillator U18, PALs U20 and U23,
and counters U19, U21, U22, and U24. Counters U19, U21, and U22 count down
from the oscillator providing sequencing inputs to PALs U20 and U23, as well as the
144 kHz and 72 kHz clocks used to synchronize the monitor’s switching power
supplies. The PALs U20 and U23 decode the counter inputs to produce output
signals that control the ultrasound transmitter (TRANS), select the active transducer
(PIN1* and PIN2*), provide the detector carrier (DETQ and DETQ*), and switches
the detector output to the Channel A or Channel B filters (CH1* and CH2*). An
inverted version of the 4.604 MHz clock is used to clock the registered outputs of
PAL U23 giving the inputs one half clock cycle to settle before the outputs are
switched.
Signals A* and B* are additional inputs to PAL U20 and U23 and provide
information on the number of transducers connected to the monitor. If only one
ultrasound channel is selected, U23 outputs the appropriate signals for that channel
only, at a pulse repetition frequency of 4 kHz. If both ultrasound channels are
selected, the rate for each channel is 2 kHz. This repetition rate is determined by the
states of A* and B* as well as the state of the signal USMODE.
The signal USMODE is generated by counter U24 and used by PAL U23 when both
Channel A and Channel B are selected. The USMODE signal is ignored if only one
channel (A* or B*) is selected. When A* and B* are both active, the state change of
USMODE every 1024 clock cycles will alternately activate signals PIN1* and
PIN2*.
Signals PIN1* and PIN2* are used to control the pin diode switches which in turn
selects which of the two channels is active for transmitting and receiving.
The signal TRANS is a gated square wave with a frequency of 1.151 MHz which
controls the ultrasound transmitter circuitry. It is initiated four clock cycles after
reset and continues up to 428 clock cycles.
The signals DETQ and DETQ* are gated square waves of 1.151 MHz which are 180
degrees out of phase with each other, and provide both phases of the detector carrier
required by the balanced ring demodulator. The signals begin 640 clock cycles after
the counter chain is reset and continue up to 1020 clock cycles.
CH1* and CH2* control the switching of the output of the demodulator to the track
and hold filter of Channel A and Channel B, respectively. They are controlled by
A*, B*, and USMODE (similar to PIN1* and PIN2*), however they are switched
active low from 640 to 1020 clock cycles following the same active timing as DETQ
and DETQ*.
U23, pin 12, goes to a low state every 1151 clock cycles which resets the binary
counters U19, U21, and U22 restarting the timing decoders at state 0.
Mode control signals (A*, B*, TEST*, and TELMCON) for the digital section of the
ultrasound board are generated on the DSP Board and latched from the processor
data bus by U15. U16, pin 6, provides the latch clock by decoding a processor chip
select (CS0*) and the processor write pulse (BWR*). Mode information
(US1EN*,US2EN*, and USMODE) is read by the processor through buffer U17.
The output enable for U17 is generated by U16, pin 8, which decodes CS0* and the
processor read pulse (BRD*).