2. THEORY OF OPERATION
Service information Phyaction 190 / 190i - Page 7
Countdown of the treatment time is stopped when the microcontroller notes that there is no or not enough
contact between treatment head and patient. The intensity of the ultrasound can be adjusted by the user using
the intensity knob. Depending on the voltage
Vpotus
the microcontroller computes and sets the required voltage
Vus
with the controlling voltage
VSetUs
in order to generate the desired amount of ultrasound.
2.3.1 PLL
The required ultrasound frequency
Fus
of 0.8 MHz or 3.3 MHz cannot be generated by the microcontroller
itself. For this reason a phase locked loop (PLL) is used to transform a low-frequent signal
RefClock
, that the
microcontroller can generate, up to the desired steering frequency of
2XFus
. The modulator divides this double
frequency and mixes it with the desired duty-cycle.
The frequency generated by the microcontroller is always around 400 Hz. In order to obtain ultrasonic
frequencies of both 0.8 MHz and 3.3 MHz a programmable divider is used in the PLL. Whenever the PLL is out
of lock (e.g. when changing the frequency for contact control calculations) the
Lock-error
signal is used to stop
the modulator in order to prevent erroneous frequencies being produced by the treatment head.
2.3.2 Modulator
To obtain a symmetrical steering signal for the power stage the modulator switches its outputs with half the
input frequency. Also the modulator mixes the
duty-cycle
signal synchronously with the ultrasound frequency to
prevent spikes. An amplitude modulation with 100% modulation depth (the signal being switched on and off) is
the result of this mixing. With the
Lock-error
signal the modulator can be forced to shutdown. This action can be
taken by the PLL when it is out of lock or by the microcontroller. The
Lock-error
line can be used as an input as
well as an output by the microcontroller.
2.3.3 Voltage controller
A pulse controlled downward converter voltage controller is used to convert the unstab29V to the
desired ultrasound voltage. The voltage controller is made in such a way that its output voltage is linear
proportional to its input voltage
VSetUs
. Thus the microcontroller can adjust the ultrasonic power by setting
VSetUs
proportional to the square root of the desired output power. The microcontroller constantly monitors the
output voltage of the voltage controller. If the output voltage is too high (because of a hardware failure), the
microcontroller will shut down the ultrasound unit and give an error message.
Figure 2.2, Functional block diagram of the ultrasound unit
