Valleylab CUSA EXcel-8 - Driver Stage; Phase Compensator

Ultrasonics Board-Ultrasonic Generator
* The input signal levels
* The minimum dynamic range required for the handpiece stroke levels
under load and start-up conditions.
RV8 sets the gain of the amplifier, while RV9 sets the gain for the feedback
signal levels.
The control function has an offset and a gain which limit the operating
range capabilities (start-up and load) of the handpiece. At minimum level
settings the control function must have a start-up gain of at least two times
or greater the nominal error output. Under maximum load, the gain
should be sufficiently large to allow stroke, even if the stroke is at reduced
levels. If the gain is too large, however, a large enough feedback will force
the error amplifier output to 0, whereby control is lost.
RV11 is for calibration purposes only.
Phase Compensator
The two-pole, active filter, U9, provides loop phase compensation. This is
a Bessel filter characteristic implemented with a “low sensitivity”
configuration. It is “low sensitivity” in that it has low operational
amplifier (op-amp) slew rate and drive requirements, while component
temperature and tolerance sensitivities also remain low. The phase for the
23 kHz circuit depends on the RC value of RV5, R52, and C41 selected by
the analog switch U5A; the phase for the 35.75 kHz circuit depends on
RV4, R300, and C41 selected by U5D. The nominal -3 dB point of the filter
is 37.30 kHz.
The alignment of these filter poles with the BPF determines the amount of
phase at the undesired longitudinal modes. Too little phase from the BPF
makes it difficult to adjust these modes out; too much phase allows the
center frequency to remain mis-adjusted and, therefore, may sacrifice mid-
band gain flatness.
Driver Stage
The driver stage consists of the following components:
시
0941
31031
* Apre-amp, U1B
e A push-pull rectifier or phase splitter, ULA
* A positive half-cycle driver, U2B
* A negative half cycle driver U2A.
For the 23 kHz circuit, output stage gain depends on the combination of
two gains:
e The pre-amp gain (R10 and R11)
* The phase-splitter gain (R19, R20, R24, and RV3).
For the 35.75 kHz circuit, output stage gain depends on RV2, selected by
switches U5C and USB.
Ji
È
CUSA
EXcel-8
System
Service
Manual
7-11