Valleylab CUSA EXcel-8 - Bandpass Filter (BPF)

Ultrasonics Board—-Ultrasonic Generator
A tip, designed as an ultrasonic velocity transformer or horn connected to
the transducer or motor, makes use of the energy contained in the
vibration of the handpiece. The horn also increases the stroke and velocity
with
minimal losses. The amount of transformation is referred to as “tip
gain.”
Temperature and load affect the frequency of vibration. If the transducer
heats, its frequency decreases; if the transducer cools, its frequency
increases. Transducer inefficiencies can cause “self-heating.” Loading of
the transducer can move the frequency up or down depending on
hardness, viscosity, and elasticity. Either case requires a change in the
amount of gain and phase from the generating circuit. Additionally, this
circuit must be able to react as quickly as possible to the load changes;
otherwise, the delivered power from the generator could exceed the elastic
limit of the tip material and cause fractures.
The Main Frequency Loop
The frequency loop consists of the following items:
* ACUSA EXcel handpiece
e Abandpass filter (BPE)
+ An AGC/error amplifier circuit
e A phase compensator
e A driver stage
‧ Feedback signal
+ Bias circuitry
e Frequency selection circuitry.
The handpiece has many potential operating points. The console must
choose one of these by way of some additional BPF function to select the
phase and gain that meet the oscillation criteria over the desired frequency
range of operation. Flat phase and gain over this region of interest is
desirable; otherwise, poor power factors and/or unstable operation could
result. The AGC loop controls the multiplier and, therefore, adjusts the
frequency loop gain to control the amplitude under varying load and
handpiece operating conditions. Linear operation of these two loops in
conjunction is imperative to avoid handpiece operation at an undesired
point.
AMOU
UNI
The output driver stage is a Class B push-pull current drive coupled to the
handpiece via a transformer and DC bias circuitry. Most magnetostrictive
drives use a current drive because the magnetic field intensity is linearly
proportional to the drive current—and, therefore, stroke—when the
handpiece is properly biased.
Bandpass Filter (BPF)
The input bandpass filter consists of an instrumentation amplifier stage
centered around U7 and an active two-pole filter centered around U12.
The instrumentation amplifier has a common / differential mode loss pad
x
d
E
CUSA
EXcel-8
System
Service
Manual
79