The Main Frequency Loop; Bandpass Filter (Bpf) - Integra Cusa Excel Service Manual

CUSA EXcel Ultrasonic Surgical Aspiration System Service Manual
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:
• A CUSA EXcel handpiece
• A bandpass filter (BPF)
• An AGC/error amplifier circuit
• A phase compensator
• A driver stage
• Feedback signal
• Bias circuitry
• 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.
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 and RF filter
consisting of R48, R49, R67, R68, C43, C46, and C53. The filter and pad pass the
differential mode ultrasonic frequencies at about 16 dB of midband loss due to the
large feedback voltage swings of up to 90 Vp-p of the 23 kHz handpiece/tip. The
gain set resistor R66, selected by the analog switch U10C when the 35.75 kHz
Ultrasonics Board–Ultrasonic Generator
7-9