United States
Environmental Protection
Agency
Health Effects Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S1-85/021 Jan. 1986
Project Summary
A System for Measurement of
Small Vibrations at Material
Interfaces Induced by
Electrostrictive Forces
Joseph S. All and William T. Joines
The mechanisms of interaction of ELF
and ELF-modulated RF fields with bio-
logical systems is presently an active
area of research. Some models propose
that field-induced forces may influence
certain observed biological effects such
as RF hearing and calcium ion efflux. To
investigate the validity of the field-
induced force model for the calcium-ion
efflux effect, a system is needed which
is capable of exposing samples to ELF
fields or to ELF-modulated RFfields. At
the same time the induced vibration
caused by the forces of electrostriction
must be monitored preferably by a
noncontacting method.
A microwave phase-sensitive receiver
was designed to sense the small vibra-
tions. Limitations on-the receiver sensi-
tivity imposed by phase noise is dis-
cussed. Phase noise measurement
systems were designed and used to
characterize the key receiver compo-
nents. A limiting amplifier in the IF
section of the receiver eliminates the
need for knowledge of the reflection
coefficient of the object of interest for
quantitative vibration measurements.
A special exposure cell is described
which is a section of X-band waveguide
which has been modified by the addition
of a center conductor to form a rec-
tangular transmission line. The center
conductor with a sample placed under
the waveguide broad wall is excited by a
low-frequency or low-frequency ampli-
tude modulated signal. In addition an
X-band signal is fed through the wave-
guide, scattered by the sample and
detected by the phase-sensitive re-
ceiver.
The fields and the resulting forces on
certain standard shaped samples are
derived. With the measurement of the
Young's modulus and Poisson's ratio of
the samples, the resulting vibration
amplitude of the field-induced vibration
is estimated. Actual vibration measure-
ments verified with a piezoelectric
crystal indicate a vibration sensitivity of
about 1 nanometer peak-to-peak. Vibra-
tions in this range were predicted to
occur in certain calcium-ion efflux
studies. This system therefore promises
to help unravel some of the uncertainties
surrounding the interaction of RF fields
with biological systems.
This Project Summary was developed
by EPA's Health Effects Research
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering in-
formation at back).
Conclusions
The characteristics of an RF field scat-
tered from a linearly vibrating object were
derived. The scattered signal was shown
to be phase modulated with, for sinusoidal
vibration, the phase modulation index
proportional to the amplitude of the vibra-
tion. The spectral characteristics of this
signal were derived and it was shown
that for a small modulation index, only
one pair of sidebands is significant. When
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the ratio of one sideband to the carrier
was formed, this suggested a method of
extracting the vibration amplitude infor-
mation without a knowledge of the re-
flection coefficient of the vibrating object.
The sideband to carrier ratio was shown
to be dependent only on the vibration
amplitude and the wavelength of the
incident field.
Several receivers were considered for
the recovery of the vibration information
from the received signal. The Doppler
transceiver offered the advantages of
simplicity and good sensitivity; however,
the recovery of the vibration amplitude
would require a knowledge of the reflec-
tion coefficient of the target. In addition,
the receiver would have to be a certain
distance from the target to establish
phase quadrature between the transmit-
ter reference signal and the received
signal. The homodyne receiver incorpo-
rated a manual phase shifter which elim-
inates the special target-receiver spacing
requirement; however, knowledge of the
reflection coefficient is still required for
quantitative vibration measurements.
The sideband heterodyne receiver with
an IF limiting amplifier was selected for
the final receiver design because this
design permitted quantitative vibration
measurements. It was initially thought
that this receiver would also provide
enhanced sensitivity over the Doppler
and homodyne receivers because of the
expected improvement in noise figure.
However the sensitivity was not improved.
Noise figure was not found to be a good
measure of receiver sensitivity when the
carrier to sideband ratio is large and
when the sideband is close to the carrier.
When the sensitivity is limited by flicker
noise, as it is in this receiver, the actual
sensitivity can be considerably less than
predicted by the noise figure.
An analysis of the factors limiting
sensitivity was performed. Phase noise
theory was reviewed as well as modern
techniques for its measurement. The
quadrature phase detection method for
high sensitivity phase noise measure-
ments was described. A system for mak-
ing quantitative phase noise measure-
ments in terms of the single-sideband
phase noise (L(f)) was assembled. The
ultimate sensitivity of the quadrature
phase detection method is limited by the
flicker noise of the phase detector diodes.
The sensitivity for the measuring systems
used were presented graphically. Key
receiver components such as an RF
amplifier, the limiting amplifier, the low
noise IF amplifier, the RF mixer and the
phase detector were all characterized in
terms of their residual single-sideband
phase noise. The RF amplifier produced
an excessive level of residual phase noise.
The RF amplifier was therefore not used
in the final receiver design because
receiver sensitivity would have been
degraded severely. The low noise IF
amplifier had negligible residual phase
noise compared to the limiting amplifier.
The phase detector RF drive level was
found to be important for minimizing
residual phase noise. For a fixed LO level
the absolute level of the noise at the
output at baseband frequencies increased
with increasing RF drive level. However
the phase detector constant increases
with an increasing RF drive level. When
the residual phase noise was measured,
which is computed from the ratio of the
absolute noise level to the phase detector
constant, the lowest noise performance
was obtained with an RF drive level
between 0.6 and 5.0 mW for a 5.5 mW LO
drive level.
All oscillators exhibit phase noise and
furthermore the oscillators used in this
receiver have greater SSB phase noise
than most of the other active components.
However, because of the correlation effect
and the short transit time experienced by
a signal travelling from the transmitter to
the target and other reflecting objects in
the waveguide and back to receiver, the
apparent oscillator phase noise is reduced
substantially below the level of the phase
noise of the other active components in
the receiver. Therefore, the oscillators do
not limit the sensitivity of the receiver in
this application. Examination of the single
sideband phase noise data revealed that
themicrowavemixer(M2)andthe limiting
amplifier were the components that lim-
ited the sensitivity of the receiver.
A unique exposure cell was designed to
simultaneously expose samples to an ELF
or an ELF-modulated RF field while
providing for the non-contact vibration
monitoring of the sample with an X-band
signal. The cell has a return loss of
greater than 20 dB up to 1 GHz at the
exposure field input port. Properties of
the exposure cell however, do place limi-
tations on the size of the sample that can
be accommodated. If the sample is larger
than about one-third of the center plate to
wall spacing, charge redistribution in-
creases the effective field to which the
sample is exposed. The effective field
then becomes a strong function of the
sample size and the gap between the
object and the center plate and this tends
to add large uncertainties to the calcula-
tions. If the sample is too small, on the
other hand, the return loss of the sample
begins to approach the return loss of the
empty exposure cell. When this happens,
quantitative measurements become im-
possible because of an unknown amount
of carrier cancellation or augmentation of
the signal scattered from the vibrating
sample.
Starting with the Lorentz force law, the
force per unit area at a dielectric interface
due to electrostrictive forces was derived
for the cases of the electric fields normal
and tangent to the dielectric interfaces. It
was shown that for both the electric fields
normal or tangent to the dielectric inter-
face, the force at the interface is always
directed from the region with the higher
dielectric constant to the region with the
lower dielectric constant and does not
depend on the sign of the electric field.
The electric field normal to the surface of
a high dielectric constant or conducting
prolate spheroid was calculated. From
this expression the vertical force on a
half-prolate spheroid with its base on a
ground plane was derived. Agar gel was
used as a mechanical model for tissue.
The Young's modulus of agar gel was
measured as a function of concentration
and the modulus was found to increase
linearly with the concentration of agar in
deionized water. Poisson's ratio was also
experimentally determined for a 0.6% c of
agar gel. With these values, estimates of
the expected vibration amplitude and the
expected receiver signals were computed
for three sizes of half-prolate spheroid
samples exposed to an ELF electric field.
Measured values of vibration amplitude
for the half-prolate spheroid samples in a
sub-resonant ELF field agreed most
closely with the calculated values for the
3.5 mm base sample size. Samples of
other sizes differed from the calculated
values for the reasons stated above. The
measured vibration frequency was always
at twice the frequency of the exposure
field as predicted by the theory.
The major resonance frequency of both
the half-prolate spheroid and cubical
samples were measured. Simple models
including the simple harmonic oscillator
model and others derived for the trans-
verse or longitudinal vibration of continu-
ous systems of standard shapes were
applied. The resonance frequencies ob-
served for the cubical-shaped samples
agreed most closely to the calculated
resonance frequencies for the transverse
vibration of a prismatic bar. For the half-
prolate spheroid samples it was impos-
sible to conclude with certainty from the
simple models employed whether the
vibration at the major resonance fre-
quency was longitudinal or transverse.
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A receiver with an exposure cell has
been designed and characterized that is
capable of vibration measurement in the
nanometer range. Vibrations in this range
were predicted to occur in certain calcium
ion efflux studies which demonstrated a
biological effect. This system therefore
promises to help unravel some of the
uncertainty surrounding the interaction
of RF fields with biological systems.
The full report covers a period from
October 1, 1982 to April 15, 1985 and
work was completed as of April 15,1985.
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The EPA authors, Joseph S. AH (also the EPA Project Officer, see belowl and
William T. Joines, are with Health Effects Research Laboratory. Research
Triangle Park. NC 27711.
The complete report, entitled "A System for Measurement of Small Vibrations at
Material Interfaces Induced by Electrostrictive Forces," (Order No. PB 86-116
530/AS; Cost: $16.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Health Effects Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use S300
EPA/600/S1-85/021
0000329 PS
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