New Microwave Bridge Design for Reducing Microphonics in In
Vivo L-band EPR Spectroscopy
1Walczak, T., 1Lesniewski, P., and 1Swartz, H.M.
1EPR Center for the Study of Viable Biological Systems, Dartmouth Medical
School,
Hanover, NH 03755 – USA
INTRODUCTION: Sensitivity of low frequency EPR in vivo is limited by
“microphonics”, an
incompletely understood instability that appears at higher microwave power
and higher amplitude of
modulation. It is more pronounced if the magnetic modulation exceeds
(0.3-1Gauss) and/or
microwave power is greater than 20-30mW. It is visible as a shift in the
baseline of recorded spectra
or unpredictable distortions of baseline. Therefore, the spectrum cannot be
separated adequately
from the noise. This phenomenon diminishes the ability to increase
sensitivity by increasing power
and modulation amplitude and compromised improving the spectrum by using
accumulation
procedure. We recognized that the amplitude and phase noise of the microwave
source and RF
preamplifier does not contribute to this kind of distortion. We have
established the mechanism of
this phenomenon; it is a result of the influence of the modulation coils on
the resonator. This
interaction results in microvibrations of the resonant structure due to Eddy
currents induced in the
metal parts. Due to this vibration the Q and the resonant frequency of the
resonator are modulated
with the same frequency used to detect the EPR signal.
METHOD: We investigated the effect of different bridge configurations
on the "microphonics". We
employed different compensation circuits: circulators, 180o hybrid, and
directional coupler, as the
“core” of our experimental bridge. We employed different signal detection
systems and AFC
techniques. Also, a theoretical approach was developed to estimate the
operating characteristics of
an electronically tunable resonator.
RESULTS: A theoretical analysis of the system, consisting of a
surface coil and microwave bridge,
was performed. We have found that the interfering signal is due to power
reflected from the
resonator; this is less if the resonator is perfectly tuned to the radio
frequency source. A conventional
bridge does not permit perfect tuning of the resonator, due to leakage from
the circulator. Leakage
from a circulator usually is compensated by microwave power reflected from
the resonator. The
resonator operating on the slope of the resonance curve acts as a converter
of mechanical vibrations
to the microwave voltage. When the mechanism of this phenomenon was
established, we considered
various solutions to diminish resonator detuning. This goal can be achieved
by replacing the
circulator with 180o hybrid with isolation of about 46db. Unfortunately,
this causes a two-fold
decrease in sensitivity. An alternative solution is to use an additional
compensation arm in the
circulator. This type of solution can be used without bridge readjustment
only in a narrow range of
frequencies. We have used this solution because the resonators we use are
tunable, which allows us
to operate at a fixed frequency. Based on this, an experimental microwave
bridge and new wholebody
and surface coil resonators were designed and tested. This experimental
setup included a home
made dielectric resonator-oscillator, a logarithmic detector-amplifier for
precise measurement of
reflected power from the resonator, a separate detection system for the AFC,
a new electronic circuit
for the AM system, and a signal detector based on two diodes and a 90o
hybrid. We have tested this
experimental setup and found that “microphonics” were significantly reduced
compared to our
existing systems.
REFERENCE: H. Hirata, T. Walczak and H.M. Swartz, “Characteristics of
an Electronically Tunable Surface-Coil-
Type Resonator for L-Band EPR Spectroscopy,” RSI 72:2839-2841 (2001).