Microwave Bridge that is insensitive to phase and frequency
noise
1T. Walczak, 2W. Froncisz and 1H.M. Swartz
1EPR Center for the Study of Viable Systems, Dartmouth Medical School,
Hanover, NH 03755 – USA
2 Jagiellonian University, Krakow, Poland
INTRODUCTION: The main factor limiting the sensitivity of in vivo EPR
is noise caused by the physiological
movements of the animal (breathing, heart beating and voluntary movements),
that results in
mismatching and detuning of the resonator. Usually automatic matching and
tuning systems are
employed to reduce the adverse effects of the physiological noise. The
usefulness of these automatic
systems for some rapid motions such as heart beats is limited due to the
relatively long response
time. Therefore we have investigated various bridge configurations and have
found a unique
configuration and setting described below, that makes the system almost
insensitive to frequency
and phase noise.
METHODS: EPR microwave bridges have two arms: main and reference. The
microwave power in the
main arm is reflected from the resonator and then combined with the
microwave power from the
reference arm after adjustment of its phase and amplitude.
The voltage of the microwave power reflected from the resonator and combined
with the
voltage coming from the reference arm can be represented by vectors placed
inside a circle in a
complex plane. The position of the circle center depends on the phase and
attenuation setting of the
reference arm. Fig.
1 shows the plot for VINCIDENT=VREFRENCE
Figure 1 Microwave voltage at the diode for VINCIDENT=VREFRENCE
represented by the vector
in the complex plane (left plot) at the resonant,
w0,
and lower,
w0-Dw,
frequency. |
Right plot represents the absolute value of the microwave voltage as a
function of frequency.
In this case the center of the circle is shifted up along the real part axis
of the complex plane. It
can be seen that the length of the voltage vector depends on the microwave
frequency as is plotted
in the right side of Fig.1. We are proposing a unique setting of the
reference arm that causes the
independence of the diode voltage on the frequency. This can be achieved
when the incident
microwaves are in the opposite phase to the reference microwaves and their
amplitude is twice as
high as the amplitude of the reference microwaves. On the “circular
diagram”, the center of the
circle is located in the origin of the coordination system. Thus, the length
of the voltage vector is
independent on the microwave frequency
as can be seen in Fig. 2.
Figure 2. Microwave voltage at the diode for VREFRENCE= VINCIDENT/2
represented by the vector
in the complex plane (left plot) at the resonant,
w0,
and lower, w0-Dw,
frequency.
Right plot represents the absolute value of the microwave voltage as a function
of frequency.
RESULTS: The experimental microwave bridge, based on the principle
described above, was constructed in
our laboratory. Test measurements performed with this device have shown
significant reduction
in three kinds of noise:
1. Microphonic noise, caused by the field modulation, was reduced more than
twice.
2. Noise, due to respiration, was reduced significantly (Fig. 3)
3. The phase noise of the microwave source was much less visible, due to the
flat frequency characteristics of the system.
Figure 3 Suppression of noise from
respiration by the reverse phase detection system.
A piece of charcoal was implanted subcutaneously. Top spectrum is with normal
detection configuraton;
the
spectrum obtained with the reverse phase detection system is at the bottom.
DISCUSSION: The proposed detection system will be particularly useful
in the studies involving paramagnetic
materials with narrow and easily saturable lines, like LiPc. Under some
conditions this system
can work without an AFC. The described system compensates for the effects of
the resonator
detuning which is the main source of noise and lineshape distortion,
although it does not reduce
the effects of resonator mismatching.