Note: Descriptions are shown in the official language in which they were submitted.
Introduction and General Discussion
The present invention relates to an electronic circuit
for determining the external magnetic field measured by a
super-conducting device known as a SQUID.
Known electronic circuits bias the SQUID by applying
r.E. power to the SQUID via a tuned circuit located in
very close proximity thereto. Changes in the current
flowing in the tune circuit are detected and these changes
are related to the magnetic field being measured.
A SQUID iS a super-conducting device which can be made
to measure magnetic field strength. The SQUID device is
generally in the form of a loop, with the material of the
loop having some predetermined cross sectional area. A
"weak link" is located at some point in the material
forming the loop. When a SQUID is placed in a magnetic
field, a current flows in the loop which is proportional
to the intensity of the field within certain bounds. As
the field strength increases the current flowing in the
loop increases until it reaches a critical level Ic. At
this point, the SQUID undergoes a flux transition, whereby
the current undergoes a quantum drop and then begins to
rise until it reaches Ic once again. At this point,
another flux transition takes place. As a result, if a
SQUID is calibrated at some external magnetic field
strength level, say ~cal, the exact magnetic field
strength of an unknown field can be determined by
counting, either up or down~ the number of times the
critical current Ic was reached, thereby determining the
number of quantum flux transitions the SQUID has undergone
and then by adding to that the instantaneous value of the
current circulating in the SQUID which, is proportional to
the flux in the SQUID at that instant in time.
The SQUID and its associated electronics is meant to
be used as an airborne magnetometer and as a result, it
must be capable of measuring rapid changes in the magnetic
field. This phenomenon is known as slew and it is an
object of the present invention to increase the detectable
slew rate of the system.
With the prior art circuit, the excitation or bias
signal, at a ~requency of about 30 mHz, is pumped to the
SQUID with a bandwidth of about 300 kHz. The r.f. voltage
on the tank circuit must be of sufficient amplitude to
cause flux transitions in the SQUID which in turn causes
the extraction of energy from the tank circuit. The
variation of the external magnetic field being measured
modulates the extraction of energy from the tank circuit
and thus causes small variations in the tank circuit
voltage. In order that these small variations on a large
r.f. tank voltage can be detected, the external field is
modulated by a 100 kHz "audio" signal. As a result, the
slew rage must fit within the response curve of the tank
circuit and not exceed the "audio" frequency of 100 kHz.
Since frequency response in the tank circuit is nonlinear~
the frequency of the magnetic field strength must be
fairly low with respect to the audio fre~uency modulating
the r.f. signal. Finally, the prior art system uses a
flux-locked loop which must have a time constant which is
sufficiently long to allow for an averaging of many audio
signal time periods. These constraints limit the useful
magnetic signal frequency and therefore the slew rate to a
range much less than 100 kHz~ for example, about 3 kHz.
Summary of the Invention
In the system according to the present invention~ two
radio frequency signals are generated, summed and fed to a
pumping coil located in close proximity to the SQUID. The
SQUID is an inherently nonlinear device and therefore,
when it exhibits a current change due to a change in the
magnetic field, energy at many frequencies is radiated.
As a result, a tuned LC circuit, tuned to the difference
frequency o~ the two generators is placed in close
proximity to the SQUID and has a current induced therein
which is linearly proportional to changes in the external
magnetic field being measured. If the external magnetic
field does not change rapidly, the difference frequency
detected by the tuned LC circuit is modulated between 0
and some maximum value. As a result, it can be seen that
there is no need to modulate the pumping frequency with a
constant "audio" signal.
In the known system, energy which is proportional to
the external magnetic ~ield is extracted from a "full"
tank circuit and the tank voltage is only slightly altered
by changes in the external magnetic field being measured.
In the system according to the present invention the
energy is picked up by an l'empty" tuned circuit and the
voltage varies between 0 and some maximum value depending
on the amplitude of the applied field.
The slew rate is limited only by the bandwidth of the
tuned LC circuit and as a result, is improved
considerably. The signal is distributed symmetrically
about the difference frequencyO The limitation on the
slew rate imposed by the averaging procedure in the
flux-locked loop is removed because an audio frequency of
100 k~z has been replaced by an r.f. difference
frequency. Therefore, many periods of the difference
frequency may be averaged even if the time constant of the
flux-locked loop is relatively short.
Since the new system does not detect a small signal
superimposed on a large signal it can be seen that the new
system has an improved signal-to-noise ratio.
In accordance with one aspect of the present invention
there is provided an electronic circuit for determining
magnetic field strength using a SQUID, said circuit
comprising: first and second radio frequency generators
producing first and second signals having frequencies fl
and f2 respectively; first adding means to produce a SQUID
bias signal having a frequency fl + f2; a pumping coil
placed in close proximity to said SQUID, said coil being
connected to said adding means; a tuned circuit placed in
close proximity to said SQUID and tuned to the difference
frequency fl - f2, said tuned circuit having a measurement
signal induced therein; and a detector means connected to
said tuned circuit, said detector means counting flux
transitions in said SQUID due to changes in magnetic field
strength and an analog signal for changes in field
strength less than a predetermined level which ~ould cause
a flux transition.
In accordance with another aspect of the present
invention there is provided a method of measuring the
strength of an external magnetic field using a SQUID
comprising the steps of: generating a first signal having
a frequency fl; generating a second signal having a
frequency f2; adding said first and second signals to
provide a SQUID bias signal having a frequency fl + f2;
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8 ~ ~
deriving a measurement signal in a t-tned circuit placed in
close proximit~ to the SQUID, said measurement frequency
being symmetrical about a difference frequency fl - f2;
detecting said measurement frequency to produce a signal
representative of the number of flux transitions of said
SQUID and an analog signal representative of changes in
the magnetic field strength which are less than a
predetermined level which would cause a flux transition.
In the Drawings
In drawings which illustrate embodiments of the
invention:
Figure l is a block diagram of a "prior art" system;
Figure 2 is a frequency spectrum diagram illustrating
the useful signal range of the "prior art" system of
Figure l;
Figure 3 is a block diagram o~ a system according to
the present invention; and
Figure 4 (appearing on the same sheet of drawings as
Figure l) is a frequency spectrum diagram illustrating the
useful signal range of the device according to Figure 3.
Detailed Description
Figure 1 illustrates the "prior art" circuit. The
$QUID is illustrated schematically at ln and is sensitive
to external field ~ex. R. L . generator 14 produces an r.f.
signal of sufficient strength to force the SQUID lO to
undergo flux transitions. The frequency of the generator
14 is not critical, however, tank circuit 12 must be tuned
to the frequency of the generator 14 in order to ensure a
maximum transfer of energy. For the sake of illustration,
the frequency of the generator 14 will be considered to be
30 mHz,
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The r.f. signal is amplitude modulated by an "audio"
_ signal frequency of 100 kHz produced by audio oscillator
16. The amount o~ energy in the tank circuit 12 changes
with changes in the magnetic field being measured by SQUID
10. These changes in energy are detected as changes in
voltage by detector circuit 18. A flux-locked loop is
maintained by feedback path 19. The detector circuit
provides an output count for flux transitions where the
external field changes by amounts greater than ~p, the
field which produces a critical current Ic in the SQUID.
Figure 2 shows the range of useful signal for the
circuit of Figure 1. The bandwidth of the tank circuit is
approximately 300 kHz. Since the signal of interest is
superimposed on a 100 kHz "audio" signal it can be seen
that the useful signal range is necessarily narrowed in
order to be substantially linear.
Figure 3 shows a particular system according to the
present invention. Oscillator 20 produces a first
frequency fl and oscillator 22 produces a second frequency
f2. Attenuators 24 provide the correct level o outputs
fl and f2 to summing circuit 26. The summed frequency
fl + f2 is fed to pumping coil 28 arranged in very close
proximity to SQUID 10. The outputs fl and f2 are also fed
to a mixing circuit 30 to produce a difference frequency
fl - ~2. The difference frequency is fed through an
appropriate bandpass filter 32. A local oscillator 3~ is
provided which produces a signal having a frequency f3
which, in turn, is mixed with the difference frequency at
mixer 36 to produce a frequency f4 which is equal to
(fl - f2) - f3. Frequency f4 is amplified by amplifier 38.
A tuned ~C circuit 40 is located in close proximity to
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SQUID 10~ The SQUID is an inherently nonlinear device and
so energy changes which are a result of changes in the
magnetic field to be measured are radiated by the SQUID
over a large Erequency spectrum, one frequency of which is
the diEference Erequency fl - f2 of the two oscillators 20
and 22~ As a result, tuned circuit 40 is tuned to the
difference frequency fl - f2. The signal having frequency
fl - f2 is then modulated by changes in the magnetic
field. This signal is amplified by buffer amplifier 41
and fed to mixer 42. Frequency f4 is mixed with the
modulated difference frequency from tuned circuit 40 and
an IF signal is produced which is fed to IF amplifier 44O
The frequency content of the output of amplifier 41 is
(fl - f2) + f(~ex), where f(~ex) represents the frequency
spectrum of the signal representing the magnetic field
strength being measured. The frequency content of the
output of mixer 42 is (fl - f2) + f(~ex) - f4 but, as
mentioned abovel f4 = fl - f2 - f3. Therefore, the
frequency content of the signal handled by the IF
amplifier 44 is merely f(~ex) + f3.
This signal is finally mixed in mixer ~6 to obtain
f(~ex). This signal has a frequency content ranging from
dc to some low frequency level, say 100 kHz. This is the
slew rate and is fed through low pass filter 48.
The amplitude of this signal is a measure of the field
strength and ranges from 0 to some maximum level equalling
a flux transition. Detector 50 produces an up count or a
down count of flux transitions and an analog signal level
for th~t portion of the signal ~ex < + ~p~ where as
mentioned before, ~p is the field strength necessary to
cause a flux transition. A portion of the detected signal
is fed back to the output tuned circuit 40 via summer 52
to provide a flux-locked loop.
Figure 4 shows the usable frequency range of the
signal representing measured field strength. It can be
seen tha~ the limitation is really only the bandwidth
restriction of the tuned circuit and that the usable
frequency range and therefore the slew rate is in the
neighbourhood of 100 kHz.
In the example shown in Figure 3, oscillators 20 and
22 can produce a frequencies of 80 and 50 mHz,
respectively, so that the difference frequency fl - f2 is
in the neighbourhood of 30 mHz. The frequency of the
oscillator 34 can be 12 mHz. It should be understood that
the present invention is not limited to these frequencies.
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