CA1181812A - Dual r.f. biased squid electronic circuit - Google Patents
Dual r.f. biased squid electronic circuitInfo
- Publication number
- CA1181812A CA1181812A CA000312435A CA312435A CA1181812A CA 1181812 A CA1181812 A CA 1181812A CA 000312435 A CA000312435 A CA 000312435A CA 312435 A CA312435 A CA 312435A CA 1181812 A CA1181812 A CA 1181812A
- Authority
- CA
- Canada
- Prior art keywords
- frequency
- signal
- squid
- circuit
- detector
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
- G01R33/035—Measuring direction or magnitude of magnetic fields or magnetic flux using superconductive devices
- G01R33/0354—SQUIDS
Abstract
DUAL R. F. BIASED SQUID ELECTRONIC CIRCUIT
Abstract of the Disclosure The present invention relates to an electronic circuit for determining magnetic field strength using a SQUID.
The circuit is comprised of a first and a second radio frequency generator producing signals having frequencies f1 and f2 respectively. An adder is provided to produce a SQUID bias signal having a frequency f1 - f2. A pumping coil is placed in close proximity to the SQUID and is connected to the adder. A tuned circuit is placed in close proximity to the SQUID and is tuned to the difference frequency f1 - f2. A detector is provided, connected to the tuned circuit. The detector counts flux transistions in the SQUID due to changes in magnetic field strength. In addition, the detector produces an analog signal which is proportional to changes in the field strength which are less than a predetermined level which would cause a flux transition.
Abstract of the Disclosure The present invention relates to an electronic circuit for determining magnetic field strength using a SQUID.
The circuit is comprised of a first and a second radio frequency generator producing signals having frequencies f1 and f2 respectively. An adder is provided to produce a SQUID bias signal having a frequency f1 - f2. A pumping coil is placed in close proximity to the SQUID and is connected to the adder. A tuned circuit is placed in close proximity to the SQUID and is tuned to the difference frequency f1 - f2. A detector is provided, connected to the tuned circuit. The detector counts flux transistions in the SQUID due to changes in magnetic field strength. In addition, the detector produces an analog signal which is proportional to changes in the field strength which are less than a predetermined level which would cause a flux transition.
Description
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;
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, 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 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.
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;
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, 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 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.
Claims (11)
1. 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 f1 and f2 respectively, first adding means to produce a SQUID bias signal having a frequency f1 + 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 f1 - 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 would cause a flux transition.
first and second radio frequency generators producing first and second signals having frequencies f1 and f2 respectively, first adding means to produce a SQUID bias signal having a frequency f1 + 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 f1 - 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 would cause a flux transition.
2. The circuit according to claim 1 wherein a portion of the signal detected by said detector means is fed to the input of said detector via a second adding means located between said tuned circuit and said detector to thereby form a flux-locked loop.
3. The circuit according to claim 2 wherein a first attenuator is inserted between said first r.f. generator and said first adding means and a second attenuator is inserted between said second r.f. generator and said first adding means, so that the amplitude of the first and second signals can be controlled prior to being added in said first adding means.
4. The circuit according to claim 3 further including a first mixer which produces a difference frequency f1 - f2; a local oscillator producing a third signal having a frequency f3; a second mixer mixing said difference frequency and said third signal to produce a fourth signal having a frequency f4; and a third mixer connected between said second adding means and said detector, said third mixer producing an IF frequency by mixing said measurement signal and said fourth signal.
5. The circuit according to claim 4 including an IF
amplifier connected between said third mixer and said detector.
amplifier connected between said third mixer and said detector.
6. The circuit according to claim 5 further including a fourth mixer connected between said IF amplifier and said detector, said fourth mixer mixing said IF frequency and said third signal to produce a signal which is directly proportional to said magnetic field strength.
7. The circuit according to claim 6 wherein a bandpass filter is connected between said first and second mixer, said filter having a center frequency tuned at f1 - f2.
8. The circuit according to claim 7 wherein a lowpass filter is connected between said fourth mixer and said detector.
9. The circuit according to claim 8 wherein a buffer amplifier is connected between said second adding means and said third mixer.
10. The circuit according to claim 8 wherein the frequency of the first r.f. generator is 80 mHz, the frequency of said second r.f. generator is 50 mHz, the frequency of the local oscillator is 12 mHz and the bandwidth of the tuned circuit is 300 kHz.
11. 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 f1;
generating a second signal having a frequency f2;
adding said first and second signals to provide a SQUID bias signal having a frequency f1 + f2;
deriving a measurement signal in a tuned circuit placed in close proximity to the SQUID, said measurement frequency being symmetrical about a difference frequency f1 - 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.
generating a first signal having a frequency f1;
generating a second signal having a frequency f2;
adding said first and second signals to provide a SQUID bias signal having a frequency f1 + f2;
deriving a measurement signal in a tuned circuit placed in close proximity to the SQUID, said measurement frequency being symmetrical about a difference frequency f1 - 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.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000312435A CA1181812A (en) | 1978-10-02 | 1978-10-02 | Dual r.f. biased squid electronic circuit |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000312435A CA1181812A (en) | 1978-10-02 | 1978-10-02 | Dual r.f. biased squid electronic circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1181812A true CA1181812A (en) | 1985-01-29 |
Family
ID=4112489
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000312435A Expired CA1181812A (en) | 1978-10-02 | 1978-10-02 | Dual r.f. biased squid electronic circuit |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1181812A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002097462A1 (en) * | 2001-06-01 | 2002-12-05 | Commonwealth Scientific And Industrial Research Organisation | Method and apparatus for magnetic field measurement |
-
1978
- 1978-10-02 CA CA000312435A patent/CA1181812A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002097462A1 (en) * | 2001-06-01 | 2002-12-05 | Commonwealth Scientific And Industrial Research Organisation | Method and apparatus for magnetic field measurement |
| GB2392733A (en) * | 2001-06-01 | 2004-03-10 | Commw Scient Ind Res Org | Method and apparatus for magnetic field measurement |
| GB2392733B (en) * | 2001-06-01 | 2005-02-02 | Commw Scient Ind Res Org | Method and apparatus for magnetic field measurement |
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