FI68129C - FOERFARANDE FOER TIDDELAD KANALISERING AV FLERA RF-SQUID DETEKTORER - Google Patents

Description

1 68129 METHOD FOR CONTROLLING MULTIPLE RF SQUID SENSORS IN TIME-TIMER

AATTEELLA

The present invention relates to a method for simultaneously monitoring a plurality of superconducting quantum interferometers or rf-SQUIDs pumped with radio frequency current on a time division principle, in which method a rf voltage acting across the tank circuit of each SQUID is connected to a synchronous voltage across each tank circuit. a modulation signal for a common rf amplifier, and wherein all SQUIDs are kept in operation using continuous feedback.

10 Biomagnetic research measures the weak magnetic fields generated by the electrical action of different parts of the body (eg heart, brain, skeletal muscles). Normally, a very sensitive magnetic flux and electric current detector, or SQUID, is used for the measurement. Such a SQUID magnetometer is usually a single-channel apparatus, in which case three separate measurements are required to determine the magnetic field vector. Currently, the growing interest in biomagnetism has created the need to develop multichannel magnetometers suitable for measurement, allowing the three components of a magnetic field vector to be measured simultaneously and thus seeing changes in vector direction and magnitude in real time. Of course, such measuring devices can also be used for other magnetic field research, such as geomagnetic measurements. To date, such multi-channel SQUID magnetometers have been constructed with two different principles: 1) A multi-channel system is assembled from several single-channel SQUID magnetometers, with each rf-SQUID having its own continuous control unit. The measuring coils, with which the external magnetic flux to be measured is connected to the signal coil of the SQUID, are normally wound on a common coil body, or the flux to be measured is connected directly to the SQUID sensor. Each control unit includes a radio frequency oscillator used to pump the SQUID. The oscillators all operate at different frequencies. Ehnholm, G.J., Ilmoniemi, R.J. and Wiik, T.O., A seven channel 2 68129 SQUID magnetometer for brain research. Physica 107 B, 1981, pp. 29-30. and

Varpula, T., Karp, P., Katila, T., Poutanen, T. and Seppänen, M., A three-channel superconducting vector magnetometer. Proc.

5 XVI Annual Conference of the Finnish Physical Society, Espoo, Finland, Feb 1982, pp. 8:23.

2) Another principle that has emerged is to use a specially made SQUID sensor. The sensor consists of three rf-SQUIDs made using thin film technology on the same substrate 10 so that they can be monitored using one tank circuit and thus also one rf amplifier. The pumping of the SQUID is also done with the same radio frequency signal. As modulating signals, different frequency sinusoidal audio signals have been used, which are inductively connected together with the back-15 connection to SQUID. Each channel is indicated by a phase detector tuned to the modulation frequency. Shirae, K., Furukawa, H. and Kishida, K., A new multielement rf SQUID system and its application to the magnetic vector gradiometer. Cryogenics 1981, pp. 707-710.

20 The solutions presented above have the following disadvantages: 1) When using several separate control units, the number of components is large and the handling of the equipment is cumbersome.

2) When using RF pumps of different frequencies, interference interference occurs when the rf signal is connected, either directly or via 25 measuring coils, to the SQUID tank circuit of adjacent channels. Interference is difficult to eliminate.

3) The construction of a custom-made SQUID requires mastery of the thin-film technique and associated tuning technique. Tuning is necessary because in this case all SQUIDs 30 must have identical properties.

The method according to the present invention provides a decisive improvement over the above drawbacks. To achieve this, the method according to the invention is characterized by what is stated in the characterizing parts of the claim. The main advantages of the method according to the invention are:

Conventional, commercially available 3 68129 rf-SQUIDs can be used. Only one rf-SQUID is connected to the rf amplifier at a time, while the others retain their operating state due to continuous feedback. Because common rf and audio control frequencies are used for all rf-SQUIDs, all 5 mutual interference disturbances are avoided. In addition, the number of components required by the method is clearly smaller than when using separate units.

The invention will now be described in detail with reference to the accompanying drawings, in which Figure 1 shows a block diagram of a conventional continuous control method, and Figure 2 shows a block diagram of an apparatus implementing the method according to the present invention.

Figure 1 shows the so-called commonly used control of rf-SQUID. the soap loop works as follows:

The radio frequency current required to pump the Rf-SQUID is obtained from the rf oscillator 1. The rf voltage generated by the current over the tank circuit is amplified by a low noise rf amplifier 2. A rectangular waveform modulation flux with a frequency of a few tens of kilohertz to a few hundred kilohertz φ / 2 (φ -15 oo is a flux quantum, 2.0678 * 10 Wb), is connected from the audio oscillator 3 to SQUID 4. This signal frequency is also connected to the phase detector 5 as a reference. When the external magnetic flux 25 is connected to the SQUID 4 via the measuring coil 6, its operating point changes and the rf voltage across the tank circuit is modulated by a rectangular wave. The phase of modulation depends on the direction of flux change. The amplitude modulated rf signal is connected to an am detector 7 and a phase-sensitive amplifier 5, 30, the output of which has a DC voltage, the value of which depends on the degree of modulation of the signal. The voltage is integrated in the integrator 8 and reconnected as current to the tank circuit of the SQUID. This feedback current compensates for externally coupled magnetic flux and drives the SQUID back to the original 35 operating points. The voltage generated at the feedback resistor 9 is proportional to the external flux change and is connected to the output via a low-pass filter 10. The coupling thus linearizes the response of the SQUID.

The operation of the time division multiplexed sooted loop shown in Figure 2 is as follows: 5 Each of the three measuring coils 11 wound on a common coil body is connected to one of the three rf-SQUIDs 12. This so-called the vector radiometry sensor 11 and rf-SQUIDs 12 are in the measuring devar in liquid helium. On each channel, the rf-SQUID tank circuit is connected to the input of the rf-10 preamplifier 13. The preamplifiers are housed in an rf unit attached to the deck cover. The oscillator 14 required to generate the radio frequency pump current and the audio frequency modulation signal oscillator 15 are common to all rf-SQUIDs. The Rf pump current, the modulation current 15, and the feedback current from the feedback full unit 16 are conventionally connected to the rf-SQUID tank circuit on each channel separately. The rf voltage across the tank circuit of the Rf-SQUID is amplified in the preamplifier and then alternately connected to the main amplifier 20 17 using high speed electronic rf switches 18. The switches are controlled by the time division multiplexing control unit 19 so that the switching frequency is one third of the frequency of the modulation signal. Thus, each channel is connected to the main amplifier for one period of the modulation signal 25. Thus, the time division multiplexed rf signal is amplified in the main amplifier 17, detected in the am detector 20, and the resulting audio frequency signal is coupled to a phase detector 21, the reference signal of which is the output signal of the modulation oscillator 15. The output of the phase detector 21 thus has a voltage 30 which contains successive voltage levels proportional to the magnetic flux connected to the SQUID of each channel. The channels are then separated using sampling and holding circuits 22. The sampling circuit switches the output of the phase detector to the integrator 35 23 of the channel in question for the period of the modulation signal. Thus, while the channel is on, the flow-locked loop operates as in the continuous method. When the output of the phase detector is capable of the next channel, the integrator of the previous channel integrates 5 68129 the constant voltage at the output of the holding circuit of this channel, which corresponds to the value of the signal at the end of the sample. The flow-locked loop closes through the feedback circuit 16, which keeps the integrator output continuously connected to the SQUID tank circuit and prevents the lock from opening, which would likely cause a new lock at a different level. If the frequency of the modulation signal is selected to be large enough so that the sampling frequency is at least twice the bandwidth of the soaped loop, the folding phenomenon is prevented and the constant voltage present in the holding circuit output between the samples approximates the signal well. The sampling frequency also determines the maximum signal frequency to be measured. For example, if the sampling frequency is 10 kHz, magnetic fields with a frequency of less than 5 kHz can be measured simultaneously on all channels with a magnetometer. The voltage from the integrator 23 is directly proportional to the magnetic flux coupled to the SQUIDs, and the output signal of each channel is obtained after the low-pass filter 24. When the output voltage of the integrator exceeds the range ± 5 V, the reset circuit 20 resets the integrator 23, the reset circuit includes a voltage comparator which is connected to the output of the currently switched channel.

Claims (2)

6 68129 A method for simultaneously controlling a plurality of superconducting quantum interferometer, i.e. rf-SQUID, pumped with radio frequency current by the time division principle, which method uses conventional rf-SQUIDs in construction, wherein one SQUID at a time is connected to a sooted loop in a state known in continuous operation. -The rf voltage across the tank circuit of the SQUID (12) is alternately connected to a common rf amplifier (17) using electronic rf switches (18) whose switching frequency is synchronized to the modulation frequency connected to the rf-SQUID, and that all rf-SQUIDs use the same rf pump frequency and the same modulation frequency. A method according to claim 1, characterized in that in each channel the output of the integrator (23) is continuously enabled via the feedback circuit (16) to the tank circuit of the rf-SQUID (12).