JP2014153263A - Magnetic field detection probe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic field detection probe with a simple configuration, configured to prevent influence of an electric field and improve detection signal sensitivity based on a magnetic field.SOLUTION: In a magnetic field detection probe 1, impedance ZL of a terminal resistor 6 is made coincident with impedance of a transmission line (a signal conductor 4, and a constant-potential conductor 5) constituting a sensor unit 2, while the impedances are set to be smaller than input impedance (characteristic impedance of a connection unit 3) of a detection circuit. In the magnetic field detection probe 1, a division ratio of the detection circuit is made larger than a case where the impedance of the transmission line or the impedance ZL of the terminal resistor 6 is made coincident with the input impedance Zs of the detection circuit, thereby improving detection sensitivity.

Description

  The present invention relates to a magnetic field detection probe using a shielded loop antenna.

  Conventionally, a magnetic field detection probe using a shielded loop antenna is known. In the shielded loop antenna, a coaxial cable or a semi-rigid cable is formed in a loop shape, and one end of the cable is connected to a detection circuit. The other end of the cable includes an unbalanced output type in which the internal conductor is directly grounded (connected to the external conductor) and a balanced output type in which the internal conductor is grounded (connected to the external conductor) via a termination resistor. In either case, a gap using an external conductor is formed in the loop, and an induced voltage generated in the gap by a magnetic field linking the loop is detected by a detection circuit.

  By the way, in the unbalanced output type, there is a problem that the voltage generated by the electric field is detected by the detection circuit. On the other hand, in the balanced output type, the detection sensitivity of the electric field can be suppressed. It is known that the voltage is divided by the input impedance of the detection circuit and the impedance of the termination resistor, and there is a problem that the detection voltage at the detection circuit is reduced to ½ (becomes −6 dB).

  On the other hand, two gaps are formed in the loop, and signals based on the induced voltage generated in both gaps are individually detected, and one signal is inverted by a phase shifter and added to the other signal. Thus, there is known a double gap type shielded loop antenna that detects only a magnetic field by canceling a signal (induced voltage) generated by an electric field (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 10-268209

  However, in the conventional technique described in Patent Document 1, the configuration of the magnetic field detection probe becomes complicated, such as providing two gaps in the loop portion and providing a phase shifter to extract the detection signal. There was a problem that.

  In order to solve the above-described problems, an object of the present invention is to provide a magnetic field detection probe that realizes suppression of the influence of an electric field and improvement of sensitivity of a detection signal based on a magnetic field with a simple configuration.

  The present invention is premised on a magnetic field detection probe configured using a balanced output type shielded loop antenna. The balanced output type shielded loop antenna is composed of a signal line conductor and a constant potential conductor partly formed in a loop shape by a transmission line having a gap part, and a transmission line constituting the sensor part. And a termination resistor connected to one end. In the present invention, the impedances of the transmission line and the termination resistor are set to match each other with a value smaller than the input impedance of the detection circuit connected to one end different from the connection end of the transmission line termination resistor. Yes.

  According to the magnetic field detection probe of the present invention configured as described above, the configuration is the same as that of a well-known balanced output type shielded loop antenna except that the impedances of the transmission line and the termination resistor are different. In addition, the influence of the electric field can be suppressed. In addition, according to the present invention, since the impedance of the termination resistor is smaller than the input impedance of the detection circuit, the detection voltage in the detection circuit is larger than ½ of the induced voltage, and the detection sensitivity can be improved.

It is a perspective view which shows the external appearance of a magnetic field detection probe. It is sectional drawing which showed typically the internal structure of the magnetic field detection probe. It is the III-III sectional view in FIG. It is a circuit diagram which shows the equivalent circuit of a balanced output type shielded loop antenna. It is explanatory drawing which shows the structure of a measurement system. It is explanatory drawing which shows the structure of an antenna about the shield dead loop used as a comparative example, (a) is an unbalanced output type, (b) is a balanced output type. It is a graph which shows the measurement result of magnetic field detection sensitivity. It is a graph which shows the measurement result of electric field detection sensitivity. It is a graph which shows the result of having measured the magnetic field detection sensitivity by changing the impedance of a transmission line and termination resistance.

Embodiments of the present invention will be described below with reference to the drawings.
<Overall configuration>
As shown in FIG. 1, the magnetic field detection probe 1 includes a sensor unit 2 that generates a signal corresponding to the magnitude of the magnetic field, and a connection unit 3 that extracts a signal from the sensor unit 2.

The connecting portion 3 includes a center conductor 31 and a cylindrical outer conductor 32 formed around the center conductor 31, and is formed of a known SAM connector having a characteristic impedance of 50Ω.
The sensor unit 2 is configured by using a flexible multilayer substrate formed in a loop shape, and includes a balanced output type shielded loop antenna formed by using a strip line printed on the multilayer substrate.

  As shown in FIG. 2, the multilayer substrate constituting the sensor unit 2 includes four dielectric layers 21 (211 to 214) and three conductor layers 22 (221) formed between the dielectric layers 21. 223). In the following, the dielectric layer located on the innermost side of the loop is the first layer 211, the dielectric layer located on the outermost side is the fourth layer 214, and the two dielectric layers located between them are the second layer 212 and It shall be called the third layer 213. The signal conductor 4 is formed in the signal conductor layer 222 which is a conductor layer located between the second layer 212 and the third layer 212, and is located between the first layer 211 and the second layer 212. The constant potential conductors 5 (51, 52) are formed on the inner conductor layer 221 that is the conductor layer to be conductive and the outer conductor layer 223 that is the conductor layer located between the third layer 213 and the fourth layer 214, respectively. ing.

  However, as shown in FIG. 3, the constant potential conductor 5 has a line width (in the Y-axis direction in the figure) so that the transmission line formed by the signal conductor 4 and the constant potential conductor 5 functions as a strip line. The width is formed to be sufficiently wide compared to the line width of the signal conductor 4. That is, assuming that the width of the signal conductor 4 is W, the thickness of the signal conductor 4 is L, the interval between the constant potential conductors 51 and 52 is H, and the relative dielectric constant of the dielectric layer 21 is ε, these parameters are By setting appropriately, the impedance of the transmission line (strip line) composed of the signal conductor 4 and the constant potential conductor 5 can be set to an arbitrary magnitude.

  Returning to FIG. 2, the signal conductor 4 is formed so as to make one round of the loop along the shape of the multilayer substrate. One end of the signal conductor 4 is connected to the central conductor 31 constituting the connecting portion 3, and the other end is terminated to the constant potential conductor 52 formed in the outer conductor layer 223 in the vicinity of the connecting portion 3. The resistor 6 is connected.

  The constant potential conductors 51 and 52 are wired so as to form a loop along the shape of the multilayer substrate, and are opposed to the formation portion of the connection portion 3 with the hollow portion of the loop formed by the multilayer substrate interposed therebetween. A gap (interval of the constant potential conductors 51 and 52) is provided at a portion to be performed. The part of the sensor part 2 provided with this gap is hereinafter referred to as a gap part 7. The constant potential conductor 52 is connected to the external conductor 32 that constitutes the connection portion 3.

  That is, the transmission line formed by the signal conductor 4 and the constant potential conductor 5 has a shielding effect against the electric field and the magnetic field by the constant potential conductor 5 at a portion other than the gap portion 7. The gap portion 7 where the signal conductor 4 is exposed has a shielding effect against an electric field, but the magnetic field has a structure that links the signal conductor 4.

  Further, the impedance of the transmission line formed by the signal conductor 4 and the constant potential conductor 5 and the impedance ZL of the termination resistor 6 are the characteristic impedance Zs of the connection portion 3 (and consequently the detection circuit connected to the connection portion 3). It is set to a value smaller than (input impedance), and is set to 10Ω in this embodiment. In particular, the impedance of the transmission line is 10Ω by using a substrate with ε = 3.5 and designing H = 0.418 mm, t = 18 μm, and W = 1.81 mm.

<Operation>
The sensor unit 2 configured as described above has a configuration equivalent to a well-known balanced output type shielded loop antenna (see FIG. 6B) configured using a coaxial cable or a semi-rigid cable. As a transmission line forming a loop, a strip line formed on a multilayer substrate is used instead of a coaxial cable or a semi-rigid cable. However, the code | symbol attaches | subjects the same code | symbol to the structure corresponding to each part of the magnetic field detection probe 1 of this embodiment.

  The loop formed by the transmission line of the sensor unit 2 is formed on the XZ plane in the figure (see FIGS. 1 and 2). That is, the magnetic field detection probe 1 is configured to output a detection signal corresponding to the magnitude of the component along the Y-axis direction of the magnetic flux passing through the sensor unit 2 (particularly the hollow portion of the loop).

  An equivalent circuit of the balanced output type shielded loop antenna is shown in FIG. 4, and the gap between a and f depends on the magnitude of the magnetic flux (and hence the magnetic field) passing through the sensor unit 2 (the loop antenna formed by the transmission line). An induced voltage is generated in the detection circuit, and a detection signal having a magnitude obtained by dividing the induced voltage by the characteristic impedance (input impedance of the detection circuit) Zs of the connection portion 3 and the impedance ZL of the termination resistor 6 is input to the detection circuit. It will be. That is, when the induced voltage is Ve, the input voltage Vi of the detection circuit is expressed by the equation (1).

Vi = Ve × Zs / (Zs + ZL) (1)
In the equivalent circuit, L represents the self-inductance of the loop antenna, and R represents the resistance of the loop antenna. Further, points indicated by symbols a to f in the equivalent circuit correspond to points indicated by symbols a to f in FIG.

<Effect>
As described above, in the magnetic field detection probe 1, not only the impedance ZL of the termination resistor 6 is matched with the impedance of the transmission line that constitutes the sensor unit 2, but also these impedances are converted into the input impedance (connection unit 3) of the detection circuit. Characteristic impedance) Zs. As a result, the magnetic field detection probe 1 detects the detection circuit because the voltage dividing ratio on the detection circuit side is larger than when the impedance of the transmission line and the impedance ZL of the termination resistor 6 are matched (matched) with the input impedance Zs of the detection circuit. Sensitivity can be improved.

  Here, since the input impedance of the detection circuit is set to 50Ω and the impedance of the termination resistor and the transmission line is set to 10Ω, the reduction of the input voltage Vi with respect to the induced voltage Ve is suppressed to 17% (−1.6 dB). be able to.

<Experiment>
The measurement results of the magnetic field detection sensitivity and the electric field detection sensitivity are shown in FIGS. However, as shown in FIG. 5, as a measurement system, a magnetic field detection probe 1 is arranged on a substrate on which a microstrip line is formed, and a signal is supplied from the port 1 of the network analyzer to the microstrip line, thereby detecting the magnetic field detection probe 1. The connection is made so that the detection signal output from the connection unit 3 is input to the port 2 of the network analyzer.

  However, the sensor unit 2 in the magnetic field detection probe 1 is formed such that the size Lx in the X-axis direction is 35.4 mm, the size Ly in the Y-axis direction is 10 mm, and the size Lz in the Z-axis direction is 5.4 mm (see FIG. 1 and 2) and arranged so that the X-axis direction is along the wiring direction of the microstrip line.

  FIG. 7 is a graph showing magnetic field detection sensitivity. Specifically, when the frequency of the signal supplied to the microstrip line via port 1 is swept by the network analyzer in the range of 1 MHz to 1 GHz, the S parameter from port 1 to port 2 (S21: passage loss) Was calculated by simulation.

  As a comparative example, an unbalanced output type shielded loop antenna (see FIG. 6A) was used (Comparative Example 1), and a balanced output type shielded loop antenna (see FIG. 6B) was used. The same measurement was performed on the sample (Comparative Example 2). However, in Comparative Examples 1 and 2, both are well-known ones configured using a semi-rigid cable (50Ω), and in particular, the latter uses a terminal resistance matched to the impedance of the semi-rigid cable (50Ω). ing. As shown in FIG. 7, the magnetic field detection sensitivity of the magnetic field detection probe 1 is improved by 6 dB compared to that of the comparative example 2, and is comparable to that of the comparative example 1.

  FIG. 8 is a graph showing the electric field detection sensitivity. Specifically, the output voltage of the connection part 3 when a plane wave was irradiated to the magnetic field detection probe installed in the space was calculated by simulation. However, the plane wave to irradiate is formed of a perpendicular magnetic field and an electric field, and the magnetic field is in the Z-axis direction (see FIGS. 1 and 2) and the electric field is in the X-axis direction (see FIGS. 1 and 2) in consideration of the shape of the magnetic field detection probe. 2), the magnetic field is most difficult to detect and is easily affected by the electric field (only the electric field is easily detected). As shown in FIG. 8, the electric field detection sensitivity of the magnetic field detection probe 1 is −10 dB or less (however, 1 MHz to 20 MHz) compared to those of Comparative Examples 1 and 2.

That is, it can be seen that the magnetic field detection probe 1 can suppress the influence of the electric field and can obtain the magnetic field detection sensitivity equivalent to that of the first comparative example.
Next, in FIG. 9, in the magnetic field detection probe 1, the impedances of the transmission line (signal conductor 4, constant potential conductor 5) and termination resistor 6 are changed, and the impedances of the input impedance Zs of the detection circuit and the impedance of the termination resistor 6 are changed. It is the result of changing the partial pressure ratio and calculating the passage loss S21 by simulation. Here, the partial pressure ratio was determined for 1: 1 (ZL = 50Ω, ie, conventional product), 2.5: 1 (ZL = 20Ω), 5: 1 (ZL = 10Ω), 10: 1 (ZL = 5Ω). . As shown in FIG. 9, it can be seen that the detection sensitivity is improved as the voltage division ratio is increased, but the linearity is disturbed and the frequency characteristics are deteriorated, and the usable frequency range is narrowed. Incidentally, the disturbance of linearity requires that the width of the constant potential conductor 5 be made closer to infinity as the impedance of the transmission line is brought closer to 0Ω, but in reality, there is a limit to the width of the constant potential conductor 5. Based on something. Therefore, it is desirable that the voltage division ratio is about 2: 1 to 10: 1, that is, the impedance of the transmission line and the termination resistor is about 1/2 to 1/10 of the input impedance of the detection circuit. .

<Other embodiments>
As mentioned above, although embodiment of this invention was described, it cannot be overemphasized that this invention can take a various form, without being limited to the said embodiment. For example, a part of the configuration of the above embodiment may be replaced with a known configuration having a similar function.

  In the above embodiment, the sensor unit 2 (balanced output type sealed dead loop antenna) is configured using a strip line configured on a multilayer substrate, but a coaxial cable or semi-rigid having an impedance smaller than the input impedance of the detection circuit. You may comprise using a cable.

  DESCRIPTION OF SYMBOLS 1 ... Magnetic field detection probe 2 ... Sensor part 3 ... Connection part 4 ... Signal conductor 5 (51, 52) ... Constant potential conductor 6 ... Termination resistor 7 ... Gap part 21 (211 to 214) ... Dielectric layer 22 (221) 223) Conductor layer 31 Central conductor 32 External conductor

Claims (3)

The gap portion (7) is composed of a signal conductor (4) for transmitting a signal and a constant potential conductor (5) for electromagnetically shielding the signal conductor, and partially exposing the signal conductor. A sensor part (2) formed in a loop by a transmission line having;
A terminating resistor (6) connected to one end of the transmission line;
A magnetic field detection probe (1) configured using a balanced output type shielded loop antenna with
The impedance of the transmission line and the termination resistor are set to match each other with a value smaller than the input impedance of the detection circuit connected to one end (3) different from the connection end of the termination resistor of the transmission line. A magnetic field detection probe characterized by comprising:   The magnetic field detection probe according to claim 1, wherein impedances of the transmission line and the termination resistor are ½ to 1/10 of an input impedance of the detection circuit.   The said sensor part is comprised using a flexible multilayer board | substrate (21, 22), The said transmission line consists of a stripline (4, 5) comprised on the said multilayer board | substrate. Or the magnetic field detection probe of any one of Claim 2.