JPH03120796A - Low-noise shield device - Google Patents

Abstract

PURPOSE:To obtain a low-noise shield device which can easily be sunk under liquid nitrogen even when the device is used in the liquid nitrogen so as to minimize the influence of noise from the outside by respectively enclosing resins in the first inner and second outer shield cases. CONSTITUTION:A low-noise FET amplifier substrate 181 is fitted to an inner shield case 182 by means of spoke members 196. In addition, the case 182 is fitted to an outer shield case 183 by means of spoke members 196. A resin is enclosed in the inner shield case 182 and another resin 190 is enclosed in the outer shield case 183. As a result, vent holes provided through the cases 182 and 183 can be blocked and the shielding characteristics of this device can be improved. In addition, even when the device is sunk under liquid nitrogen, the device can be sunk quickly. Moreover, when a resin is enclosed in the first case and conductive paste is enclosed in the second case, the shielding characteristics of this device can be further improved.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a low-noise shielding device that houses a low-noise amplifier and shields noise from the outside world, and particularly relates to a low-noise shielding device that houses a low-noise amplifier and the like and shields noise from the outside world. The present invention relates to a low-noise shielding device that is suitable for use with low-noise amplifiers with a wide operating temperature range, such as those that operate at low temperatures.

[Prior art] In recent years, as it has become possible to achieve a superconducting state at liquid nitrogen temperature, there has been an increase in the number of cases where it is desired to measure the shielding characteristics of magnetically shielded containers and electroencephalogram measurements at the liquid nitrogen temperature of 77 K. . For this reason, very small detection signals are often handled.

FIG. 5 shows a schematic configuration of an apparatus for measuring shielding characteristics of a magnetically shielded container at liquid nitrogen temperature.

The apparatus shown in the figure measures the magnetic shielding effect of the superconducting magnetically shielded container 4. 1 is the reference coil, 2
is an excitation coil, 3 is a pickup coil, and 5 is a low noise F
It is an ET amplifier. Reference coil 1 , excitation coil 2 , pickup coil 3 , shield container 4 , and low-noise FET amplifier 5 are placed in liquid nitrogen 6 .

A signal generated by the function generator 7 is power amplified by a power amplifier 8, and the excitation coil 2 is driven by this signal. Among the magnetic flux generated by driving the excitation coil 2, the magnetic flux leaking from the outside to the inside of the shield container 4 is detected by the pickup coil 3. The detected minute signal is amplified by a low-noise FET amplifier 5 and input to a preamplifier 9 placed at room temperature. The signal amplified by preamplifier 9 is input to lock-in amplifier 10.

On the other hand, a reference coil 1 is provided alongside the excitation coil 2. The magnetic flux generated by driving the excitation coil 2 is detected by the reference coil 1, and this detection signal is input to the lock-in amplifier 10 via the preamplifier 11.

The lock-in amplifier 10 extracts the same frequency component in the leakage flux detection signal from the preamplifier 9 in synchronization with the frequency of the reference signal from the preamplifier 11. This is because the leakage magnetic flux detection signal from the preamplifier 9 contains many frequency components due to noise from the surroundings, so the magnetic flux from the excitation coil 2 is A reference coil 1 is provided to detect the
The same frequency component in the leakage flux detection signal (minimal signal) from the preamplifier 9 is extracted in synchronization with the frequency of the signal from the reference coil 1 (equal to the frequency of the signal generated by the function generator 7), thereby eliminating noise. This is to remove the amount. At this time, nanovolts (n V) of the relevant frequency are selected from noise on the order of microvolts (μV) to millipol I□ (mV).
The order signal will be extracted. Reference numeral 12 denotes an oscilloscope that displays measurement results based on the output signal from the lock-in amplifier 10.

The low-noise FET amplifier 5 immediately amplifies the differential input (signal on the order of nV) from the pickup coil 3 and delivers it to the preamplifier 9. This low noise FET amplifier 5
If there is no such thing, you would need to connect the pickup coil 3 to the preamplifier 9 with a cable of an appropriate length (for example, about 1.5 m), but if you route the cable in this way, it will pick up a lot of external noise, and the S /N ratio deteriorates. To counter this, a low noise FET amplifier 5 is provided to reduce the input equivalent noise voltage of the entire amplifier. Furthermore, a shielding device is applied to such a low-noise FET amplifier to prevent outside noise from entering.

FIG. 6 shows a top view of the above-described shielding device for a low-noise FET amplifier, and FIG. 7 shows a perspective view thereof.

The perspective view of FIG. 7 shows the internal structure of the shield device as seen through the shield case and the like.

In these figures, 151 is the board of the above-mentioned low-noise FET amplifier, 152 is the inner (first) shield case, 1
53 is an outer (second) shield case. Low noise F
ET amplifier board 151 is attached to inner shield case 152. 154 is a hole for attaching a connector, 155
158 to 158 are cable penetration holes, 159 to 162 and 159' to 162' are air vent holes in the inner shield case 152, and 163 to 166 and 163' to 166' are air vent holes in the outer shield case 153. An input cable 167 consisting of a twisted pair shielded wire is connected to the substrate 151 through cable penetration holes 158 and 157. Further, an output cable 168 made of a twisted pair cable is connected to the connector 171 from the board 151 through the cable penetration hole 156. 169 is a power cable (3-core cable for +, GND, -), and 170 is a bushing.

The double-shielded low-noise FET amplifier is used by submerging it in liquid nitrogen as shown in FIG. The air vent holes 159 to 162 and 159' to 162' of the inner shield case 152 and the air vent holes 163 to 166 and 163' to 166' of the outer shield case 153 are provided when the FET amplifier board 151 is placed in liquid nitrogen together with the shield case. , an opening is made in the case in advance in order to quickly release the air inside the shield case to the outside of the case. These air vents are low noise FE
It is also necessary to remove the liquid nitrogen when the T amplifier is taken out from the liquid nitrogen into the air. The size and number of holes for connector attachment, cable penetration holes, and air vent holes in each case are determined as necessary.

[Problems to be Solved by the Invention] However, when using the above-mentioned shield case, the shield case has many holes of various types, so the influence of external noise is large, and even at room temperature, the input of the amplifier cannot be accessed through the holes. There were times when the intruding noise was reflected.

In view of the problems with the conventional type described above, the present invention provides a low-noise shield that can be easily submerged in liquid nitrogen and minimizes the influence of noise from the outside world. The purpose is to provide equipment.

[Means for Solving the Problem] In order to achieve the above object, the present invention includes a first shield case (inner case) that stores a circuit body, and a first shield case (inner case) that stores a circuit main body.
and a second shield case (outer case) for storing the shield case, and is characterized in that resin is sealed in the first shield case and the second shield case.

Furthermore, a first shield case that stores the circuit body;
The present invention is characterized in that it comprises a second shield case that stores the first shield case, a resin is sealed in the first shield case, and a conductive paste is sealed in the second shield case.

[Function] According to the above configuration, resin is sealed in the first shield case and the second shield case and the cases are filled, so the air vent hole can be omitted and when submerged in liquid nitrogen. can also be easily submerged.

Further, if a conductive paste is sealed in the second shield case, this conductive paste exhibits a shielding effect and the shielding characteristics are further improved.

[Example] Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a top view of a low-noise shield device according to an embodiment of the present invention, and FIG. 2 is a perspective view thereof (the shield case etc. are seen through).

In these figures, 181 is a substrate of a low-noise FET amplifier, 182 is an inner (first) shield case, and 183 is an outer (second) shield case. Low noise FET amplifier board 181 is attached to inner shield case 182 via spoke members 196. Further, the inner shield case 182 is attached to the outer shield case 183 via spoke members 197. 184 is a hole for attaching a connector, and 185 to 188 are holes for passing a cable through. Input cable 191 is cable penetration hole 1
It is connected to the substrate 181 through 88 and 187. Further, the output cable 192 is connected to the connector 195 from the board 181 through the cable penetration hole 186. 19
3 is a power cable, and 194 is a bushing. A resin 189 is sealed in the inner shield case 182 (shaded area in the figure), and a resin 190 is sealed in the outer shield case 183 (shaded area in the figure). After the substrate 181 and the like are placed inside the shield case, these resins are poured and cured while removing air from the case.

When a shield case filled with resin is submerged in liquid nitrogen in this way, it submerges much faster than in the conventional case.

Furthermore, after a certain amount of time has passed, the temperature becomes uniform throughout the interior, and the characteristics become stable. Further, since the air vent holes 159 to 166 and 159' to 166' shown in Fig. M66 and 7 can be eliminated, noise coming from the outside world is reduced.

On the other hand, even in the shielding devices shown in FIGS. 1 and 2, the resin molded portion does not have a shielding effect, and the shielding characteristics are deteriorated due to the cable penetration holes and the connector mounting holes.

FIG. 3 is a top view of a low-noise shielding device with improved shielding characteristics according to another embodiment of the present invention, and FIG. 4 is a perspective view thereof (the shielding case etc. are seen through).

In these figures, 351 is a low-noise FET amplifier substrate, 352 is an inner shield case, and 353 is an outer shield case. Low noise FET amplifier board 351 is attached to inner shield case 352 via spoke members 380.

Further, the inner shield case 352 is attached to the outer shield case 353 via spoke members 381. 362 and 363 are shield plates. A resin 364 is sealed in the inner shield case 352 (shaded area in the figure).
, Similarly, the resin 36 is also applied to the inside of the shield plates 362 and 363.
5,366 is enclosed (shaded area in the figure). After the substrate 351 and the like are placed inside the shield case, these resins are poured and cured while removing air from the case.

354 is a hole for attaching a connector, and 355 to 361 are holes for passing a cable through. Input cable 368 is connected to substrate 351 through cable penetration holes 358, 361, and 357. In addition, the output cable 369 passes from the board 351 through the cable penetration holes 356 and 360 to the connector 37.
Connected to 2. 370 is a power cable, and 371 is a bushing. A conductive paste 367 is sealed in the outer shield case 353 (shaded area in the figure). After the inner shield case 352 and the like are arranged in the outer shield case 353, the conductive paste 367 is poured little by little and dried.

By sealing the conductive paste 367 in the outer shield case 353 in this way, the inner silt case 352
and outer shield case 353 and shield plate 362,
Since the space surrounded by 363 is filled with conductive paste, the shielding characteristics are improved compared to the embodiment shown in FIG. 1.2.

Furthermore, when the shield case is submerged in liquid nitrogen, it sinks very quickly, and after a certain amount of time, the temperature becomes uniform throughout the interior, and its characteristics become stable.

In the above embodiment, aluminum was used as the material for the shield case and the shield plate, but other metals having a shielding effect may be used.

Further, for the input cable, output cable, and power cable, twisted pair shield, twisted pair, and three-core cable were used, respectively, but other cables having a shielding effect may be used.

1 2 FIG. 8 shows an example of a suitable low-noise FET amplifier to which the low-noise shielding device of the present invention is applied. This is a circuit diagram of a low noise FET amplifier using FETs as active elements to operate at liquid nitrogen temperatures. At liquid nitrogen temperatures, ordinary bipolar transistors do not operate due to the low temperature, so FETs are used as active elements.

In the figure, signals input from differential input sections 205 and 206 (signals from pickup coil 3) are grounded through input termination resistors 207 and 208, respectively. In addition, the signal grounded by the input termination resistor 207 is connected to the dual FET 20
, 21 and is grounded by the input termination resistor 208.
212 gates. Here, the sources of the two FETs 201 and 202 of the dual FET 20 are connected to the FET 203 that constitutes the constant current circuit 213.
The sources of the two FETs 211 and 212 of the dual FET 21 are connected to the drain of the FET 204 constituting the constant current circuit 214. As a result, the dual FETs 20 and 21 are each driven with a constant current. Therefore, the differential output section 209.
Between 210 and 210, differential signals input to the differential input sections 205 and 206 are amplified and output. Note that, in the figure, the input protection resistor, the stage after each of the differential output section 209210, and the feedback signal from the stage after the differential output section 209210 are omitted.

In the above circuit, dual FET1 is used in one constant current circuit.
(two FETs). One FET in dual FETs 20 and 21 (
201, 202, 211, or 212) is ID, the current value flowing through one constant current circuit is only 2ID.

Therefore, the FE constituting the constant current circuits 213 and 214
A certain amount of current can be passed through each of the dual FETs 20 and 21 without exceeding the absolute maximum ratings of T203 and T204, and amplification can be performed at a low noise level.

FIG. 9 shows a circuit diagram of a low-noise FET amplifier that eliminates the need for adjustment due to FET variations, which is required in the circuit of FIG. 8. In the circuit shown in FIG.
2 is used. Two FETs in Dual FET30
No. 303 and No. 304 have the same electrical characteristics including the drain current-gate-source voltage characteristics, and are also temperature matched. Therefore, the constant current circuit 313,
Fixed resistor 21.9,2 for adjusting the current amount of 314
20 (FIG. 8) can be omitted. Furthermore, it reduces the temperature drift of the offset voltage and increases the gain versus temperature with dual F
Since it is determined by the temperature coefficient of the mutual conductance of ET301, 302, 309.310, gain stabilization can be achieved.

[Effects of the Invention] As explained above, according to the present invention, resin is sealed in the first shield case that stores the circuit body and the second shield case that stores the first shield case. , the air vent hole in the shield case can be eliminated, improving shielding characteristics. Also, when submerged in liquid nitrogen, it can be submerged quickly.

Furthermore, resin is sealed in the first shield case, and the second shield case is filled with resin.
If a conductive paste is sealed in the shield case, the shielding characteristics will be further improved.

[Brief explanation of drawings]

FIG. 1 is a top view of a low-noise shielding device according to an embodiment of the present invention, FIG. 2 is a perspective view of the shielding device of FIG. 1, and FIG.
A top view of a low-noise shielding device with improved shielding characteristics according to another embodiment of the present invention, FIG. 4 is a perspective view of the shielding device of FIG. 3, and FIG.
A schematic configuration diagram of a measuring device for measuring shielding characteristics of a magnetically shielded container at liquid nitrogen temperature, FIG. 6 shows a conventional low-noise FE.
FIG. 7 is a perspective view of the shielding device of FIG. 6, and FIGS. 8 and 9 are a top view of the shielding device of the T amplifier; FIG. 7 is a perspective view of the shielding device of FIG. 6; and FIGS. FIG. 2 is a circuit diagram of a FET amplifier. 181.351...Low noise FET amplifier board, 182
．． 352...Inner shield case, 183,353.
...Outer shield case, 184,354...Connector mounting hole, 185-188,355-361...Cable penetration hole, 189...Inner mold resin, 19
0... Outer mold resin, 191,368... Input caple, 192,369... Output caple, 195
．． 372... Connector, 362, 363... Shield plate, 364-366... Mold resin, 367...
・Conductive paste.

Claims (2)

[Claims] (1) A first shield case that stores a circuit body and a second shield case that stores the first shield case, and a resin is provided in the first shield case and the second shield case. A low noise shielding device characterized by being sealed. (2) A first shield case that stores the circuit body and a second shield case that stores the first shield case, a resin is sealed in the first shield case, and the second shield case is provided with a resin. A low-noise shielding device characterized by encapsulating conductive paste in a shielding case.