Variable attenuator, step attenuator, and test device

Abstract

The present invention provides a variable attenuator comprising: a transmission line having an open stub connected between an input and an output of the variable attenuator; a first switch connected between the input and a reference potential; and a second switch connected between the output and the reference potential.

Description

Variable attenuator, step attenuator, and test equipment

[0001] The present invention relates to a variable attenuator, a step attenuator, and a test apparatus.

[0002] Patent Document 1 states, "In the microwave circuit 1A, the line width step portion 32A is formed at the center of the transmission line 25A in the longitudinal direction (X direction), and the line width step portion 32A is formed on one side of the transmission line 25A, i.e., biased in the width direction (Y direction)" (paragraph 44). Patent Document 2 states, "The high-frequency device further includes terminals P1 and P2 for inputting and outputting high-frequency signals on the pattern conductor 1, and a ground terminal GND on the pattern conductor 5. The pattern conductors 1, 4, and 5 and the via conductors 2 and 3 connect the terminals P1 and P2 and the ground terminal GND to form a signal line for transmitting high-frequency signals...A portion of the signal line has resistances R1 and R2 to high-frequency signals, so that the high-frequency device functions as an attenuator" (paragraphs 27 and 30). Patent Document 3 states, "Many semiconductor RF switches today stack multiple low BVds transistors in series to improve the breakdown performance of the entire switch" (paragraph 6). [Prior art documents] [Patent documents] Patent document 1: JP 2014-241482 A Patent document 2: WO 2014 / 030469 A Patent document 3: JP 2010-527179 A

[0003] In a first aspect of the present invention, there is provided a variable attenuator comprising: a transmission line having an open stub connected between an input and an output of the variable attenuator; a first switch connected between the input and a reference potential; and a second switch connected between the output and the reference potential.

[0004] The variable attenuator may include a first resistor connected in series with a first switch between the input and a reference potential, and a second resistor connected in series with a second switch between the output and the reference potential.

[0005] In the variable attenuator, the first resistor may be connected to an end of the first switch on the reference potential side, and the second resistor may be connected to an end of the second switch on the reference potential side.

[0006] In any of the above variable attenuators, the open stub may be connected to the transmission line at a central position between the input and output of the variable attenuator.

[0007] In any of the above variable attenuators, the open stub may have a length less than the length of the transmission line between the input and output of the variable attenuator.

[0008] In a second aspect of the present invention, there is provided a step attenuator comprising a plurality of cascaded variable attenuators according to the first aspect.

[0009] A third aspect of the present invention provides a test apparatus comprising a test signal generating circuit that generates a test signal for testing a device under test, a comparison circuit that compares a response signal output by the device under test in response to the test signal with an expected value, and a variable attenuator of the first aspect that attenuates the test signal or the response signal.

[0010] The test apparatus may include a gain amplifier connected between the test signal generating circuit and the variable attenuator, for amplifying the test signal.

[0011] Any of the above test devices may include a mixer connected between the comparison circuit and the variable attenuator to convert the frequency of the response signal.

[0012] The above summary of the invention does not list all of the necessary features of the present invention, and subcombinations of these features may also constitute inventions.

[0013] 1 shows the configuration of a variable attenuator 10 according to the present embodiment. FIG. 2 shows the configuration of a modified example of the variable attenuator 10 according to the present embodiment. FIG. 3 shows the configuration of a step attenuator according to the present embodiment. FIG. 4 shows the configuration of a test apparatus 400 according to the present embodiment. FIG. 5 shows the Smith chart output results of the variable attenuator 10 at each position of the variable attenuator 10 according to the present embodiment. FIG. 6 shows the Smith chart output results of the variable attenuator 10. FIG. 7 shows the Smith chart output results of the variable attenuator 10. FIG. 8 shows the Smith chart output results of the variable attenuator 10. FIG. 9 shows the Smith chart output results of the variable attenuator 10. FIG. 10 shows the Smith chart output results of the variable attenuator 10. FIG. 11 shows the Smith chart output results of the variable attenuator 10. FIG. 12 shows the Smith chart output results of the variable attenuator 500. FIG. 13 shows the relationship between the impedance and length of an open stub as viewed from the connection position of a transmission line. 1 shows a simulation result of a state error of a variable attenuator with an attenuation amount of 8 dB. 2 shows a simulation result of an insertion loss of a step attenuator in Example 2, Reference Example 2, and Reference Example 3. 3 shows a simulation result of a state error of a step attenuator in Example 2, Reference Example 2, and Reference Example 3.

[0014] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention as claimed. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0015] 1 shows the configuration of a variable attenuator 10 according to this embodiment. The variable attenuator 10 has an input terminal P10 and an output terminal P11. The variable attenuator 10 attenuates, for example, a high-frequency signal input to the input terminal P10 and outputs the attenuated signal from the output terminal P11. The variable attenuator 10 can change the amount of signal attenuation by changing the internal resistance value.

[0016] The variable attenuator 10 includes a transmission line 100, a first switch 110, a first resistor 120, a second switch 130, and a second resistor 140. The transmission line 100 is connected between an input terminal P10 and an output terminal P11. The transmission line 100 outputs a signal input to the input terminal P10 from the output terminal P11. The transmission line 100 can attenuate a high-frequency signal such as an RF signal by transmitting the high-frequency signal. The transmission line 100 has a straight line connecting the input terminal P10 and the output terminal P11. The length (electrical length) of the straight line is l. ｓｅｒｉｅｓ is λ / 8<l where λ is the wavelength of the signal to be transmitted. ｓｅｒｉｅｓ <λ / 4.

[0017] The transmission line 100 has an open stub 105 connected between the input terminal P10 and the output terminal P11 of the variable attenuator 10. The open stub 105 branches off from the straight line of the transmission line 100 between the input terminal P10 and the output terminal P11, and has an open end. The open stub 105 may be connected to any position on the straight line between the input terminal P10 and the output terminal P11 of the transmission line 100, for example, it may be connected to the center position of the transmission line 100 between the input terminal P10 and the output terminal P11 of the variable attenuator 10. The open stub 105 is provided for impedance conversion of the variable attenuator 10. The open stub 105 is provided to convert the impedance of the variable attenuator 10. The open stub 105 is provided along the length l of the straight line of the transmission line 100 between the input terminal P10 and the output terminal P11 of the variable attenuator 10. ｓｅｒｉｅｓ Length less than l (electrical length) ｓｔｕｂ For example, the open stub 105 may have a length l from the connection position (branch position) with respect to the wavelength λ of the signal to be transmitted. ｓｔｕｂ However, the impedance of the open stub 105 seen from the connection position is capacitive, i.e., λ / 4>1. ｓｔｕｂ (In particular, λ / 8≧l ｓｔｕｂ ) and l ｓｅｒｉｅｓ >l ｓｔｕｂ may be in the range of

[0018] The first switch 110 is connected between the input terminal P10 and a reference potential. The first switch 110 switches whether or not the input terminal P10 and the reference potential are connected. The first switch 110 may be a shunt switch such as a HEMT (High Electron Mobility Transistor). The first resistor 120 is connected in series with the first switch 110 between the input terminal P10 and the reference potential. The first resistor 120 may be connected to the end of the first switch 110 on the reference potential side. Here, the reference potential may be a fixed potential such as ground potential (0 V), and the same applies below.

[0019] The second switch 130 is connected between the output terminal P11 and the reference potential. The second switch 130 switches whether or not the output terminal P11 and the reference potential are connected. The second switch 130 may be a shunt switch such as a HEMT. The second resistor 140 is connected in series with the second switch 130 between the output terminal P11 and the reference potential. The second resistor 140 may be connected to the end of the second switch 130 on the reference potential side.

[0020] The first switch 110 and the second switch 130 are connected to a control circuit or the like that controls switching. The variable attenuator 10 can increase the attenuation amount by turning on at least one of the first switch 110 and the second switch 130 using the control circuit.

[0021] FIG. 2 shows the configuration of a modified example of the variable attenuator 10 according to the present embodiment. The variable attenuator 10 shown in FIG. 2 has the same configuration and function as the variable attenuator 10 shown in FIG. 1 , except that it further includes a third switch 200 and a fourth switch 210. The third switch 200 is connected to the end of the first switch 110 on the reference potential side. The third switch 200 may be turned on at the same time as the first switch 110 is turned on. The fourth switch 210 is connected to the end of the second switch 130 on the reference potential side. The fourth switch 210 may be turned on at the same time as the second switch 130 is turned on. In FIG. 2 , the first switch 110, the second switch 130, the third switch 200, and the fourth switch 210 may each be a HEMT. The modified variable attenuator 10 includes shunt switches stacked in multiple stages, thereby further reducing off-capacitance and improving voltage resistance.

[0022] The variable attenuator 10 is not limited to the one in which two switches are connected as shown in FIG. 2, but may be one in which three or more switches are connected.

[0023] Fig. 3 shows the configuration of a step attenuator according to this embodiment. The step attenuator 300 includes a plurality of cascaded variable attenuators 310 to 370. In the embodiment shown in Fig. 3, the step attenuator 300 includes seven variable attenuators 310 to 370. Each of the variable attenuators 310 to 370 may have the same configuration as the variable attenuator 10 according to this embodiment shown in Fig. 1 or 2.

[0024] The first-stage variable attenuator 310 has an input terminal P10 connected to the input terminal P30 of the step attenuator 300, and an output terminal P11 connected to the input terminal P10 of the second-stage variable attenuator 320. The second- to sixth-stage variable attenuators 320 to 360 have one of their input terminals P10 and output terminals P11 connected to the other of the input terminal P10 and output terminal P11 of the adjacent variable attenuators 320 to 360. The output terminal P11 of the seventh-stage variable attenuator 370 is connected to the output terminal P31 of the step attenuator 300.

[0025] The step attenuator 300 may include variable attenuators 310 to 370 that all have the same configuration, or may include one or more variable attenuators 310 to 370 that have different amounts of attenuation.

[0026] 4 shows the configuration of a test apparatus 400 according to this embodiment. The test apparatus 400 may be ATE (Automated Test Equipment) that tests a DUT (Device Under Test) 470. The test apparatus 400 may determine that the DUT 470 is a good product if a response signal output by the DUT 470 in response to a test signal supplied to the DUT 470 during the test matches an expected value for the response signal. The test apparatus 400 may determine that the DUT 470 is a defective product if a response signal output by the DUT 470 in response to any of the test signals supplied to the DUT 470 during the test does not match the corresponding expected value.

[0027] The test apparatus 400 includes a test signal generating circuit 410, a comparing circuit 420, and a connecting device 430. The test signal generating circuit 410 generates a test signal for testing a DUT 470. The comparing circuit 420 compares a response signal output by the DUT 470 in response to the test signal with an expected value.

[0028] The connection device 430 is connected to the test signal generation circuit 410 and the comparison circuit 420. The connection device 430 functions as a switch device that switches input / output paths for signals such as analog radio frequency signals (RF signals). For example, the connection device 430 may be an RF test front end that switches between a transmission circuit that outputs a transmission signal and a reception circuit that inputs a reception signal. The connection device 430 supplies the test signal from the test signal generation circuit 410 to the DUT 470 and supplies a response signal output by the DUT 470 to the comparison circuit 420.

[0029] The connection device 430 includes a first mixer 442 , a gain amplifier 444 , a first variable attenuator 446 , a first power amplifier 448 , a connection section 450 , a second variable attenuator 460 , a second mixer 462 , and a second power amplifier 464 .

[0030] The first mixer 442 is connected to the test signal generating circuit 410. The first mixer 442 converts the frequency of the test signal from an intermediate frequency (IF) to a radio frequency (RF) by mixing the test signal generated by the test signal generating circuit 410 with a carrier wave. The gain amplifier 444 is connected between the test signal generating circuit 410 and the first variable attenuator 446 and is connected to the first mixer 442. The gain amplifier 444 amplifies and transmits the test signal frequency-converted by the first mixer 442. The first variable attenuator 446 is connected to the gain amplifier 444 and attenuates and transmits the test signal amplified by the gain amplifier 444. The first variable attenuator 446 may have a configuration similar to the variable attenuator 10 shown in FIG. 1 or 2, or may have a configuration similar to the step attenuator 300 shown in FIG. 3. The first power amplifier 448 is connected to the first variable attenuator 446, and amplifies and transmits the test signal attenuated by the first variable attenuator 446.

[0031] The connection unit 450 is connected to the first power amplifier 448. The connection unit 450 is a switch that switches whether or not to connect the terminal P41 or the terminal P42 to the terminal P43. The terminal P41 is connected to the test signal generating circuit 410, the terminal P42 is connected to the comparison circuit 420, and the terminal P43 is connected to the DUT 470.

[0032] The second variable attenuator 460 is connected to the terminal P42 and attenuates and transmits the response signal received from the DUT 470. The second variable attenuator 460 may have a configuration similar to the variable attenuator 10 shown in FIG. 1 or 2, or may have a configuration similar to the step attenuator 300 shown in FIG. 3. The second mixer 462 is connected between the comparison circuit 420 and the second variable attenuator 460. The second mixer 462 converts the frequency of the response signal from RF to IF by mixing the response signal received from the DUT 470 with a carrier wave. The second power amplifier 464 is connected between the second mixer 462 and the comparison circuit 420 and amplifies and transmits the response signal frequency-converted by the second mixer 462.

[0033] Fig. 5 is an explanatory diagram for explaining the output results of the Smith chart at each position of the variable attenuator 10 of this embodiment. Fig. 5 shows the variable attenuator 10 of this embodiment shown in Fig. 1 in a state where the first switch 110 and the second switch 130 are turned on. In Fig. 5, an open stub 105 is connected to the center of the transmission line 100 between the input terminal P10 and the output terminal P11 of the variable attenuator 10. The output results of the Smith chart at multiple positions of the variable attenuator 10 are shown in Figs. 6 to 10, respectively.

[0034] Fig. 6 shows the output result of the Smith chart at position a of the variable attenuator 10 shown in Fig. 5. Fig. 7 shows the output result of the Smith chart at position b of the variable attenuator 10 shown in Fig. 5. Fig. 8 shows the output result of the Smith chart at position c of the variable attenuator 10 shown in Fig. 5. Fig. 9 shows the output result of the Smith chart at position d of the variable attenuator 10 shown in Fig. 5. Fig. 10 shows the output result of the Smith chart at position e of the variable attenuator 10 shown in Fig. 5. The Smith charts in Figs. 6 to 10 show the impedance distribution in the frequency range of 0 GHz (DC) to 90 GHz of the signal transmitted to the variable attenuator 10 with bold lines.

[0035] In the variable attenuator 10, the open stub 105 acts as a shunt capacitor, and therefore, by combining the open stub 105 with the λ / 8 transmission line 100 disposed between positions b and c, it is possible to compensate for the reactance component at high frequencies (around 90 GHz in this case). On the Smith chart, in FIGS. 6 to 8, the distribution near high frequencies approaches the horizontal axis representing the resistance component. As shown in FIGS. 7 and 8, the open stub 105 shifts the impedance of the circuit counterclockwise on the Smith chart, and the variable attenuator 10 of this embodiment uses the electrical length l of the transmission line 100 between the input terminal P10 and the output terminal P11 to compensate for the reactance component. ｓｅｒｉｅｓ Therefore, the electrical length of the transmission line 100 between the input terminal P10 and the output terminal P11 can be shortened, and the chip size can be reduced.

[0036] FIG. 11 is an explanatory diagram illustrating the output results of the Smith chart at each position of the variable attenuator 500 of the reference example. The variable attenuator 500 of the reference example includes a transmission line 510, a fifth resistor 520, and a sixth resistor 530. The transmission line 510 has the same configuration as the transmission line 100 (particularly the straight line of the transmission line 100) between the input terminal P10 and the output terminal P11 of the variable attenuator 10 of FIG. 5, except that it does not have an open stub. The fifth resistor 520 is connected between the input terminal IN and the reference potential and has the same configuration as the first resistor 120 of the variable attenuator 10 of FIG. 5. The sixth resistor 530 is connected between the output terminal OUT and the reference potential and has the same configuration as the second resistor 140 of the variable attenuator 10 of FIG. 5. The output results of the Smith chart at multiple positions of the variable attenuator 500 of the reference example are shown in FIGS. 12 to 14, respectively.

[0037] Fig. 12 shows the output result of the Smith chart at position a of the variable attenuator 500 shown in Fig. 11. Fig. 13 shows the output result of the Smith chart at position b of the variable attenuator 500 shown in Fig. 11. Fig. 14 shows the output result of the Smith chart at position c of the variable attenuator 500 shown in Fig. 11. The Smith charts in Figs. 12 to 14 show the impedance distribution in the frequency range of the transmitted signal from 0 GHz (DC) to 90 GHz with bold lines.

[0038] Between positions b and c, the impedance near high frequencies is distributed away from the horizontal axis on the Smith chart shown in Fig. 12 due to the transmission line 510. Therefore, the transmission line 510 alone cannot bring the impedance near high frequencies sufficiently close to the horizontal axis, and therefore cannot convert the impedance to the target impedance.

[0039] FIG. 15 shows the relationship between the impedance of the open stub 105 and the length I ｓｔａｂ 15, the horizontal axis represents the length I of the open stub 105 from the connection position of the transmission line 100 between the input terminal P10 and the output terminal P11 in FIG. ｓｔａｂ15, the vertical axis represents the impedance of the open stub 105 as viewed from the connection position of the transmission line 100. As shown in FIG. ｓｔａｂ <λ / 4 (especially λ / 8≧1 ｓｔｕｂ ), the open stub 105 exhibits capacitive properties and operates as a shunt capacitor. Therefore, the electrical length of the open stub 105 in the variable attenuator 10 of this embodiment is set to 0<I ｓｔａｂ <λ / 4, particularly λ / 8≧1 ｓｔｕｂ The range is preferred.

[0040] Fig. 16 shows the results of a simulation of the state error of the variable attenuator 10 with an attenuation of 8 dB. In Fig. 16, the horizontal axis represents the frequency of the transmitted signal, and the vertical axis represents the state error (variation from the nominal value of the attenuation). In Fig. 16, Example 1 shows the results of a simulation of the state error of the variable attenuator 10 of the present embodiment shown in Fig. 2, and Reference Example 1 shows the results of a simulation of the state error of a variable attenuator having the same configuration as the variable attenuator 10 of the present embodiment shown in Fig. 2 but without an open stub.

[0041] The maximum state error in Example 1 is 0.5 dB, and the maximum state error is 0.9 dB in Reference Example 1. It can be seen that by using the variable attenuator 10 of this embodiment, the state error is improved by 0.4 dB compared to Reference Example 1.

[0042] Compared to a variable attenuator with a 4 dB attenuation, a variable attenuator with an 8 dB attenuation has a larger attenuation, so in Reference Example 1, it is necessary to widen the gate width of the shunt HEMT used as a switch from the viewpoint of allowable current and distortion characteristics. However, widening the gate width of the shunt HEMT increases the capacitance of the circuit, making it difficult to perform impedance conversion (impedance matching) for the large capacitance, and increasing the state error of the variable attenuator. For example, when simulating the state error of a variable attenuator without an open stub, with a configuration with an attenuation of 4 dB and a configuration with an attenuation of 8 dB, the state error of the variable attenuator with an attenuation of 4 dB (designed with a gate width of 25 μm) was found to be a maximum of 0.4 dB, while the state error of the variable attenuator with an attenuation of 8 dB (designed with a gate width of 50 μm) was found to be a maximum of 0.9 dB. As described above, there is a trade-off between the attenuation amount of the variable attenuator and the state error. However, in the variable attenuator 10 of this embodiment, the open stub 105 is used to perform impedance conversion, and the state error can be reduced even with a large attenuation amount.

[0043] FIG. 17 shows simulation results of insertion loss during thru (attenuation amount 0 dB) of the step attenuators in Example 2, Reference Example 2, and Reference Example 3. In FIG. 17, the horizontal axis represents the frequency of the transmitted signal, and the vertical axis represents insertion loss (dB). The step attenuator 300 of Example 2 is configured by cascading five variable attenuators 10 of the present embodiment shown in FIG. 2. The step attenuator 300 of Example 2 has an operating frequency of 50-90 GHz and includes one variable attenuator 10 with an attenuation amount of 4 dB and three variable attenuators 10 with an attenuation amount of 8 dB, for a total attenuation amount of 28 dB. Each of the five variable attenuators 10 has an electrical length of λ / 4 between the input terminal P10 and the output terminal P11, and the first switch 110 and the second switch 130 are HEMTs.

[0044] Reference example 2 uses a step attenuator having a configuration similar to that of the step attenuator 300 of Example 2, except that each variable attenuator of the step attenuator of Reference example 2 does not have an open stub. The step attenuator of Reference example 3 is configured by cascading seven variable attenuators 10 of the present embodiment shown in FIG. 2, except that each variable attenuator of the step attenuator of Reference example 2 does not have an open stub. The step attenuator of Reference example 3 has an operating frequency of 50-90 GHz and includes seven variable attenuators with an attenuation of 4 dB, for a total attenuation of 28 dB. Each switch in the variable attenuators of Reference examples 2 and 3 is a HEMT.

[0045] In Reference Example 3, the gate width of the shunt HEMT used in each variable attenuator is smaller, so the capacitive effect of the switch is smaller, but the number of cascaded variable attenuators required to achieve the desired attenuation of 28 dB is greater than in Reference Example 2. If the insertion loss of each variable attenuator in Reference Example 3 during Thru (when attenuation is 0 dB) is just over 0.5 dB, connecting seven stages in cascade will result in the insertion loss of the entire step attenuator being just over 3.5 dB. In Example 2, the insertion loss during Thru (when attenuation is 0 dB) is significantly improved by 1.4 dB compared to Reference Example 3, which has a larger number of cascaded connections.

[0046] Fig. 18 shows simulation results of the state error of the step attenuators in Example 2, Reference Example 2, and Reference Example 3. In Fig. 18, the horizontal axis represents the frequency of the transmitted signal, and the vertical axis represents the state error (dB). Example 2, Reference Example 2, and Reference Example 3 in Fig. 18 are step attenuators with the same configuration as the simulation in Fig. 17.

[0047] In Reference Example 2, a variable attenuator with an attenuation amount of 8 dB is used, so the state error is larger than in Reference Example 3, and the state error as shown in Figure 16 is further increased by the cascade connection. In Example 2, the state error is improved compared to Reference Examples 2 and 3.

[0048] Furthermore, the length of the transmission line between the input terminal and output terminal of the variable attenuator in Reference Example 2 was 300 μm for the variable attenuator with an attenuation amount of 4 dB and 320 μm for the variable attenuator with an attenuation amount of 8 dB, whereas in Example 2, it was 255 μm for the variable attenuator 10 with an attenuation amount of 4 dB and 270 μm for the variable attenuator 10 with an attenuation amount of 8 dB. Therefore, by using the step attenuator in Example 2, the chip size is reduced by about 15% compared to the step attenuator in Reference Example 2. Note that the open stub 105 in Example 2 is sufficiently shorter in length than the shunt HEMT and resistor, and does not contribute to an increase in chip size.

[0049] With the variable attenuator 10 of this embodiment as described above, impedance can be adjusted by the transmission line 100 having the open stub 105, so that the insertion loss during Thru (when the attenuation amount is 0 dB) can be reduced and also the state error can be reduced. Furthermore, with the variable attenuator 10 of this embodiment, a design that shortens the transmission line 100 between the input terminal P10 and the output terminal P11 becomes possible, which reduces the chip size and contributes to high-density packaging of the RF test front end.

[0050] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

[0051] It should be noted that the order of execution of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and that the processes can be performed in any order unless the output of a previous process is used in a subsequent process. Even if the operational flow in the claims, specifications, and drawings is described using "first," "next," etc. for convenience, this does not mean that the processes must be performed in this order.

[0052] 10 variable attenuator 100 transmission line 105 open stub 110 first switch 120 first resistor 130 second switch 140 second resistor 200 third switch 210 fourth switch 300 step attenuator 310 variable attenuator 320 variable attenuator 330 variable attenuator 340 variable attenuator 350 variable attenuator 360 variable attenuator 370 variable attenuator 400 test device 410 test signal generating circuit 420 comparison circuit 430 connection device 442 first mixer 444 gain amplifier 446 first variable attenuator 448 first power amplifier 450 connection section 460 second variable attenuator 462 second mixer 464 second power amplifier 470 DUT 500 Variable attenuator 510 Transmission line 520 Fifth resistor 530 Sixth resistor

Claims

1. A variable attenuator comprising: a transmission line having an open stub connected between an input and an output of the variable attenuator; a first switch connected between the input and a reference potential; and a second switch connected between the output and a reference potential.

2. The variable attenuator according to claim 1, comprising: a first resistor connected in series with the first switch between the input and the reference potential; and a second resistor connected in series with the second switch between the output and the reference potential.

3. The variable attenuator according to claim 2, wherein the first resistor is connected to an end of the first switch on the reference potential side, and the second resistor is connected to an end of the second switch on the reference potential side.

4. The variable attenuator according to claim 1, wherein the open stub is connected to the transmission line at a central position between the input and output of the variable attenuator.

5. The variable attenuator of claim 1, wherein the open stub has a length less than the length of the transmission line between the input and output of the variable attenuator.

6. A step attenuator comprising a plurality of variable attenuators according to claim 1 connected in cascade.

7. A test apparatus comprising: a test signal generating circuit that generates a test signal for testing a device under test; a comparison circuit that compares a response signal output by the device under test in response to said test signal with an expected value; and a variable attenuator according to claim 1 that attenuates said test signal or said response signal.

8. The test apparatus according to claim 7, further comprising a gain amplifier connected between said test signal generating circuit and said variable attenuator for amplifying said test signal.

9. The test apparatus according to claim 7, further comprising a mixer connected between said comparison circuit and said variable attenuator for converting the frequency of said response signal.

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