JPH0573573U - Sampling head - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a sampling head, and an object of the present invention is to provide a sampling head having a wide band frequency characteristic and a small input capacity. [Structure] An object to be measured is connected to an input end of a sampling circuit from which a matching resistor is removed, via a series circuit of a metal body and a resistor body.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a sampling head, and more particularly, to improvement of high input impedance.

[0002]

[Prior Art]

 Conventionally, when measuring the voltage waveform of each part of an electronic circuit with a probe having a high input impedance, a 10: 1 probe that uses a series resistance of 9 MΩ as a basic element at the input terminal of an ordinary oscilloscope with an input resistance of 1 MΩ was used. It has been done to connect the so-called probe.

FIG. 3 is an equivalent circuit diagram of an input terminal in a state where such a probe is connected. That is, a large input capacitance C in of about 15 pF is connected in parallel with the input resistance Rin of 10 MΩ. Here, the input capacitance Cin is based on the compensation capacitance provided for flattening the frequency characteristic.

[0004]

[Problems to be solved by the device]

However, when observing the voltage waveform of the circuit under measurement having a relatively low impedance of about 100Ω with such a conventional probe, the signal source resistance (resistance 100Ω of the circuit under measurement) due to the charging current of the input capacitance C in The frequency characteristic of this measurement system deteriorates due to the voltage drop, and the frequency band in which the characteristic deteriorates by 3 dB is from 10 ⁶ / (2π100 [Ω] × 15 [pF]) to 110 MHz.

In this way, the original performance of the oscilloscope may not be exhibited depending on the signal source impedance. Therefore, efforts are being made to reduce the input capacitance Cin as much as possible.

In addition, in such a 10: 1 probe, since the signal voltage becomes 1/10, the measured signal may be buried in the noise generated by the oscilloscope and the signal waveform may not be clearly observed.

In order to remedy these drawbacks, it has been practiced to use what is called a 1: 1 FET probe. As a specific example, there is a probe whose input resistance Rin is 100 kΩ and input capacitance Cin is 3 pF and the frequency band of the probe itself is about 1 GHz. Waveform observation is impossible.

[0008] In addition, as a waveform observing means of a circuit to be measured which operates at high speed, there is also a probe in which a resistance of 450Ω is connected to an input terminal of a sampling oscilloscope having an input impedance of 50Ω so that an input resistance Rin becomes 500Ω. However, since the input resistance Rin is low, accurate waveform observation cannot be performed unless the circuit has a considerably low signal source impedance.

The present invention has been made in view of such a problem, and an object thereof is to provide a sampling head having a wide band frequency characteristic and a small input capacity.

[0010]

[Means for Solving the Problems]

 The present invention is characterized in that the object to be measured is connected to the input end of the sampling circuit from which the matching resistor has been removed via a series circuit of a metal body and a resistor. ..

[0011]

[Action]

 The transmission line consisting of a series circuit of a metal body and a resistor for connecting the device under test to the input end of the sampling circuit is placed substantially in the air, and the input capacity is kept at a small value. Be drunk Then, since the matching resistance at the input end of the sampling circuit is removed, it becomes an open state, and the input resistance becomes almost infinite.

As a result, wideband frequency characteristics can be obtained.

[0013]

【Example】

 Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Generally, the sampling oscilloscope has a very wide frequency band from DC to 10 GHz to 50 GHz. Although the input impedance is 50Ω, the input capacitance Cin is designed to be extremely small in order to reduce the reflection when the transmission line is connected to the input terminal.

Therefore, in the present invention, 50Ω is removed by utilizing only this wide band and low input capacitance, and the input resistance Rin is made infinite (open) and the frequency characteristic is made flat as much as possible. To do.

FIG. 1 is a sectional view showing the configuration of an embodiment of the present invention. In the figure, 1 is a metal plate to which the input connector of the sampling head is attached. The left side of the drawing is the inside of the sampling head, and the right side is the outside to which the part to be measured is connected. Reference numeral 2 is a glass bead that constitutes an input connector, and a metal rod 3 is fixed through it. One end (left end) of the metal rod 3 is connected to a sampling head circuit (not shown), and the other end (right end) is connected to the ceramic substrate 4. The terminal resistance of 50Ω inside the sampling head circuit is removed.

FIG. 2 is a block diagram of the ceramic substrate 4. A metal pattern 5 made of, for example, gold and a resistor pattern 6 are formed in series on one surface, and a metal pattern is formed on the other surface. Not not.

Referring again to FIG. 1, 7 is a metal tube, one end of which is fixed to the metal plate 1 so as to enclose the ceramic substrate 4. The metal pattern 5 and the resistor pattern 6 formed on the ceramic substrate 4 form a transmission line having the metal tube 7 as a reference potential (earth). An insulator 8 formed in a concentric shape is attached to the other end of the metal tube 7. An input terminal 9 is fixed through the central portion of the insulator 8. One end (left end) of the input terminal 9 is connected to the right end side of the resistor pattern 6 formed on the ceramic substrate 4, and the other end (right end) is brought into contact with an object to be measured (not shown). Reference numeral 10 denotes a metal ring formed so as to cover the outer circumference of the metal tube 7. The voltage waveform of the measurement target is measured with a part of the metal ring 10 being in contact with the reference potential (earth) of the measurement target. Set.

In general, when a high frequency voltage is applied to a metal having a certain length, remarkable frequency characteristic disturbance occurs near the frequency at which the length of the metal becomes about ¼ of the wavelength. For example, for a metal having a length of 7.5 mm, its frequency is 10 GHz. Therefore, the distance between the measurement circuit, the sampling bridge, and the circuit under test must be as short as possible. When the space between the metal transmission line and the ground for connection between them is filled with a substance having a dielectric constant εs, the frequency that becomes 1/4 of the above wavelength is 1 / ( εs) It drops to ^1/2 . Therefore, it is necessary to keep the above connecting line in the air as much as possible. In the structure of the present invention, the reason why the metal pattern is not provided on the lower surface of the ceramic substrate 4 is to make it substantially equivalent to the connection line that is effectively arranged in the air.

Further, the reason why the resistor pattern 6 is provided on the ceramic substrate 4 is to dampen the resonance generated at a high frequency so that the frequency characteristic becomes as flat as possible. Further, the width of the metal pattern 5 formed on the ceramic substrate 4 is widened or narrowed because the characteristic impedance value of the transmission line is changed depending on the place to flatten the frequency characteristic. Is.

The frequency characteristic of the sampling oscilloscope is substantially flat from DC to 10 GHz to 50 GHz because all the characteristic impedances of the input transmission lines including the connector are unified to a constant value (generally 50Ω) and have the resistance value. This is because it is matched by terminating with a fixed resistor to prevent standing waves.

According to the result of computer simulation, the characteristic of the embodiment when the input circuit of the actual sampling head is connected is ± 1.3 dB from DC to 16 GHz when the signal source resistance Rg is 25Ω. When the signal source resistance Rg is 50Ω, the band is 7 GHz. In addition, the input capacitance Cin at 2.3 GHz is 0.7 pF, and the input resistance Rin is determined by the leak resistance of the diode bridge inside the sampling head, so it is 1 GΩ (10 ⁹ Ω) or more (however, the diode bridge Ignore the current that flows when J is turned on).

Although the metal pattern 5 and the resistor pattern 6 are formed on the ceramic substrate 4 in the above-mentioned embodiment, it is not limited to the ceramic substrate, and a small fixed resistor and a metal are connected in series, It is sufficient if there is a fixed resistor having an appropriate shape, and the point is that the permittivity of the main dielectric of the transmission line is close to that of air.

[0023]

[Effect of the device]

 According to the present invention described in detail above, the main dielectric of the transmission line has a dielectric constant close to that of air, has wide-band frequency characteristics, and can realize a sampling head with a small input capacitance.

[Brief description of drawings]

FIG. 1 is a sectional view showing the configuration of an embodiment of the present invention.

FIG. 2 is a structural example diagram of a ceramic substrate used in FIG.

FIG. 3 is an equivalent circuit diagram in a state where a conventional probe is connected.

[Explanation of symbols]

 1 Metal Plate 2 Glass Bead 3 Metal Rod 4 Ceramic Substrate 5 Metal Pattern 6 Resistor Pattern 7 Metal Tube 8 Insulator 9 Input Terminal 10 Metal Ring

Claims (1)

[Scope of utility model registration request] 1. A sampling head configured to connect an object to be measured to an input end of a sampling circuit from which a matching resistor is removed via a series circuit of a metal body and a resistor.