JPH0648427Y2 - Oscilloscope probe capacitance calibration circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a capacitance calibration circuit for a probe used in an oscilloscope.

[Outline of device]

Of the probes used in the oscilloscope, for example, 10: 1
Since the attenuation ratio probe has high impedance, the attenuation ratio is determined by the capacitance ratio at high frequencies.
This capacitance ratio must be adjusted before measurement by adjusting the variable capacitor of the probe termination circuit. However, for this work, it is necessary to use a low-capacity driver and observe the calibration square wave while adjusting so that the square wave characteristics become flat. This work is troublesome. Therefore, in order to perform this work automatically in the present invention, the input capacitance of the oscilloscope body is placed under the control of the microcomputer and the input capacitance of the oscilloscope is matched to the probe.

[Conventional technology]

Conventionally, the probe has a probe head 1, as shown in FIG.
It consists of three parts, a coaxial cable 2 and a termination circuit 3. Of these, the attenuation ratio at high frequency is the capacitor C1 and the cable capacitance,
Determined by the capacitor C2 and the input capacitance of the oscilloscope body. The attenuation ratio is determined by the ratio of the capacity of the cable, the capacity of the capacitor C2, and the input capacity of the oscilloscope to the capacity of the capacitor C1. If the ratio is 1: 9, the attenuation ratio is 10: 1 (1/10). Here, adjustment of the probe capacitance is performed by the calibration square wave signal 13
Was input to the probe and the variable capacitor C2 was adjusted so that the square wave characteristics were flat on the CRT tube surface.

[Problems to be solved by the device]

The above-mentioned conventional technique has a drawback in that it is necessary to use a dedicated driver when adjusting the variable capacitor C2, and it is necessary to always have the dedicated driver, so that extra management must be performed. . In addition, there are many occasions when the probe is attached or removed, and it is troublesome to check and readjust each time with the calibration signal. The present invention is
In order to solve these problems, a means for automatically adjusting the input capacitance on the oscilloscope body side to the capacitance on the probe side is provided to simplify the above-mentioned adjustment work.

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides a variable capacitance diode in parallel with the input section of the oscilloscope body, and the control voltage thereof can be controlled by a microcomputer incorporated in the body. The microcomputer AD-converts the input signal that has passed through the amplifier, processes the data, and supplies an optimum control voltage to the variable capacitance diode.

[Action]

As a result, when the probe is attached, the probe capacitance can be automatically corrected by connecting the probe to the calibration square wave oscillator and activating the program for detecting the optimum correction voltage of the microcomputer.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. The capacitor C2 in the probe termination circuit 2 which was a variable capacitor in the past is fixed, and the variable capacitor diode 7 is provided in the oscilloscope body 3 instead. Here, the switch S1 inserted in series with the variable capacitance diode is the contact of the relay that turns on only when the probe of the present invention is connected. This is the input capacitance of the oscilloscope when the 10: 1 probe is not used. It is provided to prevent the change. Another capacitor C3, which is also inserted in series, is to cut the DC component, and a capacitor with a large capacitance value is used. The variable capacitance diode 7 receives a control voltage from the DA converter 9 controlled by the microcomputer 11 and has an appropriate capacitance value. On the other hand, an AD converter 10 for AD-converting the signal passed through the attenuator 5 and the first stage amplifier 6 is provided as a detecting means for optimizing the capacitance value. The converted data is supplied to the microcomputer 11. Normally, the DA converter 9 only outputs the control voltage when the previous calibration was performed, but during the calibration, the control voltage is changed according to the data of the AD converter 10 to obtain a quantitative value.

Next, an example of this calibration algorithm will be described. At the time of calibration, connect the probe to the calibration square wave oscillator (usually the CAL terminal equipped on the oscilloscope) and turn on the calibration start switch S2. Then, the microcomputer 11 starts executing the calibration program and activates the AD converter 10. The AD converter performs AD conversion of 1 KHZ calibration square wave signal for about 10 cycles at a speed of about 1 Msample / sec. As shown in FIG. 2, out of the converted data, the data D1 immediately after the rising edge and the data D2 immediately before the falling edge are extracted, and their average values are calculated and compared to determine whether they match. If they do not match, the data of the DA converter 9 is increased or decreased by 1 LSB, and the above process is repeated until they match. On the other hand, when the average values of the data D1 and D2 match, it means that the control voltage of the variable capacitance diode 7 has reached the optimum value, and the calibration is completed. At this time, an example when the control voltage is not appropriate is shown in Fig. 2 data Dmax, Dmin.
Indicate. The algorithm described above is a rudimentary example, and it is clear that it will take some time to complete, but there are many possible methods for completing it quickly by advanced methods.

The calibration method described so far is valid only when, for example, a 10: 1 probe is connected. When not connected, as described above, the switch S1 must be turned off so that the oscilloscope body 3 has a predetermined input capacitance. The BNC connector J1 with a probe sensor is used as a means of detecting whether or not a 10: 1 probe is connected. A connector whose connector J1 has a structure in which the contact is short-circuited to the ground by a protrusion provided on the connector of the 10: 1 probe is used. When the 10: 1 probe is connected, the relay drive circuit 8 turns on, a current flows through the relay coil K1, and the switch S1 turns on. This
When a 10: 1 probe is connected, the variable capacitance diode 7 is automatically activated. Needless to say, the switch S1 may be manually operated according to the probe when the probe is replaced.

[Effect of device]

According to the present invention, when the probe is calibrated, the probe is connected to the calibration terminal and the calibration start button is pressed without using a low-capacity driver for calibration and observing the calibration signal waveform on the tube surface. Calibration can be ended.

[Brief description of drawings]

FIG. 1 is a block diagram showing the construction of an embodiment of the present invention, and FIG.
The figure is a waveform diagram showing an example of AD variable data when a calibration square wave signal is input. FIG. 3 is a block diagram of a conventional example. 1: Probe Hend, 2: Probe termination circuit, 3: Oscilloscope body, 5: Attenuator, 6: First stage amplifier, 7: Variable capacitance diode, 12: Vertical signal, S2: Calibration start switch, K1, S1: Relay, 14: Ground clip.

Claims (1)

[Scope of utility model registration request] 1. A variable capacitance diode provided between a signal path of a signal under measurement in an oscilloscope body and a ground, an A / D converter for converting the signal under measurement into a digital signal, and the digital signal A signal processing circuit for obtaining the optimum bias value of the variable capacitance diode, a D / A converter for converting the bias value from the signal processing circuit into a bias voltage and supplying the bias voltage to the variable capacitance diode, and the variable capacitance diode connected in series Switch, a detection means for detecting that the probe to be calibrated is connected to the input plug of the signal under test of the oscilloscope main body, and a control for closing the switch by the probe probe detection signal from the detection means. A probe capacitance calibration circuit for an oscilloscope, comprising: