JPH0682140B2 - TDR device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention applies TDR (Tim (Tim)
e-Domain-Reflectometer (Time Domain Reflectometer) equipment, especially A / D with high accuracy of measurement results
It relates to a TDR device suitable for converting.

[Conventional technology]

The TDR device inputs an impulse or step pulse having an ultrahigh-speed rising characteristic to a transmission line such as a coaxial cable and measures the reflected wave with a broadband oscilloscope. From this result, the failure point and the failure content of the coaxial cable or the like can be known. Recently, TDR devices for optical fibers that apply this principle have also appeared.

For the conventional TDR device, see, for example, Kiyotaka Okada: Oscilloscopes, pages 257-261, Kyoritsu Publishing (1983
Year).

FIG. 3 is a principle diagram of the TDR device described in the above document. In FIG. 3, 1 is a palace generator and 2 is an oscilloscope. The pulse generator 1 inputs an impulse or step pulse having an ultrahigh-speed rising characteristic into the transmission line to be measured. The pulse traveling on the transmission line having the characteristic impedance Z ₀ is reflected at a fault point corresponding to the disturbance of the characteristic impedance, and then runs backward on the transmission line again and returns to the pulse generator side. By measuring this reflected wave with a broadband oscilloscope such as a sampling oscilloscope, the impedance and distance of the fault point can be known.

Now, assuming that the impedance at the obstacle point is Z and the distance to the obstacle point is 1, the reflection coefficient ρ showing the ratio of the reflected wave is expressed by the following equation.

Therefore, by measuring ρ, Z can be known from the equation (1). On the other hand, if the relative permittivity of the dielectric material forming the transmission line is ε _r , the propagation time t _d of the electromagnetic wave propagating in the transmission line is smaller than that in the case of propagating in a vacuum. growing. In addition, the reflected wave displayed on the oscilloscope is the time for the pulse to reciprocate the distance 1 from the input end of the pulse to the failure point. Therefore, letting the speed of light (3 × 10 ⁸ m / sec) be v, l is Desired.

FIG. 4 is a block diagram of a conventional TDR device. In FIG. 4, 3 is a slow ramp generator, 4 is a ramp comparator, 5 is a sampling head, 6 is a horizontal amplifier, 7 is a fast ramp generator, and 8 is a vertical amplifier. The slow ramp generator 3
Generates a slow ramp wave for the horizontal sweep of a CRT. The timing pulse for driving the pulse generator 1 and the sampling head 5 is generated at the point where the levels of the low speed and high speed ramp waves coincide. Therefore, it is possible to generate a timing pulse that satisfies the sampling condition by changing the repetition period and the slope of both ramp waves.

[Problems to be solved by the invention]

In the above conventional technique, the sampling pulse is generated by detecting the coincidence point of the levels of the low-speed and high-speed ramp waves with the ramp comparator. In this conventional method, the reproduction accuracy of the observed waveform reproduced on the CRT depends on the linearity of the ramp portion of the ramp wave. Therefore, this method is not suitable for highly accurate measurement. Further, since the observed waveform is displayed on the CRT, there is a problem that the waveform data cannot be recorded and analyzed as digital information.

An object of the present invention is to solve the problem of linearity of a conventional ramp wave and to have a highly accurate sampling pulse generating means,
It is to provide a highly accurate TDR device suitable for A / D conversion of an observed waveform.

[Means for solving problems]

The above object is to provide frequency synthesizing means independently for the test pulse generating means and the sampling frequency generating means for generating the sampling frequency for driving the waveform sampling means, and use the phase of the frequency generated by these two frequency synthesizing means as a reference. This is achieved by providing the A / D conversion means for synchronizing the output waveform of the waveform sampling means with the reference frequency from the frequency generating means and performing A / D conversion in synchronization with the sampling frequency.

[Action]

That is, in the present invention, the test pulse generating means and the sampling frequency generating means, by using a high-precision frequency generator for synchronizing the phase of the mutual generated frequency,
The observed waveform is sampled with high accuracy. Since the repetition frequency of the observed waveform is equal to the frequency generated by the test pulse generating means, the observed waveform is converted into a similar waveform with a low repetition frequency by sampling at a sampling frequency that is slightly deviated to a frequency that is a fraction of this integer. can do. Therefore, the waveform accuracy is determined by the frequency stability and frequency accuracy of the two frequency generating means. Therefore, by using a highly accurate frequency generator, it is possible to provide a highly accurate TDR device that does not suffer from waveform distortion due to the non-linearity of the ramp wave as in the conventional example.

Further, the output waveform of the sampling means converted into the low speed can be A / D converted with high accuracy at the sampling frequency set to the low speed, whereby the observed waveform can be digitally analyzed with high accuracy.

Example An example of the present invention will be described below.

FIG. 1 is a block diagram of an embodiment, 30 is a DUT, 9 is a pulse generator, 10 is a coaxial cable, 11 is a first frequency synthesizer, 12 is a sampling head, 13 is a sampling generator, and 14 is a first. 2 frequency synthesizer, 15 reference frequency oscillator, 16 amplifier, 17 sample / hold circuit, 18
A / D converter, 19 is a memory, and 20 is a computer.

Defining a repetition frequency f _in of the pulse generator 9 by the frequency synthesizer (1) 11. The generated pulse needs to have a fast rising characteristic in order to obtain the distance resolution, and the pulse shape needs to be selected according to the distance between the measurement object and the pulse generator 9. In general, the impulse is suitable for measuring a measurement object having a long interval and a large attenuation, and the step pulse is suitable for high-resolution measurement at a relatively short interval. The test pulse generated by the pulse generator 9 is supplied to the DUT 30 via the coaxial cable 10 having the characteristic impedance Z ₀ .
When the impedance Z ₀ of the device under test 30 is different, reflection occurs corresponding to the magnitude of the impedance. The delay time of the reflected pulse with respect to the generated pulse is
The distance can be known by knowing the delay time and the relative permittivity of the substance forming the DUT 30. The reflected pulse and the test pulse are sampled by the sampling head 12. The sampling head 12 can have wide band characteristics by forming a sampling gate with a Schottky barrier diode or the like having a high-speed switching characteristic.

Sampling frequency f _SPL is a frequency synthesizer (2)
Specified by 14. The output frequencies of the frequency synthesizer (1) 11 and the frequency synthesizer (2) 14 are synchronized in phase with each other by supplying a reference signal from the same reference frequency oscillator 15. Sampling clock generator 13
Generates a sampling clock to be supplied to the sampling head 12. The sampling waveform converted into the low speed is amplified by the amplifier 16 in order to compensate for the amplitude reduction due to the sampling efficiency of the sampling head 12. In general, the high-speed sampling head 12 has a poor droop characteristic indicating the decay rate of the hold voltage with respect to time, and it is difficult to hold the voltage with sufficient accuracy within the sampling period. Therefore, a sample / hold circuit 17 having a good droop characteristic is provided after the amplifier 16 to prevent the hold waveform from lowering. That is, since the sampling waveform is converted into a low repetition frequency which is similar to the observed waveform, it is possible to use the sample / hold circuit 17 having a good droop characteristic and improve the test accuracy. Furthermore, the waveform after sampling / holding is synchronized with the sampling clock and A / D converted by the A / D converter 18.
Convert. On the other hand, the input signal of the A / D converter 18 can also be observed by a low-speed oscilloscope. The output data of the A / D converter 18 is stored in the memory 19 and then stored in the computer 16.
Waveform analysis is performed by.

Next, the operation of the embodiment shown in FIG. 1 will be described in more detail with reference to the time chart of the signals of the respective parts shown in FIG. In FIG. 2, the vertical axis represents signal amplitude and the horizontal axis represents time. Hereinafter, the waveforms of FIG. 2 are (a), (b), (c), (d) in order from the top.
Specify. Figure 2 (a) shows a test pulse repetition frequency f _in generated by the pulse generator 9. FIG. 2 (b) shows an example of a reflected pulse reflected from an impedance having a reflection coefficient ρ = −0.66. FIG. 2C shows a sampling clock having a sampling frequency f _SPL . Second
FIG. 6D shows a sampling waveform that has been A / D converted.

Repetition frequency f _in and the sampling frequency f of the test pulse
_SPL is set according to the equation (3).

f _in = n · f _SPL + Δf (3) where n is a natural number and Δf is the frequency of the sampling waveform after sampling. In the example of FIG. 2 (b), n =
1 shows an example of the case of setting 1 and setting Δf to a condition of Δf ≦ f _SPL to reproduce a low-speed sampling waveform from an observed waveform of a plurality of cycles as shown in FIG. 2 (c). You can Therefore, the sampling head 12
It is possible to provide a high-resolution TDR device by making the bandwidth of B wide enough to observe the rise time of the test pulse.

As described above, according to the present embodiment, the accuracy of the sampling waveform in the time axis direction is determined by the frequency accuracy of the two frequency synthesizers, and a sampling waveform with less jitter can be obtained by using a synthesizer with low phase noise. it can. Further, since the data of the observed waveform can be A / D converted with high accuracy, the correction process of the electrical length caused by the difference in the relative permittivity of the dielectric material forming the DUT can be easily performed by digital calculation.

Further, it becomes possible to measure a known characteristic impedance that is a reference in advance before measurement, and use this data as correction data to digitally perform a correction calculation on data obtained by measuring an unknown impedance, Regarding the accuracy in the amplitude direction, there is an advantage that higher accuracy can be realized as compared with the analog measuring means of the conventional example. In addition, when measuring a small impedance with high resolution, it is possible to reduce random noise generated in the measurement system by performing averaging on the data measured multiple times, resulting in high accuracy. You can measure.

〔The invention's effect〕

As described above, according to the present invention, not only the measurement accuracy in the time axis direction can be improved, but also the measurement result can be digitally recorded and processed, and the error in the data in the amplitude direction can be corrected. It is possible to calculate
It is possible to provide a TDR device with higher accuracy than the conventional method.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 (a),
(B), (c), and (d) are time charts of signals of respective parts for explaining the operation of FIG. 1, FIG. 3 is an explanatory view of the principle of the conventional TDR device, and FIG. 4 is a diagram of the conventional TDR device. It is a block diagram. 9 ... Pulse generator 10 ... Coaxial cable 11,14 ... First and second frequency synthesizer 12 ... Sampling head 13 ... Sampling clock generator 15 ... Reference frequency oscillator 16 ... Amplifier 17 ... Sample / hold circuit 18 ... A / D Transducer 19 ... Memory, 20 ... Calculator 30 ... DUT.

Claims (3)

[Claims] 1. A TDR apparatus comprising a test pulse generating means for generating a test pulse to be supplied to an object to be measured, and a waveform sampling means for observing the generated test pulse and a reflection pulse from the object to be measured, The test pulse generating means and the sampling frequency generating means for generating the sampling frequency for driving the waveform sampling means are independently provided with frequency synthesizing means, and the phase of the frequency generated by both frequency synthesizing means is used as a reference from the reference frequency generating means. A TDR device characterized by comprising A / D conversion means for performing A / D conversion in synchronization with a frequency and synchronizing an output waveform of a waveform sampling means with a sampling frequency. 2. The relationship between the repetition frequency f _{in of the} pulse generated by the test pulse generating means and the sampling frequency f _SPL , where n is a natural number and Δf is a frequency smaller than f _SPL , f _in = n · The TD according to claim 1, wherein the TD is set so as to satisfy a condition of f _SPL + Δf.
R device. 3. The TDR device according to claim 1, wherein the A / D conversion means includes data processing means for storing and analyzing the output digital data.