CN104880608A - Cable dielectric loss sweep frequency test method based on correlational analysis method - Google Patents

Abstract

The invention relates to a frequency sweep test method for cable dielectric loss based on a correlation analysis method, which belongs to the technical field of dielectric loss measurement and relates to a frequency sweep test method for cable dielectric loss based on a correlation analysis method. The method adopts the dielectric loss measurement algorithm based on the correlation analysis method and the frequency sweep signal generation technology based on DDS technology. Firstly, the frequency sweep module is made. Chip, crystal oscillator and low-pass filter circuit. Connect the keyboard input module, liquid crystal display module and the main control module of the single-chip microcomputer, and the main control module of the single-chip microcomputer receives the manual input value and displays the operating state of the system, so as to realize the function of human-computer interaction. The invention adopts a dielectric loss measurement algorithm based on a correlation analysis method, realizes accurate digital measurement, and overcomes defects such as cumbersome, time-consuming, and frequency interference in fixed-frequency measurement. The invention has high cost performance and strong practicability.

Description

Frequency Sweep Test Method of Cable Dielectric Loss Based on Correlation Analysis

technical field

The invention belongs to the technical field related to dielectric loss measurement, and relates to a frequency sweep measurement method for cable dielectric loss based on a correlation analysis method.

Background technique

With the rapid development of electronic technology and state detection technology, the detection methods of on-line dielectric loss detection have become various. Based on the traditional dielectric loss detection method, combined with digital signal processing technology, the new dielectric loss detection method is more intelligent and accurate. Domestic dielectric loss factor testing instruments are mostly measured at a fixed frequency. The fixed frequency measurement is cumbersome, time-consuming, and not intuitive. The resonance phenomenon between the measurement points and the sudden change point of the network characteristics are often ignored. Therefore, the detection is not precise enough. For example, due to other error factors such as harmonic interference, the zero-crossing comparison method sometimes has a great influence on the measurement results, which limits the application of this method in high-precision phase difference detection. When using the DFT algorithm, if the number of sampling points is small, the truncation error of the window and the fence effect will have a great influence.

Contents of the invention

In order to overcome the defects in the prior art, the present invention invents a frequency sweep measurement method for cable dielectric loss based on a correlation analysis method. This method adopts the dielectric loss measurement algorithm based on the correlation analysis method and the frequency sweep signal generation technology based on the DDS technology, through the frequency sweep module, the current amplifier, and the AD conversion module, and adopts the dielectric loss measurement algorithm based on the correlation analysis method to achieve a more accurate Accurate digital measurement, and overcome the cumbersome, time-consuming, frequency interference and other defects of fixed frequency measurement.

The technical scheme that the present invention adopts is the cable dielectric loss sweep frequency testing method based on the correlation analysis method, and it is characterized in that, the method adopts the dielectric loss measurement algorithm based on the correlation analysis method and the frequency sweep signal generation technology based on the DDS technology; the present invention Specific steps are as follows:

(1) Make frequency sweep module

The frequency sweeping module is composed of a keyboard input module, a liquid crystal display module, a main control module of a single-chip microcomputer, a frequency sweep signal generation chip, a crystal oscillator and a low-pass filter circuit; The module receives the manual input value and displays the operating status of the system, so as to realize the function of human-computer interaction; then connect the main control module of the single-chip microcomputer with the frequency sweep signal generation chip to control the frequency and phase of the frequency sweep signal; the frequency sweep signal generation chip is connected with a crystal oscillator, The clock signal is provided by the crystal oscillator; the frequency sweep signal generator chip is connected to the low-pass filter circuit at the same time, so as to obtain a clean frequency sweep signal;

(2) The main control module of the single-chip microcomputer outputs the frequency sweep control word to the frequency sweep signal generation chip to control the frequency of the output signal, and the calculation process of the frequency control word is as follows:

The system output frequency is:

f _out ＝(f _r ×W)/2 ³² (1)

Among them, f _out is the output frequency, f _r is the reference clock frequency, and W is the corresponding decimal frequency control word;

The calculation formula of the frequency control word is:

W＝2 ³² ×f _out /f _r (2)

Since a 32-bit phase accumulator is used, the output signal resolution is:

Δf _out = f _r /2 ³² (3)

(3) Carry out voltage amplification to the frequency sweep signal that frequency sweep module outputs; Voltage amplifier among the present invention is a compound type amplifier, is powered by DC power supply, and the voltage of voltage amplifier output is loaded on tested cable; The current signal is amplified, and then the voltage of the cable and the amplified current signal are sampled by the AD conversion module;

(4) Send the voltage and current signals sampled in the previous step to the algorithm processing unit for dielectric loss calculation based on the correlation analysis method; the specific calculation process is as follows:

Voltage and current expressions:

$\left\{\begin{array}{c}\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span>\\ \mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span>\end{array}\right.<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Among them, u(t) and i(t) are the voltage and current respectively, U _m and I _m are the voltage and current amplitudes respectively, ω is the frequency of the voltage and current, and θ is the phase difference of the voltage and current;

The waveform has no delay, and its voltage autocorrelation function is:

$\begin{array}{c}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&Integral;</span>}_{<span\; class="notranslate">\; -</span><span\; class="notranslate">\; \&infin;</span>}^{<span\; class="notranslate">\; +</span><span\; class="notranslate">\; \&infin;</span>}\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d\&tau;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; T</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; dt</span>}\\ <span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; dt</span>}<span\; class="notranslate">\; =</span>\frac{{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\end{array}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

In the formula, T is the fundamental wave period, R _u (0) is the voltage autocorrelation, τ is the sampling time difference, and U _r is the effective value of the voltage;

Similarly, the current autocorrelation function is:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

In the formula, R _i (0) is the current autocorrelation, and I _r is the effective value of the current;

The cross-correlation function of current and voltage is:

$\begin{array}{c}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; iu</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; T</span>}}\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; dt</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; T</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; dt</span>}\\ <span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}\end{array}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

Among them, R _iu (0) is the mutual correlation between voltage and current; then the angle between current and voltage is:

$\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arccos</span>}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; iu</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}{\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

The dielectric loss angle is:

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; arccos</span>}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; iu</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}{\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

(5) In addition to the fundamental wave signal, there are DC and high-order odd harmonic harmonic components in the voltage and current signals to be measured, which form interference to the dielectric loss measurement. To overcome these interferences, it is necessary to extract the fundamental wave; The mode of wavelet multi-resolution analysis extracts the fundamental wave component of signal; By using wavelet analysis method to process the result of signal and the error of theoretical result to judge the quality of wavelet basis function, thus determine wavelet basis function; The present invention selects Daubechies 5 order wavelet (referred to as db5 wavelet);

(6) Carry out the cable dielectric loss frequency sweep test; turn on the system power supply, input the sweep frequency through the keyboard, start the test, the frequency sweep module generates the sweep signal, the voltage amplifier amplifies the voltage of the sweep signal, and the amplified voltage is loaded on The measured cable, the current of the tested cable is amplified by the current amplifier and then sampled by the AD conversion module. The AD conversion module samples the voltage signal of the tested cable at the same time, and the sampled voltage and current signals are sent to the algorithm processing unit, which is based on the correlation analysis method. Calculate the dielectric loss, and draw the dielectric loss spectrum, and turn off the system power after the test is completed.

The beneficial effect of the invention is that it can realize high-precision frequency sweep signal output, has a certain load capacity, is cost-effective, and has strong practicability.

Description of drawings

Figure 1 is a schematic diagram of the frequency sweep module.

Figure 2 is a cable test wiring diagram, where 1 is the cable guide core, and 2 is the conductive cloth.

Figure 3 is a dielectric loss spectrum diagram, the axis of abscissa represents the frequency, the axis of ordinate represents the value of dielectric loss, and the curve represents the variation trend of the dielectric loss of the tested cable with frequency.

Detailed ways

The specific implementation of the present invention will be described in detail below in conjunction with the technical solutions and accompanying drawings. FIG. 1 is a schematic diagram of the frequency sweep module. The present invention selects AD9850 and C8051F340 single-chip microcomputer to design a frequency-sweeping module. The frequency sweep module is composed of a keyboard input module, a liquid crystal display module, a single-chip microcomputer main control module, a frequency sweep signal generation chip, a crystal oscillator and a low-pass filter circuit; It can realize high-precision frequency sweep output at low frequency, its frequency resolution is 0.01Hz, and the output low voltage is 2V. Through low-pass filtering and voltage amplification module, voltage output higher than 100V can be realized.

The amplified signal is loaded on the cable. Since the current passing through the cable is very small, the current needs to be amplified before AD sampling. The sampled signal is sent to the algorithm processing unit for digital measurement based on the correlation analysis method to obtain the dielectric loss value. , and then draw the measured value into a dielectric loss spectrum diagram.

The specific steps of the cable dielectric loss frequency sweep test method are as follows:

(1) Manufacture frequency sweep module

AD9850 is selected as the frequency sweep signal generation chip in the frequency sweep module, and an external active crystal oscillator is used to provide a reference clock signal for AD9850. As shown in Figure 1, the single-chip microcomputer C8051F340 outputs the frequency sweep control word to the AD9850 to control the frequency of the output signal, the power supply pin is added with a bypass capacitor to filter out high-frequency harmonics, and the signal generated by the chip is low-pass filtered and then output to meet the frequency requirements Sweep frequency signal, low-pass filter circuit adopts 5th-order elliptic filter circuit, and single-chip computer C8051F340 is connected to keyboard input module and liquid crystal display module to receive manual input value and display system operation status, so as to realize the function of human-computer interaction;

According to the design index, the frequency resolution is 0.01Hz, and the reference clock frequency f _r <42.94MHz is calculated by formula (1)(2)(3), and f _r is taken as 40MHz.

The frequency sweep module can generate frequency sweep of any frequency band within the frequency range of 0.1Hz to 1kHz, and can realize high-precision frequency sweep output at low frequency, and its frequency resolution is 0.01Hz

(2) Voltage amplifies the frequency sweep signal output by the frequency sweep module; the voltage amplifier in the present invention is powered by a DC power supply, and OP07 and PA340 are used to form a composite amplifier, so that the input offset voltage is determined by the preamplifier OP07. By setting the closed-loop magnification of PA340 in the power supply, OP07 is connected in series with PA340 to achieve the required AC output. In the initial design, the output voltage amplitude of the AD9850 is 2V, and the AC output of plus or minus 100V can be realized after passing through the voltage amplification module.

The voltage output by the voltage amplifier is loaded on the tested cable; then the current signal of the tested cable is amplified, and then the voltage of the cable and the amplified current signal are sampled by the AD conversion module; the AD sampling module uses 24-bit high-precision A/D conversion The chip is ADS1241, the crystal oscillator is 2.4576MHz, and it is connected to the single-chip microcomputer by 4-wire mode. The power supply voltage is 3.3V, and the reference voltage is 2.5V.

(3) Carry out dielectric loss calculation based on the correlation analysis method for the above-mentioned sampled voltage and current signals, and the specific calculation process is carried out according to formulas (4) to (9);

(4) In addition to the fundamental wave signal, there are DC and high-order odd harmonic harmonic components in the voltage and current signals to be measured, which interfere with the dielectric loss measurement. To overcome these interferences, the fundamental wave needs to be extracted; The fundamental component of the signal is extracted by wavelet multi-resolution analysis. At present, we mainly judge the quality of the wavelet basis function by using the wavelet analysis method to process the signal and the error of the theoretical result, so as to determine the wavelet basis function. The present invention selects Daubechies 5th order wavelet.

(5) Carry out the cable dielectric loss frequency sweep test according to the above steps, and draw the dielectric loss spectrum diagram. The cable test wiring diagram is shown in Figure 2. The tested cable is wound with conductive cloth to form an artificial shielding layer. Turn on the system power supply during the test, input the sweep frequency through the keyboard, and start the test. The frequency sweep module generates a sweep signal, and the voltage amplifier amplifies the voltage of the sweep signal. After the current amplifier is amplified, it is sampled by the AD conversion module, and the AD conversion module samples the voltage signal of the cable under test at the same time, and the sampled voltage and current signals are sent to the algorithm processing unit, which performs the dielectric loss calculation based on the correlation analysis method, and draws the dielectric loss Loss spectrogram, turn off the system power after the test is completed.

The dielectric loss spectrogram drawn by the present invention is shown in Figure 3, in which the abscissa is the test frequency, and the ordinate is the measured dielectric loss, and the curve reflects the variation trend of the dielectric loss with frequency. Large, the overall dielectric loss tends to become smaller.

Claims (1)

1. based on a cable dielectric loss sweep check method for relevant function method, it is characterized in that, the method adopts the dielectric loss measurement algorithm based on relevant function method and the swept-frequency signal generation technique based on DDS technology; The concrete steps of sweep check method are as follows:

(1) scan module is made

Scan module is made up of input through keyboard module, LCD MODULE, single-chip microcomputer main control module, swept-frequency signal generation chip, crystal oscillator and low-pass filter circuit; Input through keyboard module, LCD MODULE are connected with single-chip microcomputer main control module, single-chip microcomputer main control module receives artificial input value and display system running status, thus realizes the function of man-machine interaction; Again single-chip microcomputer main control module is connected with swept-frequency signal generation chip, controls frequency and the phase place of swept-frequency signal; The external crystal oscillator of swept-frequency signal generation chip, provides clock signal by crystal oscillator; Swept-frequency signal generation chip connects low-pass filter circuit simultaneously, thus obtains clean swept-frequency signal;

(2) frequency sweep control word is exported to the frequency of swept-frequency signal generation chip controls output signal by single-chip microcomputer main control module, and wherein frequency control word computation process is as follows:

System output frequency is:

f _out＝(f _r×W)/2 ³²(1)

Wherein, f _outfor output frequency, f _rfor reference clock frequency, W is corresponding frequency decade control word;

The computing formula of frequency control word is:

W＝2 ³²×f _out/f _r(2)

Owing to adopting 32 phase accumulators, then outputing signal resolution is:

Δf _out＝f _r/2 ³²(3)

When frequency resolution is 0.01Hz, calculate reference clock frequency f _r<42.94MHz, chooses f _rfor 40MHz;

(3) swept-frequency signal that scan module exports is carried out voltage amplification; Voltage amplifier in the present invention is a complex type amplifier, and by DC power supply, the voltage-drop loading that voltage amplifier exports is in tested cable; Again the current signal of tested cable is amplified, afterwards by the voltage of AD conversion module sampling cable and the current signal after amplifying;

(4) flow to algorithm process unit to the voltage and current signal of previous step sampling to carry out calculating based on the dielectric loss of relevant function method; Concrete computation process is as follows:

Voltage and current expression:

$\left\{\begin{array}{c}u\left(t\right)=U_m\sin \left(\mathrm{\&omega;t}\right)\\ i\left(t\right)=I_m\sin (\mathrm{\&omega;t}+\mathrm{\&theta;})\end{array}\right.---\left(4\right)$

Wherein, u (t) and i (t) is respectively voltage and electric current, U _mand I _mbe respectively voltage and current amplitude, ω is the frequency of electric current and voltage, and θ is the phase differential of electric current and voltage;

Waveform is without time delay, and its voltage autocorrelation function is:

$\begin{array}{c}{R}_u\left(0\right)={\&Integral;}_{-\&infin;}^{+\&infin;}u\left(t\right)u(t-\mathrm{\&tau;})\mathrm{d\&tau;}=\frac{1}{T}{\&Integral;}_0^T{u}^2\left(t\right)\mathrm{dt}\\ ={\&Integral;}_0^T{\left(U_m\sin \left(\mathrm{\&omega;t}\right)\right)}^2\mathrm{dt}=\frac{{U_m}^2}{2}=U_r^2\end{array}---\left(5\right)$

In formula, T is the primitive period, R _u(0) be voltage auto-correlation, τ is sampled time skew, U _rfor voltage effective value;

In like manner, electric current autocorrelation function is:

$R_i\left(0\right)=\frac{{I_m}^2}{2}=I_r^2---\left(6\right)$

In formula, R _i(0) be electric current auto-correlation, I _rfor current effective value;

Electric current and voltage cross correlation function are:

$\begin{array}{c}{R}_{\mathrm{iu}}\left(0\right)={\&Integral;}_0^{T}i\left(t\right)u\left(t\right)\mathrm{dt}={\&Integral;}_0^T{I}_m\sin (\mathrm{\&omega;t}+\mathrm{\&theta;})\&CenterDot;U_m\sin \left(\mathrm{\&omega;t}\right)\mathrm{dt}\\ =\frac{U_m{I}_m}{2}\cos \mathrm{\&theta;}=I_r{U}_r\cos \mathrm{\&theta;}\end{array}---\left(7\right)$

Wherein, R _iu(0) be the cross-correlation of voltage and electric current; Then the angle of electric current and voltage is:

$\mathrm{\&theta;}=\arccos \frac{R_{\mathrm{iu}}\left(0\right)}{\sqrt{R_i\left(0\right)R_u\left(0\right)}}---\left(8\right)$

Dielectric loss angle is:

$\mathrm{\&delta;}=\frac{\mathrm{\&pi;}}{2}-\mathrm{\&theta;}=\frac{\mathrm{\&pi;}}{2}-\arccos \frac{R_{\mathrm{iu}}\left(0\right)}{\sqrt{R_i\left(0\right)R_u\left(0\right)}}---\left(9\right)$

(5) except fundamental signal, also there is direct current and high-order odd harmonic harmonic component in voltage to be measured, current signal, form interference to dielectric loss measurement, overcome these interference just needs to extract first-harmonic; The mode of wavelet multiresolution analysis is adopted to extract the fundametal compoment of signal; By judging the quality of wavelet basis function by the result of wavelet analysis method processing signals and the error of notional result, determine wavelet basis function thus; The present invention selects Daubechies 5 rank small echo;

(6) sweep check of cable dielectric loss is carried out; Open system power supply, by input through keyboard swept frequency, start test, scan module produces swept-frequency signal, voltage amplifier carries out voltage amplification to swept-frequency signal, voltage-drop loading after amplification is in tested cable, the electric current of tested cable is sampled by AD conversion module after current amplifier amplifies, AD conversion module is sampled the voltage signal of tested cable simultaneously, the voltage and current signal of sampling is given algorithm process unit and is undertaken calculating based on the dielectric loss of relevant function method by algorithm process unit, and draw out dielectric loss spectrogram, complete shutdown system power supply to be tested.