A Frequency Response Correction Method for the Front-End Analog Channel of a Wideband Digital Oscilloscope

Abstract

The present invention discloses a frequency response correction method for a front-end analog channel of a broadband digital oscilloscope. An ideal fast-edge signal and an expected channel frequency response characteristic are constructed in a digital domain. The output of the ideal fast-edge signal after passing through an expected frequency response model is used as an expected response of the fast-edge signal. The time domain parameters are compared with the fast-edge response output of the actual front-end analog channel to obtain the deviations of each parameter. The deviation correction indication signals corresponding to the parameter deviations are used to correct the frequency response H of the compensation filter. c (e jω ) is corrected, so there is no need to use expensive high-sampling rate, high-bandwidth, high-precision sampling oscilloscopes to perform sampling to complete the correction of the channel frequency response of the front end of the broadband oscilloscope. In addition, the present invention uses a frequency sampling method to uniformly design compensation filters for amplitude-frequency response and phase-frequency response, and finally converts all of them into FIR compensation, achieving efficient and simple compensation with high compensation accuracy, and can be applied to occasions where the channel amplitude-frequency characteristics and phase-frequency characteristics are very undesirable.

Description

Technical Field

[0001] The present invention belongs to the technical field of digital oscilloscope calibration, and more specifically, relates to a method for calibrating the frequency response of the front-end analog channel of a broadband digital oscilloscope. Background Art

[0002] As one of the most commonly used general test and measurement instruments, with the continuous increase of its sampling rate and bandwidth, in test scenarios such as high-speed signals and broadband signals, the influence of the frequency response characteristics of the front-end analog channel of a digital oscilloscope on the measurement results is becoming greater and greater. The frequency response characteristics of the front-end analog channel can be divided into amplitude-frequency characteristics and phase-frequency characteristics, which respectively reflect the amplitude change and phase change of input signals with different frequencies after passing through the front-end analog channel (the phase change corresponds to the delay of the corresponding signal after passing through the channel). Ideally, it is desired that the amplitude-frequency characteristics of the front-end analog channel show a flat characteristic within the band, while the phase-frequency characteristics show a linear phase characteristic within the band. In practice, due to non-ideal characteristics such as hardware components, the frequency response characteristics of the front-end analog channel will show certain non-ideal characteristics. Before leaving the factory, broadband digital oscilloscopes must undergo strict frequency response calibration, i.e., frequency response calibration, to ensure the measurement accuracy during their subsequent use. The purpose of frequency response calibration is to make the frequency response of the front-end analog channel after calibration approach the ideal characteristic or the characteristic expected by the user.

[0003] The frequency response calibration of the front-end analog channel of a broadband digital oscilloscope requires a fast-edge signal as its calibration signal, and the bandwidth of the fast-edge signal itself should be greater than the bandwidth of the oscilloscope to be calibrated. Since the fast-edge signal is also generated and output by a fast-edge generator, the fast-edge signal itself is also a non-ideal fast-edge signal. Therefore, a sampling device with an "ideally frequency response characteristic" is needed to sample the fast-edge signal to obtain the spectral characteristics of the fast-edge signal itself. In the prior art, a sampling oscilloscope with a higher sampling rate and higher bandwidth than the broadband digital oscilloscope to be calibrated is used as a reference device. The frequency response characteristic of this sampling oscilloscope itself is considered to be "ideal". Therefore, it is used to sample the fast-edge signal, and the sampling data is used as the standard data representing the calibration signal, so that the "true" spectral characteristics of the fast-edge signal can be obtained.

[0004] However, the foregoing prior frequency response calibration method for the front-end analog channel has two deficiencies: on the one hand, the sampling oscilloscope itself is expensive, and on the other hand, even for a sampling oscilloscope, its channel frequency response is not absolutely ideal and there will still be certain non-ideal characteristics. In this case, using its sampling data as reference data will bring unavoidable errors to the final calibration.

[0005] In the U.S. patent titled "Digital Group Delay Compensator" with patent number US7050918B2 and authorized and published on May 23, 2006, a method for compensating digital group delay was proposed to compensate the frequency response of a broadband digital oscilloscope: a high-precision sampling oscilloscope was used to sample a calibration signal, and deconvolution was performed with the sampling signal passing through the front-end analog channel of the oscilloscope to be calibrated to obtain the impulse response of the channel. Then, an ideal fast-edge signal was convolved with the impulse response to obtain an output, and the group delay characteristic of this output was compensated. The pulse characteristics (such as overshoot, preshoot, rise time, etc.) of the compensated output time-domain signal were calculated, and these parameters were further used to adjust the coefficients of its compensation filter, thus forming a closed loop. The core was to repeatedly correct the group delay characteristic of the impulse response of the front-end analog channel until the parameters of the output fast-edge signal met the specifications. The group delay compensation filter adopted the method of IIR all-pass filter. When the true group delay characteristic of the channel showed relatively many fluctuations and large fluctuation characteristics, due to the limitations of the IIR all-pass filter design method, this method was difficult to converge and the corresponding compensation filter coefficients could not be obtained, resulting in the failure of its algorithm. Summary of the Invention

[0006] The object of the present invention is to overcome the deficiencies of the prior art and provide a method for correcting the frequency response of the front-end analog channel of a broadband digital oscilloscope, without using a high-sampling-rate and high-bandwidth high-precision sampling oscilloscope as a reference device to solve the problem of high cost caused by it. At the same time, when the IIR all-pass filter compensates the group delay characteristic of the front-end channel, when the group delay characteristic has relatively many fluctuations and the fluctuations are relatively severe (with relatively many "pits"), the filter is difficult to converge, so as to avoid the failure of the algorithm.

[0007] To achieve the above object of the invention, the method for correcting the frequency response of the front-end analog channel of the broadband digital oscilloscope of the present invention is characterized by including the following steps:

[0008] (1) Obtain the initial compensation frequency response

[0009] Use a vector network analyzer to measure the frequency response of the front-end analog channel of the broadband digital oscilloscope, and record the measurement result as H(e jω ), and then take its reciprocal 1 / H(e jω ) as the compensation frequency response, that is, the frequency response H c (e jω ) of the compensation filter, where ω is the angular frequency;

[0010] (2) Calculate the compensation filter coefficients

[0011] Using the frequency sampling method, the frequency response H c (e jω ) generates the compensation filter coefficients h c (n), and outputs them to the frequency response compensation module, where n is the sampling point number;

[0012] (3) Compensate and filter the input data

[0013] Use a fast-edge signal generator to output a fast-edge signal x(t), which is input into the wideband digital oscilloscope to be calibrated. Here, t is time. After passing through the front-end analog channel, it enters the ADC. The ADC outputs the sampling sequence x(n). Set appropriate oscilloscope time base, voltage range, and trigger mode so that only one fast edge is displayed in the center of the screen of the wideband digital oscilloscope to be calibrated. The sampling sequence x(n) enters the averaging processing module. Under the control of the trigger signal, the averaging processing module averages several frames of data of the sampling sequence x(n), and then outputs the averaged data to the frequency response compensation module. The frequency response compensation module compensates and filters the input data according to the received compensation filter coefficients h c (n), and outputs the data sequence y(n);

[0014] (4) Extract time-domain parameter deviations

[0015] First, according to the bandwidth of the front-end analog channel and the ADC sampling rate, generate an ideal fast-edge signal sequence s(n), and generate a desired frequency response H d (e jω ) The filter corresponding to this frequency response H d (e jω ) filters the fast-edge signal sequence s(n) with its impulse response, and outputs the fast-edge signal sequence q(n);

[0016] Then, by comparing the time-domain parameters of the data sequence y(n) and the fast-edge signal sequence q(n), obtain the parameter deviations of each parameter and judge. If the parameter deviations of each parameter are all within the set range, end the closed-loop calibration, and the compensation filter coefficients h c (n) will no longer be updated. After that, store the compensation filter coefficients h c (n) for the frequency response compensation of the front-end analog channel when the wideband digital oscilloscope is officially used. If the parameter deviations of each parameter are not completely within the set range, output the corresponding deviation correction indication signal to the compensation frequency response correction module;

[0017] (5) Compensate and correct the frequency response

[0018] The compensation frequency response correction module, according to the input deviation correction indication signal, corrects the compensation frequency response, that is, the frequency response H c (ejω ) The amplitude-frequency characteristic and phase-frequency characteristic are corrected, and then step (2) is returned.

[0019] The invention object of the present invention is realized as follows:

[0020] In the frequency response correction method of the front-end analog channel of the broadband digital oscilloscope of the present invention, it is not necessary to use an expensive high-precision sampling oscilloscope with a high sampling rate and high bandwidth for sampling to complete the frequency response correction of the front-end channel of the broadband oscilloscope. Instead, an ideal fast-edge signal and the desired channel frequency response characteristic are directly constructed in the digital domain according to the channel bandwidth and sampling rate characteristics of the broadband digital oscilloscope to be corrected. The output of the ideal fast-edge signal after passing through the desired frequency response model is used as the desired response of the fast-edge signal to compare with the fast-edge response output of the actual front-end analog channel in the time domain parameters, and the parameter deviations (overshoot, preshoot, rise time, fall time deviation, etc.) are obtained. The frequency response H of the compensation filter is corrected by the deviation correction indication signal corresponding to these parameter deviations c (e jω ) The amplitude-frequency characteristic and phase-frequency characteristic are corrected to form a closed-loop feedback system. Finally, when each parameter deviation is less than the set threshold, the compensated channel frequency response meets the expected requirements, and the closed-loop correction ends. In addition, the present invention uses the frequency sampling method to uniformly design the amplitude-frequency response and phase-frequency response compensation filters, and finally all are converted into FIR compensation, realizing high efficiency, simplicity, and high compensation accuracy, and being applicable to the occasions where the amplitude-frequency characteristic and phase-frequency characteristic of the channel are very unsatisfactory. Description of the Drawings

[0021] Figure 1 is a flowchart of a specific implementation manner of the frequency response correction method of the front-end analog channel of the broadband digital oscilloscope of the present invention;

[0022] Figure 2 is a signal processing schematic diagram of a specific implementation manner of the frequency response correction method of the front-end analog channel of the broadband digital oscilloscope of the present invention;

[0023] Figure 3 is an example of the amplitude-frequency characteristic in the desired frequency response;

[0024] Figure 4 is a schematic diagram of the structure of the deviation extraction module;

[0025] Figure 5 is a schematic diagram of the phase characteristic rotation operation;

[0026] Figure 6 is an example diagram of two different group delay characteristics, where (a) is a fixed-value group delay, and (b) has a relatively larger delay corresponding to the high-frequency part;

[0027] Figure 7It is a diagram of a frequency response correction example. Among them, (a) is an ideal fast-edge signal, (b) is the fast-edge signal to be corrected, and (c) is the fast-edge signal after compensation, that is, after correction;

[0028] Figure 8 It is a diagram of the normal use scenario of the oscilloscope after correction. Specific implementation manner

[0029] The following describes the specific implementation manner of the present invention in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present invention. It should be particularly noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0030] Figure 1 、 2 They are respectively the flowchart and the signal processing schematic diagram of a specific implementation manner of the frequency response correction method for the front-end analog channel of the broadband digital oscilloscope of the present invention.

[0031] In this embodiment, as Figure 1 shown, the frequency response correction method for the front-end analog channel of the broadband digital oscilloscope of the present invention includes the following steps:

[0032] Step S1: Obtain the initial compensation frequency response

[0033] Use a vector network analyzer to measure the frequency response of the front-end analog channel of the broadband digital oscilloscope, and record the measurement result as H(e jω ), and then take its reciprocal 1 / H(e jω ) as the compensation frequency response, that is, the frequency response H c (e jω ) of the compensation filter.

[0034] In this embodiment, as Figure 2 shown, the compensation frequency response initialization module 1 initializes the frequency response H c (e jω ) of the compensation filter, and then outputs it to the compensation frequency response correction module 2. Since the parameter deviation has not been obtained at this time, the compensation frequency response correction module 2 directly outputs the frequency response H c (e jω ) to the compensation filter coefficient calculation module 3.

[0035] Step S2: Calculate the compensation filter coefficients

[0036] In the compensation filter coefficient calculation module 3, using the frequency sampling method, the frequency response H c (e jω ) generates the compensation filter coefficients h c(n), which is output to the frequency response compensation module 4. Specifically: for the frequency response H c (e jω ) perform an inverse FFT (IFFT) operation to obtain the corresponding compensation filter impulse response, and perform windowing processing on it to obtain the final compensation filter coefficient h c (n).

[0037] Step S3: Perform compensation filtering on the input data

[0038] Use the fast edge signal generator 5 to output a fast edge signal x(t), which is input to the broadband digital oscilloscope to be calibrated. Among them, t is time. After passing through the front-end analog channel 6, it enters the ADC 7. The ADC 7 outputs a sampling sequence x(n). Set appropriate oscilloscope time base, voltage range, and trigger mode so that only one fast edge is displayed in the center of the screen of the broadband digital oscilloscope to be calibrated. The sampling sequence x(n) enters the averaging processing module 8. The averaging processing module 8, under the control of the trigger signal, averages the sampling sequence x(n) for several frames of data, such as 8192 frames of data, and then outputs the averaged data to the frequency response compensation module 4. The frequency response compensation module 4, according to the received compensation filter coefficient h c (n), performs compensation filtering on the input data and outputs a data sequence y(n).

[0039] In the present invention, the compensation filter for the amplitude-frequency response and the phase-frequency response is uniformly designed by the frequency sampling method, and finally all are converted into FIR compensation. The reason for not using the IIR all-pass filter for the phase-frequency characteristic compensation in the present invention: The IIR all-pass filter is suitable for compensating relatively regular group delay characteristics, but not for compensating group delay characteristics with more fluctuations and more severe fluctuations.

[0040] Step S4: Extract the time-domain parameter deviation

[0041] First, according to the bandwidth of the front-end analog channel and the ADC sampling rate, generate an ideal fast edge signal sequence s(n), and generate a desired frequency response H d (e jω ), and filter the fast edge signal sequence s(n) with the filter 9 corresponding to the frequency response H d (e jω ), that is, the impulse response, and output a fast edge signal sequence q(n).

[0042] The fast edge signal can be generated as follows (taking the rising edge as an example):

[0043]

[0044] Among them, TD represents the delay of the rising edge, T s is the sampling rate of the fast edge signal, represents rounding, ω0 = 2π × P / t rise , where P is a constant related to the hardware circuit, and t rise represents the rise time, such as 20 ps (picoseconds), and this value is determined according to the front-end analog channel bandwidth of the actual wideband digital oscilloscope to be calibrated.

[0045] Desired frequency response: It can be pre-generated and stored in the machine. According to the signal bandwidth and sampling rate, various combinations of frequency response characteristics can be pre-generated. For example, the amplitude-frequency characteristics of various shapes generated, such as Figure 3 shown. The amplitude-frequency characteristic can be Butterworth type (butter), Chebyshev type I (cheby1), Chebyshev type II (cheby2), Bessel type (besself), etc., or other characteristics; the phase-frequency characteristic can be linear phase, or other phase characteristics. Here, the desired frequency response can be selected from the existing characteristic curves, or can be redefined by the user himself.

[0046] Then, in the deviation extraction module 10, by comparing the time-domain parameters of the data sequence y(n) and the fast-edge signal sequence q(n), such as overshoot, overshoot, rise time / fall time and other time-domain parameters, obtain the deviation of each parameter and judge. If the deviation of each parameter is within the set range, the closed-loop correction is ended, and the compensation filter coefficient h c (n) will no longer be updated, and then the compensation filter coefficient h c (n) is stored for the front-end analog channel frequency response compensation when the wideband digital oscilloscope is officially used. If the deviation of each parameter is not completely within the set range, the corresponding deviation correction indication signal is output to the compensation frequency response correction module 2.

[0047] In this embodiment, as Figure 4 shown, before comparing the time-domain parameters of the comparison data sequence y(n) and the fast-edge signal sequence q(n), interpolation processing of a certain ratio can be performed on these two sequences as needed to improve the parameter calculation accuracy. Specifically, as Figure 4 shown, in the deviation extraction module 10, an interpolation module 1001 is used to interpolate the data sequence y(n) and the fast-edge signal sequence q(n) respectively, and then a time-domain parameter calculation module 1002 is used to obtain the overshoot, overshoot, and rise time of the data sequence y(n) and the fast-edge signal sequence q(n) respectively. Three time-domain parameters, and then in the deviation calculation module 1003, compare the time-domain parameters of the data sequence y(n) and the fast-edge signal sequence q(n), obtain the deviation of each parameter and judge. If the deviation of each parameter is within the set range, the closed-loop correction is ended. If the deviation of each parameter is not completely within the set range, the corresponding deviation correction indication signals 1, 2,... m are output to the compensation frequency response correction module 2.

[0048] Step S5: Compensation frequency response correction

[0049] The compensation frequency response correction module 2 corrects the amplitude-frequency characteristic and phase-frequency characteristic of the compensation frequency response, that is, the frequency response H c (e jω ) according to the input deviation correction indication signal, and then returns to step S2.

[0050] For example, when it is detected that the rise time of the data sequence y(n) is greater than the rise time of the fast edge signal sequence q(n), the phase-frequency characteristic of the compensation frequency response can be corrected. Specifically, its phase-frequency characteristic can be rotated by a certain angle. An example is shown Figure 5 as follows. Suppose the initial phase delay curve is OA. Now, if you want to rotate this curve by an angle θ, the coordinate calculation formula for the rotated point B is:

[0051] x2 = (x1 - X)cosθ - (y1 - Y)sinθ + X

[0052] y2 = (x1 - X)sinθ + (y1 - Y)cosθ + Y

[0053] Figure 6 are two examples of different group delay characteristics diagrams. Figure 6 The group delay characteristic shown in (a) is a fixed-value group delay, that is, the delays of signals with different frequencies after passing through the system are the same, and its corresponding phase characteristic is a linear phase characteristic. This kind of group delay characteristic is generally used in the case where the input signal is a broadband signal. Figure 6 For the group delay characteristic in (b), the delay corresponding to the high-frequency part is relatively large. This kind of characteristic is generally used in the analysis of input signals as pulse signals, such as eye diagram analysis. For this kind of group delay characteristic, when the input is a high-speed pulse signal or a fast edge signal, the time-domain waveform will not cause excessive overshoot. Therefore, for another example, when it is detected that the overshoot value of the data sequence y(n) is significantly greater than the overshoot value of the fast edge signal sequence q(n), the group delay characteristic and amplitude-frequency characteristic of the compensation frequency response can be corrected. Specifically, the group delay corresponding to the high-frequency part can be appropriately increased and the amplitude attenuation corresponding to the high-frequency part can be reduced. An example is shown Figure 6 as (b).

[0054] Figure 7 (a) is an ideal fast edge signal, Figure 7 (b) is the fast edge signal to be corrected collected by the ADC (with a large overshoot). Using the above-mentioned present invention for correction, finally, as Figure 7 (c) shows, a fast edge signal that meets the user's expectations and is compensated, that is, corrected (with almost no overshoot) can be obtained.

[0055] Once the calibration is completed, the filter coefficients for channel frequency response compensation will no longer be updated. After that, as Figure 8 shown, these coefficients are stored and used for calibrating the frequency response of the front-end analog channels when the frequency response of the broadband digital oscilloscope is officially used.

[0056] The present invention does not require a dedicated high-precision sampling oscilloscope, greatly saving the cost required for calibration. In addition, the present invention uses the frequency sampling method to uniformly compensate for amplitude and phase, with higher compensation accuracy and a wider compensable range.

[0057] Although the above-described illustrative specific embodiments of the present invention have been described to facilitate the understanding of the present invention by those skilled in the art, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions made using the concept of the present invention are within the scope of protection.

Claims

1. A method for correcting the frequency response of a front-end analog channel of a broadband digital oscilloscope, characterized in that: The following steps are involved: (1) Obtain the initial compensation frequency response A vector network analyzer is used to measure the frequency response of the analog channel of the broadband digital oscilloscope front end. The measurement result is recorded as H(e jω ), then take its reciprocal 1 / H(e jω ) as the compensation frequency response, that is, the frequency response of the compensation filter H c (e j ω ), where ω is the angular frequency; (2) Calculate the compensation filter coefficients Frequency response H c (e jω ) performs an inverse FFT operation to obtain the corresponding compensation filter impulse response, and performs a windowing process on the compensation filter impulse response to obtain the compensation filter coefficient h c (n) and output to the frequency response compensation module, where n is the sampling point number; (3) Compensation filtering of input data A fast edge signal x(t) is outputted by a fast edge signal generator and inputted into the broadband digital oscilloscope to be calibrated, where t is the time. After passing through the front-end analog channel, the signal enters the ADC, and the ADC outputs a sampling sequence x(n). The appropriate oscilloscope time base, voltage range and trigger mode are set so that only one fast edge is displayed in the center of the screen of the broadband digital oscilloscope to be calibrated. The sampling sequence x(n) enters the averaging processing module. Under the control of the trigger signal, the averaging processing module averages several frames of data of the sampling sequence x(n), and then outputs the averaged data to the frequency response compensation module. The frequency response compensation module receives the compensation filter coefficient h. c (n), perform compensation filtering on the input data and output the data sequence y(n); (4) Time domain parameter deviation extraction First, an ideal fast-edge signal sequence s(n) is generated based on the front-end analog channel bandwidth and ADC sampling rate, and a desired frequency response H is generated. d (e jω ), the frequency response H d (e jω ) The filter corresponding to the impulse response filters the fast edge signal sequence s(n) and outputs the fast edge signal sequence q(n); Then, by comparing the time domain parameters of the data sequence y(n) and the fast edge signal sequence q(n), the deviation of each parameter is obtained and judged. If the deviation of each parameter is within the set range, the closed-loop correction is terminated, and the compensation filter coefficient h of the channel frequency response compensation is c (n) is no longer updated, and the compensation filter coefficient h c (n) stored for frequency response compensation of the front-end analog channel when the broadband digital oscilloscope is officially used. If the deviation of each parameter is not completely within the set range, the corresponding deviation correction indication signal is output to the compensation frequency response correction module; (5) Compensation frequency response correction The compensation frequency response correction module corrects the compensation frequency response, i.e. the frequency response H of the compensation filter, according to the input deviation correction indication signal. c (e jω )’s amplitude-frequency characteristic and phase-frequency characteristic are corrected, and then the process returns to step (2).

2. The frequency response correction method of the front-end analog channel of a broadband digital oscilloscope according to claim 1, characterized in that: In step (4), before comparing the time domain parameters of the data sequence y(n) and the fast edge signal sequence q(n), it also includes: using an interpolation module in the deviation extraction module to interpolate the data sequence y(n) and the fast edge signal sequence q(n), and then using a time domain parameter calculation module to obtain the three time domain parameters of the pre-shoot, overshoot and rise time of the data sequence y(n) and the fast edge signal sequence q(n).

3. The frequency response correction method of the front-end analog channel of a broadband digital oscilloscope according to claim 2, characterized in that: The fast edge signal sequence s(n) is a rising edge signal sequence: Among them, TD represents the delay of the rising edge, T s is the fast edge signal sampling rate, Indicates rounding, ω0=2π×P / t rise , P is a constant related to the hardware circuit, t rise Indicates the rise time.

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