Error rate measuring device and error rate measuring method

Abstract

To adjust a threshold voltage between signal levels in real time using a symbol count ratio of a PAM4 signal being measured.SOLUTION: An error rate measurement device 1 includes: a symbol count unit 13a for counting determination results as PAM symbols at arbitrary time in a state in which Vth_Upper, Vth_Middle, and Vth_Lower are arbitrary set, and calculates a ratio of determination frequency of signal levels 3 and 2 and a ratio of determination frequency of signal levels 1 and 0; an A / D converter 6 for detecting a voltage value of a DC average voltage for a PAM4 signal; and a control unit 8 for adjusting the Vth_Middle to be the voltage value of the DC average voltage for the PAM4 signal detected by the A / D converter 6, adjusting the Vth_Upper so that the determination frequency of the signal levels 3 and 2 becomes a desired ratio, and adjusting the Vth_Lower so that the determination frequency of the signal levels 1 and 0 becomes the desired ratio.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an error rate measurement device and an error rate measurement method for measuring the error rate of a PAM4 signal.

Background Art

[0002] Conventionally, in an error rate measurement device, when determining bits of an NRZ signal composed of two signal levels of 0 and 1 as shown in FIG. 6, it is necessary to determine a threshold voltage Vth serving as a determination criterion. Generally, as a method for obtaining the threshold voltage Vth, a method is used in which the threshold voltage Vth is swept once, and the threshold voltage Vth that gives the best bit error rate (BER) is selected from among them. This method can be expected to have the highest accuracy and the algorithm is also simple.

[0003] On the other hand, this method has drawbacks such as the need to interrupt the measurement once to sweep the threshold voltage Vth, and the inability to follow in cases where the threshold voltage Vth of the measurement signal can vary midway, lacking real-time performance.

[0004] Therefore, an error rate measurement device is provided with Auto Adjust as a function for estimating the threshold voltage Vth serving as this determination criterion in real time. This utilizes the fact that in an NRZ signal with a uniform mark rate such as a random pattern, the optimal threshold voltage Vth is equal to the DC average voltage value of the signal, and can be realized by a low-pass filter for extracting the DC average voltage of the signal and an A / D converter: ADC (Analog-to-Digital Converter) for detecting its voltage value.

[0005] Incidentally, while an NRZ signal like the one in Figure 6 has only one threshold voltage Vth, a PAM4 signal like the one in Figure 5, which consists of four signal levels (0, 1, 2, 3), requires three threshold voltages: Vth_Upper, Vth_Middle, and Vth_Lower. Of these, only Vth_Middle can be calculated using DC averaging, and Vth_Upper and Vth_Lower cannot be estimated. Therefore, although Auto Adjust is available for PAM4 signals, currently only Vth_Middle can be implemented, which presents a challenge.

[0006] Here, one method for estimating Vth_Upper and Vth_Lower is to estimate the threshold voltage Vth from the region where the digital value of the A / D converter saturates, and this method is disclosed in Patent Document 1 below. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 6025883 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, with PAM4 signals, the signal levels 0, 1, 2, and 3 are not necessarily evenly distributed with respect to the signal's peak-to-peak voltage. Furthermore, the optimal Vth_Upper is not necessarily exactly midway between signal level 3 and signal level 2, which can lead to estimation errors. Additionally, this method requires sweeping the threshold voltage Vth to determine the region where the digital value saturates, making it unsuitable for real-time estimation applications.

[0009] Therefore, the present invention has been made in view of the above problems, and aims to provide an error rate measuring device and an error rate measuring method that can adjust the threshold voltage between signal levels in real time using the symbol count ratio of the PAM4 signal being measured. [Means for solving the problem]

[0010] To achieve the above objective, the error rate measuring device described in claim 1 of the present invention determines whether the signal level of the PAM4 signal is 2 or 3. In the first bit determination unit 11C A first threshold voltage Vth_Upper for determining whether the signal level of the PAM4 signal is 1 or 2. In the second bit determination unit 11B A second threshold voltage Vth_Middle is used to determine whether the signal level of the PAM4 signal is 0 or 1. In the third bit determination device 11A A symbol counting unit 13a counts the determination results as PAM symbols over an arbitrary time period while a third threshold voltage Vth_Lower for determination is set arbitrarily, and calculates the ratio of the determination frequencies of signal levels 3 and 2, and the ratio of the determination frequencies of signal levels 1 and 0. Each of the following functions is defined as an evaluation function: if the signal level is shifted towards a higher level, it asymptotically approaches +1; if the signal level is shifted towards a lower level, it asymptotically approaches -1; and if it is balanced, it approaches 0; for the first threshold voltage, it is a function obtained by subtracting signal level 3 from signal level 2 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; for the third threshold voltage, it is a function obtained by subtracting signal level 1 from signal level 0 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; and an evaluation function calculation unit 8a calculates the evaluation function for each of the first and third threshold voltages from the detection result of the symbol count value of the symbol count unit. An operation amount calculation unit 8b calculates a voltage operation amount for each of the first and third threshold voltages from the estimation results, using the evaluation functions for each of the first and third threshold voltages calculated by the evaluation function calculation unit, A low-pass filter 5 extracts the DC average voltage of the PAM4 signal, An A / D converter 6 detects the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter, The second threshold voltage is adjusted and controlled so that it becomes the voltage value of the DC average voltage of the PAM4 signal detected by the A / D converter, and the frequency of the signal level determination between 3 and 2 is set to a desired ratio. Based on the voltage manipulation amount calculated by the aforementioned manipulation amount calculation unit The first threshold voltage is adjusted and controlled so that the frequency of the signal level being determined to be 1 and 0 is in a desired ratio. Based on the voltage manipulation amount calculated by the aforementioned manipulation amount calculation unit The system is characterized by comprising a control unit 8 that adjusts and controls the third threshold voltage.

[0011] The error rate measurement method described in claim 2 of the present invention is to determine whether the signal level of the PAM4 signal is 2 or 3. In the first bit determination unit 11CThe first threshold voltage Vth_Upper for determination, whether the signal level of the PAM4 signal is 1 or 2 In the second bit determination unit 11B The second threshold voltage Vth_Middle for determination, whether the signal level of the PAM4 signal is 0 or 1 In the third bit determination device 11A Count the determination result as a PAM symbol at an arbitrary time in a state where the third threshold voltage Vth_Lower for determination is arbitrarily set, and calculate the ratio of the determination frequencies of the signal levels 3 and 2 and the ratio of the determination frequencies of the signal levels 1 and 0 by the symbol counting unit 13a; Each of the following functions is defined as an evaluation function: if the signal level is shifted towards a higher level, it asymptotically approaches +1; if the signal level is shifted towards a lower level, it asymptotically approaches -1; and if it is balanced, it approaches 0; for the first threshold voltage, it is a function obtained by subtracting signal level 3 from signal level 2 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; for the third threshold voltage, it is a function obtained by subtracting signal level 1 from signal level 0 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; and the evaluation function is calculated by the evaluation function calculation unit 8a from the detection result of the symbol count value of the symbol count unit. The steps include: using the evaluation functions for the first and third threshold voltages calculated by the evaluation function calculation unit to estimate the difference between the first and third threshold voltages, and calculating the voltage manipulation amount for each of the first and third threshold voltages using the manipulation amount calculation unit 8b based on the estimation results; Extracting the DC average voltage of the PAM4 signal by the low-pass filter 5; Detecting the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter by the A / D converter 6; So that the determination frequencies of the signal levels 3 and 2 are at a desired ratio Based on the voltage manipulation amount calculated by the aforementioned manipulation amount calculation unit Adjusting and controlling the first threshold voltage by the control unit 8; Adjusting and controlling the second threshold voltage by the control unit so that it becomes the voltage value of the DC average voltage of the PAM4 signal detected by the A / D converter; So that the determination frequencies of the signal levels 1 and 0 are at a desired ratio Based on the voltage manipulation amount calculated by the aforementioned manipulation amount calculation unit Adjusting and controlling the third threshold voltage by the control unit, characterized by including the above steps. The error rate measuring device described in claim 3 of the present invention is an error rate measuring device according to claim 1, The second threshold voltage is characterized in that it can be set to any initial value. The error rate measurement method described in claim 4 of the present invention is the error rate measurement method of claim 2, The method is characterized by including the step of setting the second threshold voltage to an arbitrary initial value.

Advantages of the Invention

[0012] According to the present invention, the threshold voltage Vth can be adjusted in real time even in a PAM4 signal by estimation based on the symbol count ratio during measurement.

Brief Description of the Drawings

[0013] [Figure 1] It is a block diagram showing the internal configuration of the error rate measuring device according to the present invention. [Figure 2] It is a block diagram related to the automatic adjustment function (Auto Adjust) of the threshold voltage in FIG. 1. [Figure 3] It is a flowchart of the automatic adjustment method of the threshold voltage by the error rate measuring device according to the present invention. [Figure 4] It is a flowchart of the automatic adjustment method of the threshold voltage by the error rate measuring device according to the present invention following FIG. 3. [Figure 5] It is a diagram showing an example of the relationship between the signal levels 0, 1, 2, 3 of the PAM4 signal and the threshold voltages Vth_Upper, Vth_Middle, Vth_Lower. [Figure 6] It is a diagram showing an example of the relationship between the signal levels 0, 1 of the NRZ signal and the threshold voltages Vth1, Vth2, Vth3. [Mode for Carrying Out the Invention]

[0014] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings.

[0015] [Summary of the Invention] The error rate measuring device and error rate measuring method according to the present invention adopt a method focusing on the ratio of bits (symbols) determined by the receiver, and have a function of automatically adjusting the threshold voltage Vth in real time for the PAM4 signal.

[0016] First, in order to make it easier to understand the functions of the present invention, the case of using a binary NRZ signal will be described as an example with reference to FIG. 6.

[0017] In FIG. 6, when the threshold voltage Vth serving as the determination criterion is Vth1, if the target signal is a random pattern such as PRBS, the appearance frequencies of the signal level 0 and the signal level 1 should be equal, and thus the determination frequencies should also be equal.

[0018] However, if the threshold voltage Vth shifts upward as shown in Vth2 in Figure 6, the frequency of determining signal level 0 increases, and if it shifts downward, the frequency of determining signal level 1 increases.

[0019] By utilizing this, we can estimate whether the current threshold voltage Vth is shifted upward or downward by evaluating which of the judged bits is dominant. Then, by adjusting the threshold voltage Vth based on this estimation, we can consider the point where the judgment ratio is balanced as the optimal point for the threshold voltage Vth.

[0020] Furthermore, even if the frequency of occurrence of signal level 0 and signal level 1 in the target signal is not equal, if the ratio is known in advance, the desired target ratio can be changed accordingly.

[0021] Next, we apply this to the PAM4 signal. For example, using the PAM4 signal in Figure 5, the threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, and third threshold voltage Vth_Lower) can be estimated from the following symbol count ratios.

[0022] First threshold voltage Vth_Upper: Ratio of the determination frequency between signal level 3 and signal level 2. Second threshold voltage Vth_Middle: Ratio of the determination frequency of signal level 2 to signal level 1, or DC average voltage of the PAM4 signal. Third threshold voltage Vth_Lower: Ratio of the frequency of determining signal level 1 and signal level 0.

[0023] It is important to note that in this method, the threshold voltage Vth to be moved is not uniquely determined. For example, the frequency of determining signal level 2 actually correlates with both Vth_Upper and Vth_Middle, so it is not possible to uniquely determine whether Vth_Upper should be moved down or Vth_Middle should be moved up. In this regard, in the case of a PAM4 signal, if Vth_Middle is determined by estimation using the DC average voltage, all three threshold voltages Vth (Vth_Upper, Vth_Middle, Vth_Lower) can be uniquely estimated.

[0024] Therefore, if Symbol_total is the total number of symbols measured per arbitrary time, a function can be realized using the following calculation formulas (Equations (1) to (3)) that asymptotically approaches +1 if the threshold voltage Vth (Vth_Upper, Vth_Middle, Vth_Lower) is shifted upwards, approaches -1 if it is shifted downwards, and is 0 if it is balanced. In this embodiment, this function is defined as the evaluation function. Note that for the second threshold voltage Vth_Middle, which uses the DC average voltage, the value of the second threshold voltage Vth_Middle can be directly obtained, rather than using an evaluation function that evaluates the shift.

[0025] The first threshold voltage Vth_Upper: 2 × (signal level 2 - signal level 3) / Symbol_total… Equation (1)

[0026] The first threshold voltage, Vth_Upper, is calculated using equation (1) above by subtracting signal level 3 from signal level 2 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols.

[0027] The second threshold voltage Vth_Middle: DC average voltage of the PAM4 signal…Equation (2)

[0028] The second threshold voltage Vth_Middle is directly calculated as the DC average voltage of the PAM4 signal using equation (2) above.

[0029] The third threshold voltage Vth_Lower: 2 × (signal level 0 - signal level 1) / Symbol_total… Equation (3)

[0030] The third threshold voltage Vth_Lower is calculated using equation (3) above, by subtracting signal level 1 from signal level 0 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols.

[0031] [Configuration of the error rate measuring device] Next, the configuration of the error rate measurement device when automatically adjusting the threshold voltage of the PAM4 signal described above will be explained with reference to Figure 1.

[0032] As shown in Figure 1, the error rate measurement device 1 is generally configured to include a clock source 2, a variable delay unit 3, a bit error measurement unit 4, a low-pass filter (labeled LPF in Figure 1) 5, an A / D converter (labeled ADC in Figure 1) 6, an operation display unit 7, and a control unit 8. The bit error measurement unit 4 also includes a bit determination unit 11 (11A, 11B, 11C), a decoder 12, and a counting unit 13, as shown in Figure 1. Furthermore, as shown in Figure 2, a threshold voltage adjustment unit 14 (14A, 14B, 14C) is provided between the control unit 8 and each bit determination unit 11 (11A, 11B, 11C).

[0033] Clock source 2 generates a clock at a reference frequency and inputs the generated reference frequency clock to variable delay unit 3.

[0034] When the control unit 8 controls the reference frequency clock from the clock source 2 with a set phase amount, the variable delay unit 3 inputs a punching clock based on the phase amount to each bit determination unit 11 (11A, 11B, 11C).

[0035] The number of bit determination units 11 corresponds to the number of eyepieces in the PAM signal. In this case, since the PAM4 signal is the target signal, there are three bit determination units 11 (11A, 11B, 11C).

[0036] Each bit determination unit 11A, 11B, and 11C is equipped with input terminals for a bit determination threshold voltage and a punching clock, which are controlled by the control unit 8. The punching clock is a clock synchronized with the PAM signal (data signal), and the punching phase is adjusted by controlling the delay amount of the variable delay unit 3 by the control unit 8.

[0037] The decoder 12 decodes the results obtained from each bit determination unit 11 (11A, 11B, 11C) into determination results as PAM symbols.

[0038] The counting unit 13 counts symbols or errors in the PAM4 signal and comprises a symbol counting unit 13a and an error counting unit 13b.

[0039] The symbol counting unit 13a counts the determination results (determined as signal level 0, 1, 2, or 3) of PAM symbols decoded by the decoder 12 at an arbitrary time (or a preset measurement time), and calculates the ratio of the determination frequencies of signal level 3 to signal level 2, and the ratio of the determination frequencies of signal level 1 to signal level 0.

[0040] The error counting unit 13b detects errors by comparing the PAM symbol determination result decoded by the decoder 12 at an arbitrary time (or a preset measurement time) with a reference pattern sequence. Error determination involves identifying errors not only based on the number of errors, but also including information on the expected symbol and the actually determined symbol.

[0041] The threshold voltage adjustment units 14 (14A, 14C) consist of adders and subtractors, and add or subtract the voltage manipulation amount (adjustment amount) of the corresponding threshold voltage Vth calculated by the manipulated variable calculation unit 8b of the control unit 8 (described later) to the initial values ​​of the threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower). Specifically, the threshold voltage adjustment unit 14A adds or subtracts the voltage manipulation amount (adjustment amount) of the first threshold voltage Vth_Upper calculated by the manipulated variable calculation unit 8b to the initial value of the first threshold voltage Vth_Upper. The threshold voltage adjustment unit 14C adds or subtracts the voltage manipulation amount (adjustment amount) of the third threshold voltage Vth_Lower calculated by the manipulated variable calculation unit 8b to the initial value of the third threshold voltage Vth_Lower.

[0042] The low-pass filter 5 has a cutoff frequency that is sufficient to smooth the bias in the signal pattern sequence of the PAM4 signal and extract the DC voltage. On the other hand, a cutoff frequency that is too low will lead to poor responsiveness, so considering the balance, for example, for signals on the order of Gbaud, a low-pass filter 5 with a cutoff frequency of about 1 kHz to 1 MHz is desirable. The A / D converter 6 detects the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter 5.

[0043] The operation display unit 7 combines the functions of an operation unit, which consists of various keys, switches, buttons, and soft keys on the display screen equipped on the main body of the error rate measuring device 1, with the functions of a display unit, which consists of a display device such as a liquid crystal display, an EL (electroluminescent) display, or a CRT.

[0044] The operation display unit 7 allows the user to input various information necessary for automatic adjustment of the threshold voltage Vth of the PAM4 signal and measurement of the error rate (for example, the initial values ​​of the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower), phase amount (delay element operation amount), pattern sequence, and ON / OFF of the automatic threshold voltage Vth adjustment function (Auto Adjust)). The control unit 8 controls the display of setting item screens and measurement result screens for setting the above information, and displays buttons, soft keys, pull-down menus, input boxes, and other objects for operation on the setting item screen to set various conditions.

[0045] In this embodiment, the operation display unit 7 is shown as having a configuration that combines both an operation unit and a display unit, but the operation display unit 7 may also have separate configurations for the operation unit and the display unit.

[0046] As shown in Figures 1 and 2, the control unit 8 includes an evaluation function calculation unit 8a and a manipulated variable calculation unit 8b.

[0047] The evaluation function calculation unit 8a calculates an evaluation function for each threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) based on the detection results of the symbol count value from the symbol count unit 13a.

[0048] The manipulated variable calculation unit 8b uses the evaluation function for each threshold voltage Vth (Vth_Upper, Vth_Lower) calculated by the evaluation function calculation unit 8a to estimate the deviation of the threshold voltage Vth (Vth_Upper, Vth_Lower), and calculates the voltage manipulated variable (adjustment amount) for each threshold voltage Vth (Vth_Upper, Vth_Lower) from the estimation results.

[0049] The control unit 8 then comprehensively controls the variable delay unit 3, the operation display unit 7, the bit determination unit 11 (11A, 11B, 11C), and the error determination unit 13 in order to automatically adjust the threshold voltage Vth, which will be described later.

[0050] Specifically, the control unit 8 controls the operation input of various information necessary for automatic adjustment of the threshold voltage Vth of the PAM signal and measurement of the error rate (for example, initial values ​​of the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower), phase amount (delay element manipulation amount), pattern sequence, ON / OFF of the automatic adjustment function for the threshold voltage Vth (Auto Adjust), etc.) via the operation display unit 7, controls the display of the operation display unit 7 such as the setting item screen and the measurement result screen, controls the variable delay element 3 with the set phase amount (delay element manipulation amount), controls the input of the initial values ​​of the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower) and the voltage manipulation amount (adjustment amount) calculated by the operation amount calculation unit 8b to the threshold voltage adjustment unit 14 (14A, 14C) to the bit determination unit 11 (11A, 11B, 11C), and controls the input of the pattern sequence to the error determination unit 13. However, the pattern column is a setting item for when error measurement is performed simultaneously with automatic adjustment of the threshold voltage Vth, and is not a required setting item when only automatic adjustment of the threshold voltage Vth is performed.

[0051] [Method for automatically adjusting the threshold voltage Vth] Next, a method for automatically adjusting the threshold voltage Vth of the PAM4 signal using the error rate measuring device 1 configured as described above will be explained with reference to Figures 3 and 4.

[0052] To automatically adjust the threshold voltage Vth of the PAM4 signal, the operation display unit 7 is used to set the initial value of the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower), the phase amount (delay switch operation amount), the pattern sequence, and the automatic adjustment function for the threshold voltage Vth (Auto Adjust): ON (ST1).

[0053] The initial values ​​of the threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower) can be arbitrarily set by the user. In this embodiment, the bit determination unit 11 (11A, 11B, 11C) and decoder 12 are always in operation, but the user can also arbitrarily set the measurement time using the operation display unit 7.

[0054] After the above settings are made, when the measurement of the PAM4 signal is started, the control unit 8 switches the contacts of the switching unit (switch) 15 in Figure 2 in the direction shown by the dotted line so that the initial value of the second threshold voltage Vth_Middle is input to the bit determination unit 11B, and inputs the initial values ​​of the arbitrarily set threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower) to the corresponding bit determination units 11 (11A, 11B, 11C), and controls the variable delay unit 3 with the set phase amount. As a result, the variable delay unit 3 inputs a punching clock based on the phase amount to each bit determination unit 11 (11A, 11B, 11C) (ST2).

[0055] Then, each bit determination unit 11 (11A, 11B, 11C) compares the signal voltage level when the punching clock from the variable delay unit 3 punches a predetermined position in the time axis direction of the eye aperture of the PAM4 signal with the set threshold voltage Vth (first threshold voltage Vth_Upper: initial value for the first time, adjusted value after adding or subtracting a voltage manipulation amount (adjustment amount) from the second time onward, second threshold voltage Vth_Middle: initial value for the first time, DC average voltage value from the second time onward, third threshold voltage Vth_Lower: initial value for the first time, adjusted value after adding or subtracting a voltage manipulation amount (adjustment amount) from the second time onward) and makes a bit determination (ST3).

[0056] Next, the decoder 12 decodes the bit determination results of each bit determination unit 11 (11A, 11B, 11C) into determination results as PAM symbols (ST3).

[0057] Then, the symbol counting unit 13a of the counting unit 13 counts the determination results as PAM symbols decoded by the decoder 12 at an arbitrary time (or a preset measurement time), and calculates the ratio of the determination frequencies of signal level 3 to signal level 2, and the ratio of the determination frequencies of signal level 1 to signal level 0 (ST4).

[0058] Furthermore, if a pattern sequence is set by the operation display unit 7, the error count unit 13b of the count unit 13 compares the decoded judgment result of each bit determination unit 11 (11A, 11B, 11C) with the set pattern sequence at an arbitrary time (or a predetermined measurement time) to determine whether or not there is an error and detect an error.

[0059] Next, the evaluation function calculation unit 8a of the control unit 8 calculates evaluation functions for the first threshold voltage Vth_Upper and the third threshold voltage Vth_Lower, as described above, from the detection results of the symbol count value of the symbol count unit 13a. In this case, the second threshold voltage Vth_Middle is set to the voltage value of the DC average voltage of the PAM4 signal obtained from the output of the A / D converter 6 (ST5).

[0060] Then, the manipulated amount calculation unit 8b of the control unit 8 estimates the direction of the deviation and whether or not convergence occurs in the threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) using the evaluation function calculated by the evaluation function calculation unit 8a, and calculates the voltage manipulated amount (adjustment amount) for each threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) from the estimation result (ST6).

[0061] Subsequently, the control unit 8 switches the contacts of the switching unit (switch) 15 in Figure 2 in the direction shown by the solid line to input the voltage value of the DC average voltage of the PAM4 signal to the bit determination unit 11B, and inputs the voltage manipulation amount (adjustment amount) for each threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) to the bit determination units 11A and 11C via the corresponding threshold voltage adjustment units 14A and 14C (ST7).

[0062] Then, the threshold voltages Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) are automatically adjusted by adding or subtracting the corresponding threshold voltage Vth voltage manipulation amount (adjustment amount) to the currently set threshold voltages Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) (ST8). In addition, the contacts of the switching unit (switch) 15 in Figure 2 are switched in the direction shown by the solid line, and the currently set second threshold voltage Vth_Middle is automatically adjusted to the voltage value of the DC average voltage of the PAM4 signal (ST8).

[0063] Then, the control unit 8 repeats the processes from ST3 to ST7 until it determines that the judgment frequencies for signal level 3 and signal level 2, and for signal level 1 and signal level 0, calculated by the symbol count unit 13a, are in the desired ratio (ST9-Yes), and the control unit 8 controls the automatic adjustment of the first threshold voltage Vth_Upper and the third threshold voltage Vth_Lower. At this time, the contacts of the switching unit (switch) 15 in Figure 2 are switched in the direction shown by the solid line, so that the second threshold voltage Vth_Middle is automatically adjusted to the voltage value of the DC average voltage of the PAM4 signal. Then, when the user performs the operation to end the automatic adjustment of the threshold voltage Vth from the operation display unit 7 (ST10-Yes), the process of automatic adjustment of the threshold voltage Vth is terminated.

[0064] Alternatively, the ST8 determination process in Figure 4 can be omitted, and the control unit 8 can be used to repeat the ST3-ST7 processes so that the determination frequencies for signal level 3 and signal level 2, and signal level 1 and signal level 0, calculated by the symbol count unit 13a, are in a desired ratio, thereby automatically adjusting the first threshold voltage Vth_Upper and the third threshold voltage Vth_Lower. In this case, the contacts of the switching unit (switch) 15 in Figure 2 remain switched in the direction indicated by the solid line, and the second threshold voltage Vth_Middle is automatically adjusted to the voltage value of the DC average voltage of the PAM4 signal.

[0065] Furthermore, when the automatic adjustment process of the threshold voltage Vth is completed by user operation from the operation display unit 7, the contacts of the switching unit (switch) 15 may remain in the direction shown by the solid line in Figure 2, but they can also be automatically switched back to the direction shown by the dotted line in Figure 2 so that the user can set the second threshold voltage Vth_Middle to an arbitrary initial value.

[0066] Incidentally, as mentioned above, the method of this embodiment determines in real time the direction of the deviation of the threshold voltage Vth and its convergence, but it does not directly determine the position of the threshold voltage Vth (excluding Vth_Middle). Therefore, it is necessary to repeat the estimation of the direction of the deviation of the threshold voltage Vth and the adjustment of the threshold voltage Vth several times until convergence occurs, and if the initial value of the threshold voltage Vth is far from the optimal point, it may take time to converge.

[0067] Furthermore, in the above-described embodiment, since the bit determination result changes depending on the determination threshold voltage, the region where the eye is open becomes a dead zone and converges. Therefore, if the eye opening is sufficient, there is a possibility that a point shifted from the center of the eye may be estimated as the optimal point for the threshold voltage Vth and adjusted accordingly. For this reason, if it is required that the point be not only within the eye opening but also at the center of the eye opening, it is preferable to perform a conventionally known process in which the threshold voltage Vth is swept up and down until an error is detected and the midpoint of the sweep result is found.

[0068] Thus, according to this embodiment, the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower) is adjusted by estimation based on the symbol count ratio during measurement, making it possible to track and adjust the threshold voltage Vth in real time even with PAM4 signals. In principle, this method can be applied even when the signal levels 0, 1, 2, and 3 of the PAM4 signal are not evenly distributed, or when the midpoint is not the optimal point for the threshold voltage Vth.

[0069] Furthermore, instead of providing a separate circuit for detecting the threshold voltage Vth, the threshold voltage Vth is adjusted using the results of the bit determination unit 11 (11A, 11B, 11C). This allows the bit error measurement unit 4 to perform adjustment by estimating the threshold voltage Vth using the same receiver device as conventional error rate measurement devices. As a result, there is no need to consider the difference in characteristics between the circuit for adjusting the threshold voltage Vth by estimation and the bit error measurement unit 4. This enables correlated adjustment even when measuring bit errors, while also allowing for a simple and inexpensive device configuration.

[0070] The best mode of the error rate measuring device and error rate measuring method according to the present invention has been described above, but the present invention is not limited by this description and drawings. That is, other modes, examples, and operational techniques made by those skilled in the art based on this mode are all included in the scope of the present invention. [Explanation of Symbols]

[0071] 1. Error rate measuring device 2 clock sources 3. Variable delay circuit 4-bit error measurement unit 5. Low-pass filter (LPF) 6. Analog-to-digital converter (ADC) 7 Operation display section 8 Control Unit 8a Evaluation function calculation unit 8b Operation amount calculation section 11 (11A, 11B, 11C) bit determination machine 12 Decoders 13 Count section 13a Symbol count section 13b Error Count Section 14 (14A, 14C) Threshold voltage adjustment section 15. Switching mechanism (switch)

Claims

1. A symbol counting unit (13a) counts the determination results as PAM symbols over an arbitrary time period, with each of the following arbitrarily set threshold voltages: a first threshold voltage (Vth_Upper) for determining whether the signal level of the PAM4 signal is 2 or 3 using a first bit determination unit (11C), a second threshold voltage (Vth_Middle) for determining whether the signal level of the PAM4 signal is 1 or 2 using a second bit determination unit (11B), and a third threshold voltage (Vth_Lower) for determining whether the signal level of the PAM4 signal is 0 or 1 using a third bit determination unit (11A), and calculates the ratio of the determination frequencies of the signal level being 3 and 2, and the ratio of the determination frequencies of the signal level being 1 and 0. Each of the following functions is defined as an evaluation function: if the signal level is shifted towards a higher level, it asymptotically approaches +1; if the signal level is shifted towards a lower level, it asymptotically approaches -1; and if it is balanced, it approaches 0; for the first threshold voltage, it is a function obtained by subtracting signal level 3 from signal level 2 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; for the third threshold voltage, it is a function obtained by subtracting signal level 1 from signal level 0 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; and an evaluation function calculation unit (8a) calculates the evaluation function for each of the first and third threshold voltages from the detection result of the symbol count value of the symbol count unit. An operation amount calculation unit (8b) calculates a voltage operation amount for each of the first and third threshold voltages from the estimation results, using the evaluation functions for each of the first and third threshold voltages calculated by the evaluation function calculation unit, A low-pass filter (5) extracts the DC average voltage of the PAM4 signal, An A / D converter (6) detects the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter, An error rate measuring device comprising: a control unit (8) that adjusts and controls the second threshold voltage so that it becomes the voltage value of the DC average voltage of the PAM4 signal detected by the A / D converter; adjusts and controls the first threshold voltage based on a voltage manipulation amount calculated by the manipulated amount calculation unit so that the frequency of judgment of the signal level being 3 and 2 is in a desired ratio; and adjusts and controls the third threshold voltage based on a voltage manipulation amount calculated by the manipulated amount calculation unit so that the frequency of judgment of the signal level being 1 and 0 is in a desired ratio.

2. The first bit determination unit (13a) calculates the determination results as PAM symbols over an arbitrary time period, with a first threshold voltage (Vth_Upper) for determining whether the signal level of the PAM4 signal is 2 or 3 using a first bit determination unit (11C), a second threshold voltage (Vth_Middle) for determining whether the signal level of the PAM4 signal is 1 or 2 using a second bit determination unit (11B), and a third threshold voltage (Vth_Lower) for determining whether the signal level of the PAM4 signal is 0 or 1 using a third bit determination unit (11A), respectively, and the symbol counting unit (13a) calculates the ratio of the determination frequencies of signal levels 3 and 2 and the ratio of the determination frequencies of signal levels 1 and 0. Each of the following functions is defined as an evaluation function: if the signal level is shifted towards a higher level, it asymptotically approaches +1; if the signal level is shifted towards a lower level, it asymptotically approaches -1; and if it is balanced, it approaches 0; for the first threshold voltage, it is a function obtained by subtracting signal level 3 from signal level 2 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; for the third threshold voltage, it is a function obtained by subtracting signal level 1 from signal level 0 of the PAM4 signal, multiplying by 2, and dividing that value by the total number of symbols; and the evaluation function is calculated by the evaluation function calculation unit (8a) from the detection result of the symbol count value of the symbol count unit. The steps include: using the evaluation functions for each of the first and third threshold voltages calculated by the evaluation function calculation unit to estimate the deviation between the first and third threshold voltages, and calculating the voltage manipulation amount for each of the first and third threshold voltages from the estimation results using the manipulation amount calculation unit (8b); The steps include: extracting the DC average voltage of the PAM4 signal using a low-pass filter (5); The steps include detecting the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter using an A / D converter (6), The steps include: adjusting and controlling the first threshold voltage by the control unit (8) based on a voltage manipulation amount calculated by the manipulation amount calculation unit so that the frequency of determination of the signal levels 3 and 2 is in a desired ratio; The steps include: adjusting and controlling the second threshold voltage by the control unit so that it becomes the voltage value of the DC average voltage of the PAM4 signal detected by the A / D converter; An error rate measurement method characterized by including the step of adjusting and controlling the third threshold voltage by the control unit based on a voltage manipulation amount calculated by the manipulation amount calculation unit so that the frequency of determination of the signal level as 1 and 0 is in a desired ratio.

3. The error rate measuring device according to claim 1, characterized in that the second threshold voltage can be set to any initial value.

4. The error rate measurement method according to claim 2, characterized in that it includes the step of setting the second threshold voltage to an arbitrary initial value.

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