Error rate measurement device and error rate measurement 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 technology]

[0002] Conventionally, when an error rate measurement device judges the bits of an NRZ signal whose signal levels consist of two values, 0 and 1, as shown in Figure 6, it is necessary to determine the threshold voltage Vth that serves as the basis for judgment. A common method for determining the threshold voltage Vth is to sweep the threshold voltage Vth once and then select the threshold voltage Vth that gives the best bit error rate (BER), which is expected to be the most accurate and has a simple algorithm.

[0003] On the other hand, this method has the disadvantages of being unable to perform in real time, as it is necessary to interrupt the measurement and sweep the threshold voltage Vth, and it cannot keep up with cases in which the threshold voltage Vth of the measurement signal may fluctuate during the process.

[0004] Therefore, the error rate measurement device is equipped with an Auto Adjust function to estimate in real time the threshold voltage Vth, which is the reference for this judgment. This takes advantage of the fact that for NRZ signals with a uniform mark rate, such as random patterns, the optimum threshold voltage Vth is equal to the signal's average DC voltage, and can be realized by a low-pass filter to extract the signal's average DC voltage and an A / D converter (ADC) to detect that voltage value.

[0005] Incidentally, an NRZ signal such as that shown in Figure 6 has one threshold voltage Vth, but a PAM4 signal with four signal levels (0, 1, 2, and 3) as shown in Figure 5 requires three threshold voltages: Vth_Upper, Vth_Middle, and Vth_Lower. Of these, only Vth_Middle can be calculated using the DC average, and Vth_Upper and Vth_Lower cannot be estimated. Therefore, although Auto Adjust is available for PAM4 signals, there is currently an issue in that only Vth_Middle can be achieved.

[0006] One possible 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 is saturated, and this method is disclosed in Patent Document 1 listed below. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6025883 Summary of the Invention [Problem to be solved by the invention]

[0008] However, with PAM4 signals, signal levels 0, 1, 2, and 3 are not necessarily evenly distributed relative to the peak-to-peak voltage of the signal, and for example, the exact midpoint between signal level 3 and signal level 2 is not necessarily optimal for Vth_Upper, which can lead to errors in the estimation.In addition, this method requires sweeping the threshold voltage Vth to determine the region where the digital value saturates, which means it cannot be used for real-time estimation.

[0009] Therefore, the present invention has been made in consideration of the above problems, and has as its object to provide an error rate measurement device and an error rate measurement method that are capable of adjusting 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] In order to achieve the above object, the error rate measurement device according to claim 1 of the present invention comprises: a symbol counting unit 13a that counts PAM symbol determination results at an arbitrary time in a state where a first threshold voltage Vth_Upper for determining whether the signal level of a PAM4 signal is 2 or 3, a second threshold voltage Vth_Middle for determining whether the signal level of the PAM4 signal is 1 or 2, and a third threshold voltage Vth_Lower for determining whether the signal level of the PAM4 signal is 0 or 1 are each arbitrarily set, and calculates a ratio of the determination frequencies of the signal levels being 3 and 2 and a ratio of the determination frequencies of the signal levels being 1 and 0; a low-pass filter 5 that extracts the DC average voltage of the PAM4 signal; an A / D converter 6 that detects the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter; a control unit (8) that adjusts and controls the second threshold voltage so that it becomes a voltage value of the DC average voltage of the PAM4 signal detected by the A / D converter, adjusts and controls the first threshold voltage so that the frequency of determining the signal levels as 3 and 2 becomes a desired ratio, and adjusts and controls the third threshold voltage so that the frequency of determining the signal levels as 1 and 0 becomes a desired ratio.

[0011] The error rate measurement method according to claim 2 of the present invention includes the steps of counting PAM symbol determination results at an arbitrary time in a state where a first threshold voltage Vth_Upper for determining whether the signal level of a PAM4 signal is 2 or 3, a second threshold voltage Vth_Middle for determining whether the signal level of the PAM4 signal is 1 or 2, and a third threshold voltage Vth_Lower for determining whether the signal level of the PAM4 signal is 0 or 1 are each arbitrarily set, and calculating, by a symbol counting unit 13a, a ratio of the determination frequencies of the signal levels being 3 and 2 and a ratio of the determination frequencies of the signal levels being 1 and 0; extracting an average DC voltage of the PAM4 signal using a low-pass filter; detecting, by an A / D converter 6, the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter; a step of adjusting and controlling the first threshold voltage by a control unit 8 so that the frequency of determination of the signal levels 3 and 2 is a desired ratio; adjusting and controlling the second threshold voltage by the control unit so that the second threshold voltage becomes equal to the voltage value of the DC average voltage of the PAM4 signal detected by the A / D converter; and adjusting and controlling the third threshold voltage by the control unit so that the frequency of determining that the signal level is 1 and 0 has a desired ratio. [Effects of the Invention]

[0012] According to the present invention, the threshold voltage Vth can be adjusted in real time even for PAM4 signals by estimation based on the symbol count ratio during measurement. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a block diagram showing the internal configuration of an error rate measurement device according to the present invention; [Figure 2] FIG. 2 is a block diagram relating to an automatic adjustment function (Auto Adjust) of a threshold voltage in FIG. 1. [Figure 3]1 is a flowchart of a method for automatically adjusting a threshold voltage by the error rate measurement device according to the present invention. [Figure 4] 4 is a flowchart following FIG. 3 showing the method for automatically adjusting the threshold voltage by the error rate measurement device according to the present invention. [Figure 5] FIG. 1 is a diagram illustrating an example of the relationship between signal levels 0, 1, 2, and 3 of a PAM4 signal and threshold voltages Vth_Upper, Vth_Middle, and Vth_Lower. [Figure 6] 10 is a diagram showing an example of the relationship between the signal levels 0 and 1 of an NRZ signal and the threshold voltages Vth1, Vth2, and Vth3. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0015] [Summary of the Invention] The error rate measurement device and error rate measurement method according to the present invention employ a technique that focuses on the ratio of bits (symbols) determined by a receiver, and has the function of automatically adjusting the threshold voltage Vth for PAM4 signals in real time.

[0016] First, in order to make it easier to understand the function of the present invention, an example in which a binary NRZ signal is used will be described with reference to FIG.

[0017] In FIG. 6, when the threshold voltage Vth serving as the judgment standard is Vth1, if the target signal is a random pattern such as a PRBS, the occurrence frequencies of signal level 0 and signal level 1 will be equal, and the judgment frequency should also be equal.

[0018] However, if the threshold voltage Vth shifts upward like Vth2 in FIG. 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, it is possible to estimate whether the current threshold voltage Vth is higher or lower by evaluating which of the judged bits is dominant.Then, by adjusting the threshold voltage Vth based on the estimation, the point where the judgement ratio is balanced can be regarded as the optimum point for the threshold voltage Vth.

[0020] 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 target desired ratio can be changed accordingly.

[0021] Next, this is applied to a PAM4 signal. For example, taking the PAM4 signal in Figure 5 as an example, 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 frequency of determining signal level 3 to signal level 2 Second threshold voltage Vth_Middle: Ratio of the frequency of determining signal level 2 and signal level 1 or DC average voltage of the PAM4 signal Third threshold voltage Vth_Lower: Ratio of frequency of determining signal level 1 and signal level 0

[0023] It should be noted that this method does not uniquely determine the threshold voltage Vth to be adjusted. For example, the frequency of signal level 2 determination actually correlates with both Vth_Upper and Vth_Middle, so it is not possible to uniquely determine whether Vth_Upper should be adjusted downward or Vth_Middle should be adjusted upward. In this regard, for PAM4 signals, if Vth_Middle is determined by estimating the DC average voltage, all three threshold voltages Vth (Vth_Upper, Vth_Middle, Vth_Lower) can be uniquely estimated.

[0024] Therefore, if the total number of symbols measured per arbitrary time is Symbol_total, then by using the following calculation formulas (Equations (1) to (3)), it is possible to realize a function that approaches +1 when the threshold voltages Vth (Vth_Upper, Vth_Middle, Vth_Lower) are shifted upward, approaches -1 when they are shifted downward, and becomes 0 when they are balanced. In this embodiment, this function is defined as an evaluation function. Note that for the second threshold voltage Vth_Middle using a DC average voltage, the value of the second threshold voltage Vth_Middle is directly obtained, rather than using an evaluation function to evaluate the shift.

[0025] 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 by subtracting signal level 3 from signal level 2 of the PAM4 signal, doubling the result, and dividing the result by the total number of symbols using the above formula (1).

[0027] Second threshold voltage Vth_Middle: DC average voltage of the PAM4 signal...Equation (2)

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

[0029] 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 by subtracting signal level 1 from signal level 0 of the PAM4 signal, multiplying the result by two, and dividing the result by the total number of symbols using the above formula (3).

[0031] [Configuration of error rate measurement device] Next, the configuration of an error rate measurement device for automatically adjusting the threshold voltage of the PAM4 signal described above will be described with reference to FIG.

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

[0033] The clock source 2 generates a clock with a reference frequency and inputs the generated clock with the reference frequency to the variable delay device 3 .

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

[0035] The number of bit decision devices 11 provided corresponds to the number of eyes of the PAM signal. Here, since the target signal is a PAM4 signal, three bit decision devices 11 (11A, 11B, 11C) are provided.

[0036] Each of the bit decision devices 11A, 11B, and 11C has an input terminal for a determination threshold voltage for bit decision and a punching clock 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 device 3 by the control unit 8.

[0037] The decoder 12 decodes the results obtained by each bit decision device 11 (11A, 11B, 11C) into a decision result as a PAM symbol.

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

[0039] The symbol counting unit 13a counts the judgment results (judged as one of signal levels 0, 1, 2, or 3) of the PAM symbols decoded by the decoder 12 at any time (or a predetermined measurement time), and calculates the ratio of the judgment frequencies between signal levels 3 and 2, and the ratio of the judgment frequencies between signal levels 1 and 0.

[0040] The error counting unit 13b detects errors by comparing the result of the determination of the PAM symbol decoded by the decoder 12 at any time (or a preset measurement time) with a reference pattern sequence. The error determination not only counts the number of errors but also distinguishes between the expected symbols and the actually determined symbols.

[0041] The threshold voltage adjuster 14 (14A, 14C) is composed of an adder / subtractor, and adds or subtracts a voltage manipulated variable (adjustment amount) of the corresponding threshold voltage Vth calculated by a manipulated variable calculation unit 8b (described later) of the control unit 8 to or from the initial value of the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower). Specifically, the threshold voltage adjuster 14A adds or subtracts the voltage manipulated variable (adjustment amount) of the first threshold voltage Vth_Upper calculated by the manipulated variable calculation unit 8b to or from the initial value of the first threshold voltage Vth_Upper. The threshold voltage adjuster 14C adds or subtracts the voltage manipulated variable (adjustment amount) of the third threshold voltage Vth_Lower calculated by the manipulated variable calculation unit 8b to or from 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 out 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 result in poor responsiveness, so in consideration of this balance, a low-pass filter 5 with a cutoff frequency of approximately 1 kHz to 1 MHz is desirable for signals on the order of gigabits per second. 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 and display unit 7 combines the functions of an operation unit using various keys, switches, buttons, soft keys on the display screen, etc. that are equipped on the main body of the error rate measurement device 1, with the functions of a display unit using display devices such as a liquid crystal display, an EL (electroluminescence) display, a CRT, etc.

[0044] The operation display unit 7 allows the user to input various information required for automatic adjustment of the threshold voltage Vth of the PAM4 signal and error rate measurement (for example, the initial value of the threshold voltage Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower), phase amount (delay device operation amount), pattern sequence, ON / OFF of the automatic adjustment function (Auto Adjust) of the threshold voltage Vth, etc.), and, under the control of the control unit 8, displays a setting item screen and a measurement result screen for setting the various pieces of information, and displays operation objects such as buttons, softkeys, pull-down menus, and input boxes for setting various conditions on the setting item screen.

[0045] In this embodiment, the operation and display unit 7 is illustrated as having both an operation unit and a display unit, but the operation and display unit 7 may have a separate operation unit and display unit.

[0046] As shown in FIGS. 1 and 2, the control unit 8 includes an evaluation function calculation unit 8a and an operation amount calculation unit 8b.

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

[0048] The operation amount calculation unit 8b estimates the deviation of the threshold voltage Vth (Vth_Upper, Vth_Lower) using the evaluation function for each threshold voltage Vth (Vth_Upper, Vth_Lower) calculated by the evaluation function calculation unit 8a, and calculates the voltage operation amount (adjustment amount) for each threshold voltage Vth (Vth_Upper, Vth_Lower) from the estimation result.

[0049] The control unit 8 controls the variable delay unit 3, the operation and display unit 7, the bit decision units 11 (11A, 11B, 11C), and the error decision 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 required for automatic adjustment of the threshold voltage Vth of the PAM signal and error rate measurement (e.g., initial values ​​of the threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower), phase amounts (delay device operation amounts), pattern strings, ON / OFF of the automatic adjustment function (Auto Adjust) of the threshold voltage Vth, etc.) via the operation and display unit 7, controls the display of setting item screens, measurement result screens, etc. on the operation and display unit 7, controls the variable delay unit 3 using the set phase amounts (delay device operation amounts), controls the input of initial values ​​of the threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, third threshold voltage Vth_Lower) to the bit decision devices 11 (11A, 11B, 11C) and voltage operation amounts (adjustment amounts) calculated by the operation amount calculation unit 8b to the threshold voltage adjustment units 14 (14A, 14C), and controls the input of pattern strings to the error determination unit 13. However, the pattern sequence is a setting item when performing error measurement simultaneously with automatic adjustment of the threshold voltage Vth, and is not an essential setting item when only automatic adjustment of the threshold voltage Vth is performed.

[0051] [Automatic adjustment of threshold voltage Vth] Next, a method for automatically adjusting the threshold voltage Vth of a PAM4 signal using the error rate measurement apparatus 1 configured as above will be described with reference to FIGS.

[0052] To automatically adjust the threshold voltage Vth of the PAM4 signal, the operation and 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 device operation amount), the pattern sequence, the automatic adjustment function (Auto Adjust): ON for the threshold voltage Vth, etc. (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 set arbitrarily by the user. In this embodiment, the bit decision devices 11 (11A, 11B, 11C) and the decoder 12 are always in operation, but the user can also set the measurement time arbitrarily using the operation and display unit 7.

[0054] After the above settings are made, when measurement of the PAM4 signal is started, the control unit 8 switches the contacts of the switching unit (switch) 15 in Fig. 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 decision device 11B, 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 decision devices 11 (11A, 11B, 11C), and controls the variable delay devices 3 with the set phase amount. As a result, the variable delay devices 3 input punched clocks based on the phase amount to each bit decision device 11 (11A, 11B, 11C) (ST2).

[0055] Then, each bit decision device 11 (11A, 11B, 11C) compares the signal voltage level when a predetermined position in the time axis direction of the eye opening of the PAM4 signal is punched with the punching clock from the variable delay device 3 with the set threshold voltage Vth (first threshold voltage Vth_Upper: initial value the first time, adjusted value obtained by adding or subtracting a voltage manipulation amount (adjustment amount) from the second time onwards; second threshold voltage Vth_Middle: initial value the first time, voltage value of the DC average voltage from the second time onwards; third threshold voltage Vth_Lower: initial value the first time, adjusted value obtained by adding or subtracting a voltage manipulation amount (adjustment amount) from the second time onwards) and makes a bit decision (ST3).

[0056] Next, the decoder 12 decodes the bit determination results of the bit determiners 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 judgment results as PAM symbols decoded by the decoder 12 at any time (or a predetermined measurement time), and calculates the ratio of the judgment frequencies between signal level 3 and signal level 2, and the ratio of the judgment frequencies between signal level 1 and signal level 0 (ST4).

[0058] When a pattern sequence is set by the operation display unit 7, the error counting unit 13b of the counting unit 13 compares the decoded judgment results of each bit judgment device 11 (11A, 11B, 11C) with the set pattern sequence at any time (or a preset measurement time) to determine whether or not an error exists, thereby detecting the error.

[0059] Next, the evaluation function calculation unit 8a of the control unit 8 calculates an evaluation function for each of the first threshold voltage Vth_Upper and the third threshold voltage Vth_Lower from the detection result of the symbol count value of the symbol count unit 13a. At this time, the second threshold voltage Vth_Middle is set to the voltage value of the DC average voltage of the PAM4 signal acquired from the output of the A / D converter 6 (ST5).

[0060] Then, the control unit 8's operation amount calculation unit 8b uses the evaluation function calculated by the evaluation function calculation unit 8a to estimate the direction of deviation of the threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) and whether or not it has converged, and calculates a voltage operation amount (adjustment amount) for each threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) from the estimation result (ST6).

[0061] Thereafter, the control unit 8 switches the contact of the switching unit (switch) 15 in FIG. 2 in the direction indicated by the solid line to input the voltage value of the DC average voltage of the PAM4 signal to the bit decision device 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 decision devices 11A, 11C via the corresponding threshold voltage adjustment units 14A, 14C (ST7).

[0062] Then, the voltage manipulation amount (adjustment amount) of the corresponding threshold voltage Vth is added to or subtracted from the currently set threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) to automatically adjust the threshold voltage Vth (first threshold voltage Vth_Upper, third threshold voltage Vth_Lower) (ST8). Also, the contact of the switching unit (switch) 15 in FIG. 2 is switched in the direction indicated 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] The control unit 8 repeats the processes of ST3 to ST7 until it determines that the frequency of determining signal level 3 and signal level 2 and the frequency of determining signal level 1 and signal level 0 calculated by the symbol count unit 13a are equal to the desired ratio (ST9-Yes), and automatically adjusts the first threshold voltage Vth_Upper and the third threshold voltage Vth_Lower under the control of the control unit 8. At this time, the contacts of the switching unit (switch) 15 in FIG. 2 are switched in the direction indicated 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. When the user performs an operation to end the automatic adjustment of the threshold voltage Vth on the operation and display unit 7 (ST10-Yes), the automatic adjustment of the threshold voltage Vth ends.

[0064] 4, the determination process of ST8 can be omitted, and the first threshold voltage Vth_Upper and the third threshold voltage Vth_Lower can be automatically adjusted by repeating the processes of ST3 to ST7 under the control of the control unit 8 so that the determination frequencies of signal level 3 and signal level 2 and the determination frequencies of signal level 1 and signal level 0 calculated by the symbol count unit 13a reach a desired ratio. In this case, the contacts of the switching unit (switch) 15 in FIG. 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 process of automatically adjusting the threshold voltage Vth is completed by user operation on 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 may 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 any initial value.

[0066] As described above, the method of this embodiment determines in real time the direction of deviation of the threshold voltage Vth and its convergence, but 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 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 optimum point, it may take a long time to converge.

[0067] Furthermore, in the above-described embodiment, the bit decision result changes depending on the decision threshold voltage, and therefore the eye opening region becomes a dead zone, resulting in convergence. Therefore, if the eye opening is large enough, there is a possibility that a point that is off the center of the eye may be estimated as the optimal point for the threshold voltage Vth and adjusted accordingly. Therefore, if it is required that the threshold voltage Vth be at the center of the eye opening, rather than just within the eye opening, it is preferable to perform a conventionally well-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] As described above, according to this embodiment, the threshold voltages Vth (first threshold voltage Vth_Upper, second threshold voltage Vth_Middle, and third threshold voltage Vth_Lower) are adjusted by estimation based on the symbol count ratio during measurement, so that it is possible to adjust the threshold voltage Vth in real time even for PAM4 signals. In principle, this method is applicable even when the signal levels 0, 1, 2, and 3 of the PAM4 signal are not evenly distributed or when the intermediate value is not the optimum point for the threshold voltage Vth.

[0069] Furthermore, since the threshold voltage Vth is adjusted using the results of the bit decision devices 11 (11A, 11B, 11C) rather than providing a separate circuit for detecting the threshold voltage Vth, it is possible to achieve adjustment by estimating the threshold voltage Vth using the same receiver device as the bit error measurement unit 4 used in a conventional error rate measurement device. This eliminates the need to consider the difference in characteristics between the circuit for adjustment by estimating the threshold voltage Vth and the bit error measurement unit 4, making it possible to perform correlated adjustments even when measuring bit errors, while also allowing for a simple and inexpensive device configuration.

[0070] Although the best mode for implementing the error rate measurement device and the error rate measurement method according to the present invention has been described above, the present invention is not limited to the description and drawings of this mode. In other words, all other modes, embodiments, and operational techniques that can be implemented by those skilled in the art based on this mode are naturally included in the scope of the present invention. [Explanation of symbols]

[0071] 1 Error rate measurement device 2 Clock Sources 3 Variable Delay 4-bit error measurement section 5 Low-pass filter (LPF) 6 Analog-to-Digital Converter (ADC) 7 Operation display section 8 Control Unit 8a Evaluation function calculation section 8b Operation amount calculation section 11 (11A, 11B, 11C) Bit Detector 12 Decoder 13 Counting section 13a Symbol Counting Unit 13b Error count section 14 (14A, 14C) Threshold voltage adjustment unit 15 Switch

Claims

1. a symbol counting unit (13a) that counts PAM symbol determination results at an arbitrary time in a state where a first threshold voltage (Vth_Upper) for determining whether the signal level of the PAM4 signal is 2 or 3, a second threshold voltage (Vth_Middle) for determining whether the signal level of the PAM4 signal is 1 or 2, and a third threshold voltage (Vth_Lower) for determining whether the signal level of the PAM4 signal is 0 or 1 are each arbitrarily set, and calculates a ratio of the determination frequencies of the signal levels being 3 and 2 and a ratio of the determination frequencies of the signal levels being 1 and 0; a low-pass filter (5) for extracting the DC average voltage of the PAM4 signal; an A / D converter (6) that detects the voltage value of the DC average voltage of the PAM4 signal extracted by the low-pass filter; and a control unit (8) that adjusts and controls the second threshold voltage so that it becomes a voltage value of a DC average voltage of the PAM4 signal detected by the A / D converter, adjusts and controls the first threshold voltage so that the frequency of decision of the signal levels 3 and 2 becomes a desired ratio, and adjusts and controls the third threshold voltage so that the frequency of decision of the signal levels 1 and 0 becomes a desired ratio.

2. a step of counting the determination results as PAM symbols at an arbitrary time in a state where a first threshold voltage (Vth_Upper) for determining whether the signal level of the PAM4 signal is 2 or 3, a second threshold voltage (Vth_Middle) for determining whether the signal level of the PAM4 signal is 1 or 2, and a third threshold voltage (Vth_Lower) for determining whether the signal level of the PAM4 signal is 0 or 1 are each arbitrarily set, and calculating a ratio of the determination frequencies of the signal levels being 3 and 2 and a ratio of the determination frequencies of the signal levels being 1 and 0 by a symbol counting unit (13a); extracting the DC average voltage of the PAM4 signal by a low-pass filter (5); 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); a step of adjusting and controlling the first threshold voltage by a control unit (8) so that the frequency of determination of the signal levels 3 and 2 is a desired ratio; adjusting and controlling the second threshold voltage by the control unit so that the second threshold voltage becomes a voltage value of a DC average voltage of the PAM4 signal detected by the A / D converter; and a step of adjusting and controlling the third threshold voltage by the control unit so that the frequency of determining that the signal level is 1 and 0 has a desired ratio.

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