Differential skew generating device and differential skew generating method

Abstract

To enable varying a skew amount and waveform observation.SOLUTION: A differential skew generation device 1 includes: a clock oscillator 2 for oscillating and outputting a clock signal; a first pattern generator 4A for outputting a positive signal to an object W to be measured from a first output terminal 11A at timing of a signal in which a phase angle is adjusted by IQ modulation of the clock signal; a second pattern generator 4B for outputting a negative signal to the object W to be measured from a second output terminal 12B at timing of a signal in which a phase angle is adjusted by IQ modulation of the clock signal; and a monitoring device 5 connected to a second output terminal 12A of a first signal generator 4A and a first output terminal 11B of a second signal generator 4B. The first pattern generator 4A and the second pattern generator 4B operate in synchronization with each other and allocate an integer unit of a UI of a set skew amount to transmission timing shift and a decimal unit as IQ modulation to control.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a differential skew generating device and a differential skew generating method for generating a differential skew, which is a time difference between a positive signal and a negative signal. [Background technology]

[0002] Differential transmission has excellent resistance to common-mode noise, and is therefore used in many high-speed serial communications that require high waveform quality. Because differential transmission is based on the premise that the arrival timing of the positive and negative signals is equal, if the timing difference (skew) is not sufficiently small, the effectiveness of differential transmission cannot be realized and may even have a negative effect.

[0003] In particular, when differential signals pass through differential lines, capacitive, inductive, or a combination of these couplings occurs between the positive and negative signals, causing distortion and reflection in the signals due to skew, significantly degrading waveform quality.

[0004] In recent years, high-speed serial communications have been using faster modulation rates and multi-level modulation technology, for example, PCIe Gen6 uses the 32 Gbaud PAM4 modulation method. As a result, requirements for waveform quality have become more stringent, and it is becoming increasingly important to evaluate the impact of skew on transmission paths and devices.

[0005] Incidentally, a known technique for monitoring the above-mentioned skew is, for example, that using a divider as disclosed in Patent Document 1 below. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6890625 Summary of the Invention [Problem to be solved by the invention]

[0007] However, monitoring skew requires inserting a divider or pickoff-tee, as disclosed in Patent Document 1, into the signal path and monitoring the branched signal, which poses problems such as degradation of signal quality due to the insertion of the divider or pickoff-tee and the inclusion of insertion loss.To solve this problem, a method of monitoring without including loss is usually used in which the device under test and an oscilloscope are reconnected each time, but this complicates the measurement procedure and makes it impossible to simultaneously observe the behavior of the measuring instrument and the waveform, significantly reducing the efficiency of debugging the device under test, etc.

[0008] Therefore, the present invention has been made in consideration of the above problems, and aims to provide a differential skew generating device and a differential skew generating method that can change the differential skew to a desired skew amount without performing mechanical operations and that can observe the waveform. [Means for solving the problem]

[0009] In order to achieve the above object, a differential skew generating device according to claim 1 of the present invention comprises: a clock oscillator 2 that oscillates and outputs a clock signal; The clock signal is IQ-modulated to adjust the phase angle of the signal, and the positive signal is output from the first output terminal to the device under test W. The first IQ modulator 13A, the first pattern generating unit 14A, and the first control unit 15A are included. a first signal generator 4A; The first one outputs a positive signal. 3 output terminal 11B and the negative signal output terminal 11C. 4 and an output terminal 12B of the clock signal, and the clock signal is IQ-modulated and the phase angle is adjusted. 4 The negative signal is output from the output terminal of the The second IQ modulator 13B, the second pattern generating unit 14B, and the second control unit 15B are included. a second signal generator 4B; The second output terminal of the first signal generator and the second output terminal of the second signal generator 3 a monitor device 5 connected to the output terminal of the The first signal generator and the second signal generator operate in synchronization with each other, and a transmission timing shifter is configured to shift the transmission timing of the signal by an integer unit of UI of the set skew amount. to , the fractional unit of the UI of the set skew amount is applied to IQ modulation. each Allocation 、 the first control unit, when a skew amount in units of UI is set, controls at least one of the first IQ modulator and the first pattern generation unit by dividing the amount into a required IQ modulation amount and a required transmission timing shift amount; When the skew amount in units of UI is set, the second control unit divides it into a required IQ modulation amount and a transmission timing shift amount, and controls the second IQ modulator and the second pattern generation unit. The present invention is characterized by controlling at least one of the two.

[0010] The differential skew generator according to claim 2 of the present invention is the differential skew generator according to claim 1, When the skew amount is set in units of time, the skew amount set in the units of time Tobi The method is characterized by multiplying the bit rate by the amount of skew in UI units.

[0011] A differential skew generating method according to claim 3 of the present invention comprises the steps of: oscillating and outputting a clock signal; It has a first output terminal 11A that outputs a positive signal and a second output terminal 12A that outputs a negative signal. and a first IQ modulator 13A, a first pattern generating unit 14A, and a first control unit 15A. a step of outputting the positive signal from the first output terminal to the device under test W at the timing of a signal obtained by IQ-modulating the clock signal and adjusting the phase angle in the first signal generator; The first one outputs a positive signal. 3 output terminal 11B and the negative signal output terminal 11C. 4 and an output terminal 12B. and a second IQ modulator 13B, a second pattern generating unit 14B, and a second control unit 15B. In the second signal generator 4B, the clock signal is IQ modulated and the phase angle is adjusted to generate the 4 outputting the negative signal from an output terminal of the The second output terminal of the first signal generator and the second output terminal of the second signal generator 3connecting the output terminal of the a transmission timing shifter that synchronizes the first signal generator and the second signal generator and shifts the transmission timing of the signal by an integer unit of UI of the set skew amount; to , the fractional unit of the UI of the set skew amount is applied to IQ modulation. each Allocation and When the skew amount in UI units is set, the first control unit controls at least one of the first IQ modulator and the first pattern generation unit by dividing the amount into a required IQ modulation amount and a required transmission timing shift amount; When the skew amount in UI units is set, the second control unit divides it into the required IQ modulation amount and transmission timing shift amount, and controls the second IQ modulator and the second pattern generation unit. and a step of controlling at least one of the two.

[0012] The differential skew generation method according to claim 4 of the present invention is the differential skew generation method according to claim 3, further comprising: When the skew amount is set in units of time, the skew amount set in the units of time Tobi The method is characterized by multiplying the bit rate by the amount of skew in UI units. [Effects of the Invention]

[0013] According to the present invention, by generating positive and negative signals separately from two signal generators, it is possible to vary the amount of differential skew with excellent variable width and resolution without mechanical operation, and it is also possible to simultaneously observe the waveforms without modifying the paths through which the positive and negative signals are input to the device under test. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a block diagram showing the internal configuration of a differential skew generator according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0016] As shown in FIG. 1, the differential skew generating device 1 of this embodiment generates a differential skew, which is the time difference between a positive signal and a negative signal, which are repetitive signals of opposite phases that are input to a device under test W as two single-ended signals in a differential pair, and is generally configured with a clock oscillator 2, a setting unit 3, a plurality of signal generators 4 (a first signal generator 4A, a second signal generator 4B), and a monitor device 5.

[0017] The differential skew generating device 1 synchronizes the operation of the first signal generator 4A and the second signal generator 4B, manipulates the internal delay amount in each of the first signal generator 4A and the second signal generator 4B, and inputs a positive signal and a negative signal that have been varied to the desired skew amount to the device under test W, thereby making it possible to vary the differential skew to the desired skew amount and observe the waveform.

[0018] In Fig. 1, a positive signal is represented as Pos and a negative signal as Neg. Also, in Fig. 1, the first signal generator 4A and the second signal generator 4B are illustrated as separate blocks, but they can also be configured as a single module.

[0019] The clock oscillator 2 oscillates and outputs a clock signal of a required frequency using a square wave signal or a sine wave signal. The clock signal oscillated and output from the clock oscillator 2 is input to an IQ modulator 13A (described later) of the first signal generator 4A and an IQ modulator 13B (described later) of the second signal generator 4B.

[0020] The setting unit 3 is a GUI that the user operates to input information, and sets the type of pattern (positive signal and negative signal) to be input to the device under test W, the bit rate, and the amount of skew for each of the first signal generator 4A and the second signal generator 4B to obtain the differential skew, as information necessary to generate the differential skew.

[0021] Here, the skew amount can be input in UI (unit interval) or time (sec). However, in this embodiment, as in the skew amount varying method described below, the skew amount is varied by IQ modulation and transmission timing shift, so it operates in the dimension of phase ≈ UI rather than time (sec). Note that transmission timing shift means shifting the transmission timing of signal bits (symbols in PAM3 or higher) by an integer unit of UI of the skew amount to be set: N bits (symbols). Therefore, when the skew amount is input and set by the setting unit 3 in units of time (sec), the skew amount is converted into UI units in the control unit 15A (described later) of the first signal generator 4A and the control unit 15B (described later) of the second signal generator 4B, based on the formula UI = skew amount time (sec) × bit rate, and set.

[0022] In this embodiment, since the resolution of the skew is in units of 2 mUI, rounding may be necessary depending on the input value in units of time (sec) input and set by the setting unit 3, as it does not correspond to 2 mUI units. In this case, the control unit 15A of the first signal generator 4A and the control unit 15B of the second signal generator 4B, which will be described later, calculate the time (sec) of the closest skew amount that can be realized in units of 2 mUI, and perform processing to overwrite and set the input value by the setting unit 3.

[0023] The first signal generator 4A has a first output terminal 11A that outputs a positive signal and a second output terminal 12A that outputs a negative signal, and operates synchronously with the second signal generator 4B. As shown in FIG. 1, the first signal generator 4A is configured with an IQ modulator 13A, a pattern generation unit 14A, and a control unit 15A, and the second output terminal 12A is connected to the monitor device 5.

[0024] The IQ modulator 13A performs IQ modulation on the clock signal from the clock oscillator 2 using the input I and Q signals based on the decimal unit of the skew amount set by the setting unit 3 under the control of the control unit 15A.

[0025] Under the control of the control unit 15A, the pattern generation unit 14A generates positive signals and negative signals of the desired pattern based on the type of pattern and bit rate set by the setting unit 3, shifts the transmission timing of the generated positive signals and negative signals based on the integer unit of the skew amount set by the setting unit 3, and outputs the transmission timing-shifted positive signals from the first output terminal 11A and the transmission timing-shifted negative signals from the second output terminal 12A.

[0026] The control unit 15A outputs a timing synchronization signal to a control unit 15B (described later) of the second signal generator 4B so that the first signal generator 4A operates in synchronization with the second signal generator 4B, and controls the IQ modulator 13A and the pattern generation unit 14A in an integrated manner.

[0027] Specifically, the control unit 15A outputs I and Q signals corresponding to the decimal unit of the skew amount set by the setting unit 3 to the IQ modulator 13A, and adjusts and controls the phase angle of the clock signal from the clock oscillator 2. The control unit 15A also generates positive and negative signals of a desired pattern based on the type of pattern and bit rate set by the setting unit 3, and controls the pattern generation unit 14A to perform a transmission timing shift based on the integer unit of the skew amount set by the setting unit 3. Furthermore, when the skew amount is input and set by the setting unit 3 in units of time (sec), the control unit 15A converts it into units of UI based on the formula UI = skew amount time (sec) × bit rate, and if rounding is necessary, calculates the closest skew amount time (sec) that can be achieved in 2mUI units, and performs processing to overwrite and set the value input by the setting unit 3.

[0028] The second signal generator 4B has the same configuration as the first signal generator 4A, and has a first output terminal 11B that outputs a positive signal and a second output terminal 12B that outputs a negative signal. The second signal generator 4B operates in synchronization with the first signal generator 4A, and as shown in FIG. 1, is configured with an IQ modulator 13B, a pattern generation unit 14B, and a control unit 15B, and the first output terminal 11B is connected to the monitor device 5.

[0029] The IQ modulator 13B performs IQ modulation on the clock signal from the clock oscillator 2 using the input I and Q signals based on the decimal unit of the skew amount set by the setting unit 3 under the control of the control unit 15B.

[0030] Under the control of the control unit 15B, the pattern generation unit 14B generates positive signals and negative signals of the desired pattern based on the type of pattern and bit rate set by the setting unit 3, shifts the transmission timing of the generated positive signals and negative signals based on the integer unit of the skew amount set by the setting unit 3, and outputs the transmission timing-shifted positive signals from the first output terminal 11B and the transmission timing-shifted negative signals from the second output terminal 12B.

[0031] The control unit 15B outputs a timing synchronization signal to the control unit 15A of the first signal generator 4A so that the second signal generator 4B operates in synchronization with the first signal generator 4A, and controls the IQ modulator 13B and the pattern generation unit 14B in an integrated manner.

[0032] Specifically, the control unit 15B outputs I and Q signals corresponding to the decimal unit of the skew amount set by the setting unit 3 to the IQ modulator 13B, and adjusts and controls the phase angle of the clock signal from the clock oscillator 2. The control unit 15B also generates patterns (positive and negative signals) based on the type of pattern and bit rate set by the setting unit 3, and controls the pattern generation unit 14B to perform a transmission timing shift based on the integer unit of the skew amount set by the setting unit 3. Furthermore, when the skew amount is input and set by the setting unit 3 in units of time (sec), the control unit 15B converts it into units of UI based on the formula UI = skew amount time (sec) × bit rate, and if rounding is necessary, calculates the closest skew amount time (sec) that can be achieved in 2mUI units, and performs processing to overwrite and set the value input by the setting unit 3.

[0033] The monitor device 5 is a monitoring device for observing the waveforms of the patterns (positive signals and negative signals) generated by the first signal generator 4A and the second signal generator 4B, and is connected to the free ports of the output terminals of the first signal generator 4A and the second signal generator 4B, i.e., the second output terminal 12A of the first signal generator 4A and the first output terminal 11B of the second signal generator 4B.

[0034] 1, the connection configuration of the monitor device 5 may be reversed. That is, the monitor device 5 may be connected to the first output terminal 11A of the pattern generation section 14A of the first signal generator 4A and the second output terminal 12B of the pattern generation section 14B of the second signal generator 4B. In this case, the negative signal output from the second output terminal 12A of the pattern generation section 14A of the first signal generator 4A and the positive signal output from the first output terminal 11B of the pattern generation section 14B of the second signal generator 4B are input to the device under test W.

[0035] Furthermore, the cables, differential lines, etc. used in the monitor device 5 are made to have characteristics that are as close as possible to those of the cables, differential lines, etc. used between the first signal generator 4A and the device under test W and between the second signal generator 4B and the device under test W, so as to enable more accurate waveform observation.

[0036] Next, a method for varying the amount of skew using the differential skew generator 1 configured as above will be described.

[0037] First, the setting unit 3 sets the type of pattern (positive signal, negative signal) to be input to the device under test W, the bit rate, and the amount of skew for each of the first signal generator 4A and the second signal generator 4B.

[0038] Here, the skew amount can be input in UI or time (sec). If the skew amount is input and set in units of time (sec), the control unit 15A of the first signal generator 4A and the control unit 15B of the second signal generator 4B convert it into UI units and set it based on the formula UI = skew amount time (sec) × bit rate.

[0039] Furthermore, if the input value in units of time (sec) input and set by the setting unit 3 does not match 2 mUI units and rounding is required, the control unit 15A of the first signal generator 4A and the control unit 15B of the second signal generator 4B calculate the time (sec) of the closest skew amount that can be achieved in 2 mUI units, and perform a process of overwriting the input value by the setting unit 3.

[0040] Then, when the skew amount in UI units is set, the control unit 15A of the first signal generator 4A and the control unit 15B of the second signal generator 4B allocate the necessary IQ modulation amount and transmission timing shift amount to control the IQ modulators 13A and 13B and the pattern generation units 14A and 14B.

[0041] Here, transmission timing shift can only manipulate the amount of skew in 1 UI increments, but the maximum variation is large (for example, ±64 UI). In contrast, IQ modulation can vary the amount of skew in decimal units (for example, 2 mUI), but the maximum variation is small (±360° = ±1000 mUI). For this reason, in this embodiment, the integer unit portion of the required skew amount (UI) is allocated to transmission timing shift, and the decimal unit portion is allocated to IQ modulation, and both are controlled.

[0042] Specifically, when the skew amount is 1250 (mUI), it is 1 transmission timing shift (1000 mUI) + IQ modulation 90° (250 mUI), and when the skew amount is -2250 (mUI), it is -3 transmission timing shift (-3000 mUI) + IQ modulation 270° (750 mUI).

[0043] In the above examples, IQ modulation is used in the range of 0 to 360° (0 to 2π) = 0 to 1000 mUI, but the point at which the transmission timing shift is advanced is arbitrary. For example, IQ modulation can be used in the range of ±500 mUI with ±180° (±π) = 0 mUI as the center. In this case, if the skew amount is 1250 (mUI), the result is 1 transmission timing shift (1000 mUI) + IQ modulation 90° (250 mUI). Also, if the skew amount is -2250 (mUI), the result is -2 transmission timing shift (-2000 mUI) + IQ modulation -90° (-250 mUI).

[0044] In the above examples, the skew amount is varied by both transmission timing shift and IQ modulation, but depending on the set skew amount, it is also possible to vary the skew amount by either transmission timing shift or IQ modulation. For example, if the set skew amount is 1000 (mUI), the result is 1 transmission timing shift (1000 mUI), and only the transmission timing shift is controlled. Also, if the set skew amount is 250 (mUI), the result is IQ modulation 90° (250 mUI), and only the IQ modulation is controlled.

[0045] The pattern generation unit 14A of the first signal generator 4A and the pattern generation unit 14B of the second signal generator 4B can generate PRBS patterns or arbitrary patterns and can control the start timing of the patterns. Specifically, to achieve higher-rate data output from a low-rate pattern generation unit, for example, they can be configured with an FPGA that outputs 1 / N data and multiple MUXes (N:1 MUX for MSB output, N:1 MUX for LSB output, and 2:1 MUX for PAM4 output), or they can be configured with D-FFs, but are not limited to these circuit configurations.

[0046] As described above, according to this embodiment, by generating positive and negative signals separately from two signal generators (first signal generator 4A and second signal generator 4B), it is possible to vary the amount of differential skew with excellent variable width and resolution without mechanical operation. Specifically, a variable width of ±64 UI and a resolution of 2 mUI are achieved. The operating rate of this module is 2.4 Gbaud to 64.2 Gbaud, which, when converted to time units, translates to a maximum variable width of ±26.6 ns and a minimum resolution of 31.1 fs.

[0047] Furthermore, as shown in FIG. 1, the device under test W is connected to one of the two outputs (first output terminals 11A and 11B, second output terminals 12A and 12B) of the first signal generator 4A and the second signal generator 4B (first output terminals 11A and 12B, respectively) and the other output (first output terminal 11B and second output terminal 12A, respectively) is connected to a monitor device 5 and used for monitoring, so that waveform observation can be performed simultaneously without modifying the main path (path for inputting positive and negative signals to the device under test W).

[0048] The differential skew generator 1 of this embodiment is not configured to generate positive and negative signals from a single signal generator, but rather, as shown in Figure 1, is configured to generate positive and negative signals individually from two synchronously operating signal generators (first signal generator 4A and second signal generator 4B). This allows each signal to be independently manipulated for parameters other than skew. The manipulable parameters depend on the functions of the signal generators, but examples include amplitude, Tx equalizer (emphasis), and PAM linearity.

[0049] One method for evaluating the input signal to the device under test is to insert a divider or pickoff-tee into the signal path, but in this case, it is necessary to accept the degradation of signal quality caused by the insertion of the divider or pickoff-tee.

[0050] In addition, dividers and pickoff-tees generally have an insertion loss of around 14dB or 20dB, so the measuring equipment must have high S / N performance. If this is not acceptable, you can always reconnect the DUT to the oscilloscope, but in this case you cannot observe the behavior of the measuring equipment and the waveform at the same time.

[0051] In contrast, this embodiment can provide an evaluation environment that satisfies both of these requirements. Furthermore, even if there is reflection from the DUT, there is no risk of the output being affected by differential coupling on the signal generator side, making it easy for the evaluator to isolate the effects of skew. Furthermore, by using the other output for monitoring, as shown in Figure 1, it is possible to monitor the waveform and skew input to the DUT without inserting a divider or pickoff-tee into the signal path.

[0052] Furthermore, the amount of skew can be controlled by inputting it in time units or UI units in the setting unit 3, or by intuitively operating the setting unit 3 while a skewed waveform image is displayed. This makes it easier for the user to intuitively grasp what kind of skewed signal is being provided, and also makes it possible to compare the displayed waveform image with the monitored waveform image to confirm whether the intended skew has been added.

[0053] While the best mode for the differential skew generating device and 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 realized by those skilled in the art based on this mode are naturally included in the scope of the present invention. [Explanation of symbols]

[0054] 1 Differential skew generator 2 Clock oscillator 3. Settings 4. Signal Generator 4A First Signal Generator 4B Second Signal Generator 5. Monitor Device 11A, 11B First output terminal 12A, 12B Second output terminal 13A, 13B IQ Modulator 14A, 14B Pattern generation section 15A, 15B control section W Object to be measured

Claims

1. a clock oscillator (2) that oscillates and outputs a clock signal; a first signal generator (4A) having a first output terminal (11A) for outputting a positive signal and a second output terminal (12A) for outputting a negative signal, the first signal generator including a first IQ modulator (13A), a first pattern generating unit (14A), and a first control unit (15A) for outputting the positive signal from the first output terminal to a device under test (W) at the timing of a signal obtained by IQ-modulating the clock signal and adjusting the phase angle; a second signal generator (4B) having a third output terminal (11B) for outputting a positive signal and a fourth output terminal (12B) for outputting a negative signal, the second signal generator including a second IQ modulator (13B), a second pattern generating unit (14B), and a second control unit (15B) for outputting the negative signal from the fourth output terminal to the device under test at the timing of a signal obtained by IQ-modulating the clock signal and adjusting its phase angle; a monitor device (5) connected to the second output terminal of the first signal generator and the third output terminal of the second signal generator; the first signal generator and the second signal generator operate synchronously, and allocate a fractional unit of UI of the set skew amount to IQ modulation, and allocate a transmission timing shift that shifts the transmission timing of the signal by an integer unit of UI of the set skew amount, respectively; the first control unit, when a skew amount in units of UI is set, controls at least one of the first IQ modulator and the first pattern generation unit by dividing the amount into a required IQ modulation amount and a required transmission timing shift amount; A differential skew generating device characterized in that, when a skew amount in UI units is set, the second control unit controls at least one of the second IQ modulator and the second pattern generation unit by allocating the required IQ modulation amount and transmission timing shift amount.

2. 2. The differential skew generating device according to claim 1, wherein when the skew amount is set in units of time, the skew amount set in units of time is multiplied by a bit rate to convert it into a skew amount in units of UI.

3. oscillating and outputting a clock signal; a step of outputting the positive signal from the first output terminal to a device under test (W) at the timing of a signal obtained by IQ-modulating the clock signal and adjusting the phase angle in a first signal generator (4A) having a first output terminal (11A) for outputting a positive signal and a second output terminal (12A) for outputting a negative signal, and including a first IQ modulator (13A), a first pattern generating unit (14A), and a first control unit (15A); a step of outputting the negative signal from the fourth output terminal to the device under test at the timing of a signal obtained by IQ-modulating the clock signal and adjusting the phase angle in a second signal generator (4B) having a third output terminal (11B) for outputting a positive signal and a fourth output terminal (12B) for outputting a negative signal, and including a second IQ modulator (13B), a second pattern generating unit (14B), and a second control unit (15B); connecting the second output terminal of the first signal generator and the third output terminal of the second signal generator to a monitoring device (5); a step of synchronously operating the first signal generator and the second signal generator, and allocating a fractional unit of UI of the set skew amount to IQ modulation for a transmission timing shift that shifts the signal transmission timing by an integer unit of UI of the set skew amount; When the skew amount in units of UI is set, the first control unit controls at least one of the first IQ modulator and the first pattern generation unit by dividing the amount into a required IQ modulation amount and a required transmission timing shift amount; a step of controlling at least one of the second IQ modulator and the second pattern generation unit by the second control unit when a skew amount in UI units is set, by dividing the amount into a required IQ modulation amount and a required transmission timing shift amount.

4. The differential skew generation method according to claim 3, wherein when the skew amount is set in units of time, the skew amount set in units of time is multiplied by a bit rate to convert it into a skew amount in units of UI.

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