This application is based upon and claims the benefit of the priority of Japanese patent application No. 2008-314893, filed on Dec. 10, 2008, the disclosure of which is incorporated herein in its entirety by reference thereto.
The present invention relates to a transmission line loss compensation circuit and transmission line loss compensation method, and particularly to a transmission line loss compensation circuit and transmission line loss compensation method that compensate for a transmission line loss of high frequency components in high-speed serial communication, using a linear equalizer.
In high-speed serial transmission, when transmission loss increases, ISI (inter-symbol interference) jitter occurs, the eye opening of the receiver gets smaller, and the timing margin is reduced. This is due to the fact that a transmission line shows the characteristics of a low-path filter; the longer the transmission line is, the more high frequency components deteriorate.
As one of technologies for compensating for this, there is a method in which a linear equalizer that boosts high frequency components is provided for the receiving end. When the deterioration of high frequency components in the transmission line is more or less compensated by the boost of high frequency components by the linear equalizer, an optimal eye waveform can be obtained from the output of the linear equalizer.
However, if the boost of high frequency components by the linear equalizer is too weak, the jitter compensation will be insufficient, and if the boost is too much, the jitter will increase. Therefore, the compensation intensity of the linear equalizer must be adjusted optimally according to the transmission loss.
However, there are applications in which a user is not able to freely change their settings. When one needs to deal with various transmission lines having lengths and losses greatly different from each other in such an application, it is difficult to do so with a fixed linear equalizer setting. Therefore, the linear equalizer requires a function of automatically adjusting the level of compensation for the transmission line loss of high frequency components.
By supplying the ideal output signal shown in
The automatic adjustment of the intensity constitutes two modes: a reference potential adjustment mode that adjusts a reference potential Vth according to the level of Vave (low) and a linear equalizer intensity adjustment mode that adjusts the level of Vave (high) according to the adjusted reference potential. The linear equalizer 20 is controlled by switching between these two modes at a predetermined interval using a loop timer 26 of a control logic unit 31, a state indicator circuit 30, and a selector 27.
First, in the first reference potential adjustment mode, the reference potential Vth is adjusted so that the reference potential Vth equals the buffer output Vave (low) by a loop of a rectifier circuit 22b, a comparator 23, a flip-flop 24, the selector 27, an incrementer 28b, a reference potential generating circuit 29, and the rectifier circuit 22b, formed in this order.
The rectifier circuits 22a and 22b rectify the outputs of the buffer 21 and the reference potential generating circuit 29, and the comparator 23 compares output levels of the rectifier circuits 22a and 22b. When Vth<Vave (low), the comparator 23 outputs a pulse, and the output of flip-flop 24 is high level. When the flip-flop 24 outputs a high level signal before a wait timer 25 measures a predetermined amount of time, the incrementer 28b increases the setting by one step, thereby increasing the reference potential Vth, and at the same time, the flip-flop 24 and the loop timer 26 are reset.
By continuing to increase the setting, Vth reaches Vave (low), and the comparator 23 no longer outputs the pulse. If the output of the flip-flop 24 stays at low level, the incrementer 28b does not increment the setting code. After more time has elapsed, the loop timer 26 outputs a time out signal, and the circuit enters into the second linear equalizer intensity adjustment mode.
Next, in the linear equalizer intensity adjustment mode, Vave (high) is adjusted so that the buffer output Vave (high)=Vave (low)×2/π in a loop of the rectifier circuit 22a, the comparator 23, the selector 27, an incrementer 28a, the linear equalizer 20, the buffer 21, and the rectifier circuit 22a, formed in this order.
In the linear equalizer intensity adjustment mode, the reference potential Vth=Vave (low) adjusted in the reference potential adjustment mode is changed to Vth=Vave (low)×2/π, and the same adjustment as in the reference potential adjustment mode is performed. At this time, what is incremented is the setting code of the linear equalizer 20.
The rectifier circuits 22a and 22b rectify the output of the buffer 21 and the reference potential Vth, and the comparator 23 compares them. When Vave (high)<Vth=Vave (low)×2/π, the comparator 23 outputs a pulse, and the output of flip-flop 24 is high level. When the flip-flop 24 outputs a high level signal before the wait timer 25 measures a predetermined amount of time, the incrementer 28b increases the setting by one step, thereby increasing the intensity of the linear equalizer 20, and at the same time, the flip-flop 24 and the loop timer 26 are reset.
By continuing to increase the setting, Vave (high) reaches Vth=Vave (low)×2/π, and the comparator 23 no longer outputs the pulse. If the output of the flip-flop 24 stays at low level for a predetermined amount of time, the incrementer 28a does not increment the setting code. After more time has elapsed, the loop timer 26 outputs the time out signal, and the linear equalizer intensity adjustment mode is complete.
After the circuit has gone through the two modes described above and the linear equalizer intensity adjustment mode has been completed, the high frequency components of the output of the buffer 21 are adjusted to the low frequency components times 2/π. After the completion of the linear equalizer intensity adjustment mode, the circuit returns to the reference potential adjustment mode, and switches between the two modes thereafter. It is construed that this is done so that the circuit will be able to return to the normal setting if it enters into an abnormal setting.
Further, this is not mentioned in Non-Patent Document 1, but the reason why the setting code is updated only in one direction (increment) is that the adjustment code will fluctuate in a stable state if decrement is performed as well, and this needs to be avoided.
Uchiki, H., et al., “A 6 Gb/s RX Equalizer Adapted Using Direct Measurement of the Equalizer Output Amplitude,” ISSCC 2008/SESSION 5/HIGH-SPEED TRANSCERIVERS/5.4, February 2008.
The disclosure of the above Non-Patent Document is incorporated herein by reference thereto.
The present invention gives the following analysis. The transmission line loss compensation circuit described in Non-Patent Document 1 has the following problems for use in an environment in which the state of the transmission line changes. The first problem is that the linear equalizer setting cannot be turned down once it is stronger than the optimal setting. Further, the second problem is that, when the output amplitude of the linear equalizer becomes low such as when the input amplitude of the receiver is set low during operation, the compensation intensity ends up being set higher than the optimal level.
The reason for the first problem is as follows. In the linear equalizer intensity adjustment mode, the circuit operates so as to increase the linear equalizer intensity setting until the high frequency components of the buffer output exceed the reference potential. However, once they exceed the reference potential, there is no means for knowing whether the setting is optimal or the compensation intensity is too strong, nor is there means for decreasing (decrementing) the setting. As a result, when connection changes from a state with a large transmission loss to a state with a small transmission loss during the linear equalizer intensity adjustment mode or after the intensity has been adjusted, this change is not reflected on the compensation intensity of the linear equalizer, and the linear equalizer remains set high even if the transmission loss is small.
The reason for the second problem is as follows. In the reference potential adjustment mode, the reference potential is adjusted until the comparator no longer outputs the pulse (until the reference potential is higher than the low frequency component peak of the buffer=Vave (low)). However, once the reference potential has been adjusted high, even if the buffer output amplitude becomes low thereafter, the pulse is not outputted since the reference potential remains high. In other words, the reference potential is not updated. In a state in which the linear equalizer is set optimally, if the transmission loss remains the same and only the amplitude gets lower, the circuit will recognize that the high frequency components of a signal are lower than the reference potential. As a result, the linear equalizer will be set higher than the optimal setting.
The cause for the problems described above is the structure in which the reference potential and the compensation intensity of the linear equalizer can be adjusted only upwards. If a decrement function is enabled in the structure described in Non-Patent Document 1, it will be possible to make a downward adjustment, however, the setting will always be incremented or decremented and will not converge to a fixed value. In order to settle on a setting, the adjustment will have to be terminated, and in this case, since the automatic adjustment will not continue, there will be no means for adjustment when the setting temporarily gets out of the optimal state.
A transmission line loss compensation circuit according to an aspect of the present invention has a linear equalizer that compensates for a transmission line loss of high frequency components, a high-pass filter that extracts high frequency components from an output of the linear equalizer, a peak detector that compares a peak voltage of an output of the high-pass filter to first and second reference voltages, and a control circuit that controls the compensation intensity of the linear equalizer based on detection results of the peak detector so that the peak voltage becomes an intermediate voltage between the first reference voltage and the second reference voltage.
Further, in a transmission line loss compensation method for compensating for a transmission line loss of high frequency components using a linear equalizer according to another aspect of the present invention, a peak voltage of high frequency components in an output signal of the linear equalizer is detected and compared to a first reference voltage and a second reference voltage, the compensation intensity of the linear equalizer is increased when an absolute value of the peak voltage is smaller than either of absolute values of the first and the second reference voltages, the compensation intensity of the linear equalizer is decreased when an absolute value of the peak voltage is larger than either of absolute values of the first and the second reference voltages, and the compensation intensity of the linear equalizer is maintained when the peak voltage is an intermediate voltage between the first and the second reference voltages.
The meritorious effects of the present invention are summarized as follows.
According to the present invention, there are provided a transmission line loss compensation circuit and transmission line loss compensation method that compensate for a loss of high frequency components by altering the compensation intensity of a linear equalizer quickly reacting to a change when the transmission environment changes, and that maintain a stable state when there is no change in transmission loss.
A preferred mode of the present invention will be described with reference to the drawings as necessary. It should be noted that drawings and symbols referred to in the description of the preferred mode are used as examples of the mode, and variations of the mode are not limited by these.
For instance, a transmission line loss compensation circuit in a mode of the present invention has a linear equalizer 11 that compensates for the transmission line loss of high frequency components, a high-pass filter 12 that extracts high frequency components from an output of the linear equalizer 11, a peak detector 14 that compares a peak voltage of a high-pass filter output Sac to first and second reference voltages (VTH0, VTH1), and a control circuit 15 that controls the compensation intensity of the linear equalizer 11 so that the peak voltage is an intermediate voltage between the first and the second reference voltages (VTH0, VTH1) based on the detection results of the peak detector 14, as shown in
Further, for instance, the transmission line loss compensation circuit in a mode of the present invention further includes a high frequency reference level generating circuit 13 that generates the first and the second reference voltages as shown in
Further, for instance, the transmission line loss compensation circuit in a mode of the present invention controls so as to increase the compensation intensity when it detects that an absolute value of the peak voltage is smaller than either of absolute values of the first and the second reference voltages (VTH0, VTH1), and decrease the compensation intensity when it detects that the absolute value of the peak voltage is larger than either of the absolute values of the first and the second reference voltages (VTH0, VTH1) as shown
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
Further, for instance, as shown in
As in the conventional example shown in
In
When the compensation intensity of the linear equalizer 11 is insufficient, the high frequency component peak of the output signal becomes small, and conversely when the compensation is too much, the high frequency component peak gets larger. Therefore, only high frequency components are extracted from the output of the linear equalizer 11, and the intensity of the linear equalizer 11 is adjusted according to the amount of their peak level.
After the output signal of the linear equalizer passes through the high-pass filter 12, the high frequency components (Sac) of the output of the linear equalizer 11 is outputted from the high-pass filter 12. The peak detector 14 compares the relative relations between the peak level (PK) of the input signal (Sac) and the two reference voltages (VTH0, VTH1), and outputs UP, DOWN, and HOLD signals corresponding to the relative relations.
At clock timings T2 and T3, regarding the peak level (PK) of the input signal (IN), VTH0<PL<VTH1. Therefore the set signal SET0 outputted from the comparator 141 to the flip-flop 143 becomes a pulse string, and the set signal SET1 outputted from the comparator 142 to the flip-flop 144 remains at the low level. The input signal of the flip-flop 145 is set to a high level, however, since the input signal of the flip-flop 146 stays at the low level, COMP0 signal goes to a high level, COMP1 signal goes to the low level, and the decoder 147 outputs UP signal at a low level, DOWN signal at the low level, and HOLD signal at a high level at the clock timing T3.
At clock timings T4 and T5, regarding the peak level (PK) of the input signal (IN), VTH0<VTH1<PL. Therefore the set signals SET0 and SET1 outputted from the comparators 141 and 142 to the flip-flops 143 and 144 become pulse strings. Since the input signals of both the flip-flops 145 and 146 are set to the high level, both COMP0 and COMP1 signals are outputted at the high level, and the decoder 147 outputs UP signal at the low level, the DOWN signal at a high level, and the HOLD signal at the low level at the clock timing T5
Further, the peak detector 14 may be configured so as to detect both the positive-side and the negative-side peaks as shown in
Further, as shown in
The peak detector shown in
The control circuit 15 shown in
Next, the operation of Example 1 will be described. For a signal with a clear eye opening, the degree of eye opening (the high frequency components of the signal) in a voltage direction is approximately the same amount as the peak amplitude (the low frequency components of the signal). When a transmission loss is high, the degree of eye opening in a voltage direction (the high frequency components of the signal) becomes small since the high frequency components greatly deteriorate due to the low-pass characteristics of a transmission line. By increasing the high frequency gain using the linear equalizer, the high frequency components of the signal can be matched to the low frequency components, and an output having a clear eye waveform can be obtained.
In the case of the configuration shown in
Veq
—
dc=Vin
—
dc×Ae
—
dc (1)
Veq
—
ac=Vin
—
ac×Ae
—
ac (2)
Now let us look at a case where the peak amplitude of the input signal, i.e., the low frequency component (Sdc) amplitude (Vin_dc), and the low frequency gain (Ae_dc) of the linear equalizer are known, fixed values. An example of the case where the low frequency component peak amplitude (Vin_dc) of the input signal is known is, for instance, a case where the output amplitude on the transmission side is determined by a transmission standard (hence it is known), and the low frequency component loss of a transmission line is small or predictable to a certain extent. In general, the low frequency component loss of a transmission line is small, and the amount of the low frequency component loss does not vary much among different kinds of transmission lines. If Ae_dc is known in the design stage, it means that the low frequency components (Veq_dc) of the linear equalizer output are known, fixed values.
On the other hand, since the amount of the high frequency component loss of a transmission line varies greatly depending on the kind of transmission line, the high frequency components (Vin_ac) of the input signal are unknown. In order to have a linear equalizer output having a clear eye opening, the following should be done. First, set an adjustment target level (PKtarget) for the high frequency components of the linear equalizer output signal based on the peak amplitude of the low frequency components (Veq_dc) of the linear equalizer output, a known, fixed level. Observe the peak amplitude level (PK) of the high frequency components (Veq_ac) of the linear equalizer output signal, and adjust the high frequency gain (Ae_ac) of the linear equalizer so that this peak amplitude level becomes close to the adjustment target (PKtarget).
The configuration shown in
[The Operation when the Intensity of Linear Equalizer is Insufficient]
The peak (PK) of the high frequency components (Sac) of the output signal of the linear equalizer 11 is smaller than the peak level (PKtarget) in the optimal setting. Therefore, the relative relations among the high frequency component peak level (PK) of the output of the linear equalizer 11 and the two reference voltages (VTH0, VTH1) are represented by PK<VTH0<VTH1. At this time, the peak detector 14 outputs UP signal at the high level, DOWN signal at the low level, and HOLD signal at the low level. The control circuit 15 behind the peak detector changes to a signal that increases the intensity setting of the linear equalizer 11 by one step because UP signal is at the high level. As the intensity of the linear equalizer 11 increases, so does the high frequency component peak level (PK) of the output of the linear equalizer 11.
This series of operations continues until PK≈PKtarget is achieved, and the relative relations among the three levels become VTH0<PK<VTH1. At this time, the peak detector 14 outputs UP signal at the low level, DOWN signal at the low level, and HOLD signal at the high level. The control circuit 15 behind the peak detector holds and maintains the intensity setting of the linear equalizer 11 because HOLD signal is at the high level. Once the circuit enters into a stable state, the intensity setting does not change unless the relative relations among the three levels, PK, VTH0, and VTH1, change. In other words, the setting code does not fluctuate.
[The Operation when the Intensity of the Linear Equalizer is Too High]
The peak (PK) of the high frequency components (Sac) of the output signal of the linear equalizer 11 is higher than the peak level (PKtarget) in the optimal setting. Therefore, the relative relations among the high frequency component peak level (PK) of the output of the linear equalizer 11 and the two reference voltages (VTH0, VTH1) are represented by VTH0<VTH1<PK. At this time, the peak detector 14 outputs UP signal at the low level, DOWN signal at the high level, and HOLD signal at the low level. The control circuit 15 behind the peak detector changes to a signal that decreases the intensity setting of the linear equalizer 11 by one step because UP signal is at the low level. As the intensity of the linear equalizer 11 decreases, so does the high frequency component peak level (PK) of the output of the linear equalizer 11.
This series of operations continues until PK PKtarget is achieved, and the relative relations among the three levels become VTH0<PK<VTH1. At this time, the peak detector 14 outputs UP signal at the low level, DOWN signal at the low level, and HOLD signal at the high level. The control circuit behind the peak detector holds and maintains the intensity setting of the linear equalizer 11 because HOLD signal is at the high level. Once the circuit enters into a stable state, the intensity setting does not change (the setting code does not fluctuate) unless the relative relations among the three levels, PK, VTH0, and VTH1, change, such as when the transmission line changes, altering the amount of the high frequency component loss.
[The Operation when the Optimal Setting of the Linear Equalizer Changes Due to an External Factor]
For instance, if the transmission line is altered and the transmission loss changes, the linear equalizer optimal setting may change as well. In this case, the stable state (VTH0<PK<VTH1) changes to PK<VTH0<VTH1 or to VTH0<VTH1<PK. Therefore, even if the optimal setting changes, the circuit is automatically adjusted to a new optimal setting by one of the operations described above.
As described above, the intensity of the linear equalizer can be adjusted in the both cases where the intensity of the linear equalizer is insufficient or where it is too high. In a converged state, the setting code does not fluctuate and the stable state can be maintained as long as the relative relations of the three values remain within VTH0<PK<VTH1.
Example 1 is effective when the input amplitude is fixed and known in advance. However, there are cases where the input amplitude is not fixed or the value of the input amplitude is unknown. In these cases, a function (AGC: Auto Gain Control) that adjusts the output amplitude of the linear equalizer to a fixed level, regardless of the input amplitude, is added in Example 2.
The variable gain amplifier 111 amplifies the entire frequency band equal to and below the Nyquist frequency by the same gain, and its gain is adjustable externally. An example of the circuit configuration of the variable gain amplifier 111 is shown in
When the output signal of the linear equalizer 11 passes through the low-pass filter 102, the low-pass filter 102 outputs the low frequency components (Sdc) of the output of the linear equalizer 11.
The low frequency peak detector 104 is configured identically to the peak detector 14. The low frequency peak detector 104 compares the peak level (PK_dc) of the input signal (Sdc) to the two reference voltages (VTH0A, VTH1A) to obtain the relative relations among them, and outputs UP, DOWN, and HOLD signals corresponding to the relative relations. The two reference voltages (VTH0A, VTH1A) of the low frequency peak detector 104 are generated by the low frequency reference level generating circuit 103. VTH0A and VTH1A are determined so that VTH0A<PKtarget_dc<VTH1A where the positive-side peak level of the low frequency components (Sdc) of the output signal of the linear equalizer 11 is a target level (PKtarget_dc).
The control signal of the variable gain, amplifier 111 is a thermometer code as that of the linear equalizer 11. Therefore, the gain control circuit 105 that generates the m-bit gain setting signal for the variable gain amplifier 111 can be configured identically to the control circuit 15 that generates the intensity setting signal for the linear equalizer 11. Since the control signal is m bit, m number of the control circuit basic constituting units 151 are needed.
Here, the positive-side peak of the equalizer output signal is detected, however, as in the basic configuration of Example 1 shown in
Next, the operation principle of Example 2 will be described. In
In the case shown in
Veq
—
dc=Vin
—
dc×Av×Ae
—
dc (3)
Veq
—
ac=Vin
—
ac×Av×Ae
—
ac (4)
According to the expressions (3) and (4) above, when Vin_dc is unknown and Ae_dc is a fixed value, one should change Av in order to adjust Veq_dc to the target level. A concrete operation for adjusting the peak amplitude, i.e., the low frequency components (Veq_dc), of the linear equalizer output to the target level is as follows. First, set the adjustment target level (PKtarget_dc) for the low frequency components (Sdc) of the linear equalizer output signal to any value. Observe the peak amplitude of the low frequency components (Veq_dc) of the linear equalizer output signal, and adjust the voltage gain (Av) of the variable gain amplifier so that this peak amplitude becomes close to the adjustment target level (PKtarget_dc).
Then, once the operation of adjusting the peak amplitude of the linear equalizer output has converged, the automatic adjustment of the linear equalizer intensity is possible as Example 1 shown in
Further, cases are divided into a case where the input amplitude is small, a case where the input amplitude is large, and a case where the input amplitude has changed, and a concrete operation in each case will be described.
[The Operation when the Input Amplitude is Small]
When the input amplitude is small, the peak (PK_dc) of the low frequency components (Sdc) of the output signal of the linear equalizer 11 is smaller than the adjustment target level (PKtarget_dc). Therefore the relative relations among the peak level (PK_dc) of the low frequency components of the output of the linear equalizer 11 and the two low frequency reference voltages (VTH0A, VTH1A) are represented by PK_dc<VTH0A<VTH1A. At this time, the low frequency peak detector 104 outputs UP signal at the high level, DOWN signal at the low level, and HOLD signal at the low level. The gain control circuit 105 behind the peak detector changes to a signal that increases the gain setting of the variable gain amplifier 111 by one step because UP signal is at the high level.
When the gain of the variable gain amplifier 111 is increased, the peak level (PK_dc) of the low frequency components of the output of the linear equalizer 11 increases as well. This series of operations continues until PK_dc PKtarget_dc is achieved, and the relative relations among the three levels become VTH0A<PK_dc<VTH1A. At this time, the low frequency peak detector 104 outputs UP signal at the low level, DOWN signal at the low level, and HOLD signal at the high level. The gain control circuit 105 behind the peak detector holds and maintains the gain setting of the variable gain amplifier 111 because HOLD signal is at the high level. Once the circuit enters into a stable state, the intensity setting does not change (the setting code does not fluctuate) unless the relative relations among the three levels change. Then as soon as the operation of adjusting the peak amplitude of the linear equalizer output has converged, the intensity of the linear equalizer 11 is adjusted to the optimal setting by the automatic adjustment loop for the linear equalizer intensity described in Example 1.
[The Operation when the Input Amplitude is Large]
When the input amplitude is large, the peak (PK_dc) of the low frequency components (Sdc) of the output signal of the linear equalizer 11 is larger than the adjustment target level (PKtarget_dc). Therefore the relative relations among the peak level (PK_dc) of the low frequency components of the output of the linear equalizer 11 and the two low frequency reference voltages (VTH0A, VTH1A) are represented by VTH0A<VTH1A<PK_dc. At this time, the low frequency peak detector 104 outputs UP signal at the low level, DOWN signal at the high level, and HOLD signal at the low level. The gain control circuit 105 behind the peak detector changes to a signal that decreases the gain setting of the variable gain amplifier 111 by one step because DOWN signal is at the high level.
When the gain of the variable gain amplifier 111 is decreased, the peak level (PK_dc) of the low frequency components of the output of the linear equalizer 11 decreases as well. This series of operations continues until PK_dc PKtarget_dc is achieved, and the relative relations among the three levels become VTH0A<PK_dc<VTH1A. The operation thereafter is the same as the case where the input amplitude is small.
[The Operation when the Input Amplitude has Changed]
For instance, the linear equalizer output amplitude changes when the input amplitude changes due to an altered output amplitude setting on the transmitting end. In this case, the relative relations among the three levels change from the stable state (VTH0A<PK_dc<VTH1A) to PK_dc<VTH0A<VTH1A, or VTH0A<VTH1A<PK_dc. Therefore, even if the output amplitude of the linear equalizer changes, the output amplitude of the linear equalizer will be automatically adjusted to the target level by one of the operations described above.
As described, Example 1 is effective when the input amplitude is constant and the amount of the amplitude is known in advance, however, in Example 2, the intensity of the linear equalizer can be adjusted even in the cases where the input amplitude fluctuates.
In Example 2, the function (AGC function: Auto Gain Control) that adjusts the output amplitude of the linear equalizer to a fixed level regardless of the input amplitude is added so that the intensity of the linear equalizer can be adjusted when the input amplitude fluctuates. In addition, the automatic adjustment of the linear equalizer intensity can be achieved by changing the two reference voltages (VTH0, VTH1), i.e., the adjustment target level, in connection with the output amplitude level of the linear equalizer, as described below as Example 3.
The low frequency peak detector 104 is configured identically to the peak detector 14 shown in
Further, the positive-side peak of the equalizer output signal is detected by the low frequency peak detector 104, however, as in the concrete configuration examples of the peak detector 14 shown in
Next, cases are divided into a case where the input amplitude is small, a case where the input amplitude is large, and a case where the input amplitude has changed, and a concrete operation of Example 3 in each case will be described.
[The Operation when the Input Amplitude is Small]
When the input amplitude is small, the relative relations among the peak level (PK_dc) of the low frequency components of the output of the linear equalizer 11 and the two low frequency reference voltages (VTH0A, VTH1A) are represented by PK_dc<VTH0A<VTH1A. At this time, the low frequency peak detector 104 outputs UP signal at the high level, DOWN signal at the low level, and HOLD signal at the low level. The reference voltage control circuit 105A behind the peak detector increases the number of the bits of the setting code, for the high frequency reference level generating circuit 130 and the low frequency reference level generating circuit 131, that go to the high level by one bit because UP signal is at the high level. As a result, both reference voltages of the high frequency reference level generating circuit 130 and the low frequency reference level generating circuit 131 decrease. This series of operations continues until the relative relations among the three levels become VTH0A<PK_dc<VTH1A. At this time, the low frequency peak detector 104 outputs UP signal at the low level, DOWN signal at the low level, and HOLD signal at the high level. The reference voltage control circuit 105A behind the peak detector holds the control signal for the high frequency reference level generating circuit 130 and the low frequency reference level generating circuit 131 because HOLD signal is at the high level.
Once the circuit enters into a stable state, the control signal does not change (the setting code does not fluctuate) unless the relative relations among the three levels change. Then, when the operation of adjusting the low frequency reference voltages VTH0A and VTH1A has converged, the intensity of the linear equalizer 11 is adjusted to the optimal setting by the automatic adjustment loop for the linear equalizer intensity described in Example 1 since VTH0 and VTH1 have decreased following the amplitude.
[The Operation when the Input Amplitude is Large]
When the input amplitude is large, the relative relations among the peak level (PK_dc) of the low frequency components of the output of the linear equalizer 11 and the two low frequency reference voltages (VTH0A, VTH1A) are represented by VTH0A<VTH1A<PK_dc. At this time, the low frequency peak detector 104 outputs UP signal at the low level, DOWN signal at the high level, and HOLD signal at the low level. The reference voltage control circuit 105A behind the peak detector decreases the number of the bits of the setting code, for the high frequency reference level generating circuit 130 and the low frequency reference level generating circuit 131, that go to the high level by one bit because DOWN signal is at the high level. As a result, the reference voltage outputted by the high frequency reference level generating circuit 130 and the low frequency reference voltage outputted by the low frequency reference level generating circuit 131 both increase. This series of operations continues until the relative relations among the three levels become VTH0A<PK_dc<VTH1A. The operation after VTH0A<PK_dc<VTH1A has been achieved is the same as the case where the input amplitude is small.
[The Operation when the Input Amplitude has Changed]
For instance, the linear equalizer output amplitude changes when the input amplitude changes due to an altered output amplitude setting on the transmitting end. In this case, the relative relations among the three levels change from the stable state (VTH0A<PK_dc<VTH1A) to PK_dc<VTH0A<VTH1A, or VTH0A<VTH1A<PK_dc. Therefore, even if the output amplitude of the linear equalizer changes, the output amplitude of the linear equalizer will be automatically adjusted to the target level by one of the operations described above.
As described, according to each example of the present invention, the intensity of the linear equalizer can be automatically adjusted regardless of the amount of transmission loss. Further, the intensity setting signal does not fluctuate in a converged state. Further, even if an optimal setting of the linear equalizer moves out of a stable state due to a change in transmission loss, the circuit can be automatically adjusted back to a new optimal setting. Moreover, an effect that the circuit can cope with any input amplitude level, in addition to changes in transmission loss, can be obtained in the configurations of Examples 2 and 3.
Further, the variable gain amplifier 111 is provided at a stage before the linear equalizer 11 in
It should be noted that other objects, features and aspects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and claimed as appended herewith.
Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned.
Number | Date | Country | Kind |
---|---|---|---|
2008-314893 | Dec 2008 | JP | national |