JP6679635B2 - Feedforward equalizer and method for improving high frequency characteristics of feedforward equalizer - Google Patents

Description

  The present invention relates to a feedforward equalizer that can be used in a high frequency region for compensating for distortion of an input signal, and a method for improving high frequency characteristics of the feedforward equalizer.

  Conventionally, as a filter circuit that compensates for distortion of an input signal, for example, a feedforward equalizer disclosed in Patent Document 1 is known. The feedforward equalizer of Patent Document 1 has n delay devices and n weighting circuits, gives each delay device a desired delay with respect to an input signal, and further, each weighting circuit has a desired weight value. After giving the (filter value), these signals are added and output to compensate the distortion of the input signal.

  Further, Patent Document 2 below discloses a differential amplifier circuit that compensates the frequency characteristic of the differential gain of the differential amplifier section.

  By the way, in recent years, due to the spread of mobile terminals and cloud computing, the amount of data communication has been increasing and the transmission speed has been remarkably increased. In addition, the international standard for high-speed data transmission has changed from conventional NRZ transmission to PAM4 transmission along with electrical and optical interfaces.

  Therefore, in this type of PAM transmission, for example, in a BER test solution corresponding to the transmission of 56G or 64Gbaud PAM signals, compensation of frequency bands such as 28 GHz and 32 GHz is required. In addition, the frequency band for which compensation is required has been shifting to higher frequencies year by year, and there is a possibility that compensation for the 56 G or 64 GHz band will be required for 112 G or 128 Gbaud PAM transmission in the future.

Japanese Patent No. 5157907 Japanese Patent No. 5906818

  By the way, since the conventional circuits including the above-mentioned Patent Document 1 and Patent Document 2 are used in the frequency region of several GHz order, there is no need to worry about the drop in gain in the tens GHz band such as 28 GHz or 32 GHz. It was

  However, in recent years, PAM transmission measurement systems are required to compensate for high-frequency signals in the tens of GHz band, but conventional circuits including Patent Document 1 and Patent Document 2 lack compensation in the high-frequency region. However, it has not been possible to compensate for a high frequency signal in the tens of GHz band required in the PAM transmission measurement system. As a result, for example, in PAM transmission of 56G or 64Gbaud, the error rate measuring device on the receiving side of the error rate measuring device cannot compensate for the transmission distortion at the time of receiving the PAM signal of the high speed baud rate, and thus the BER (error rate) measurement. I couldn't.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a feedforward equalizer capable of improving a drop in gain in a high frequency region and a high frequency characteristic improving method using the same. There is.

In order to achieve the above object, the feed-forward equalizer according to claim 1 of the present invention comprises a cell 11 of one unit by combining an emitter follower circuit 12 and a differential circuit 13, and a plurality of cells are formed according to the delay amount. Is a feedforward equalizer 1 in which a sub-tap 3 in which the cells of FIG.
Characterized in that the terminating resistor 22 is connected a 25Ω resistor one between the ground from the output side of the differential circuit and between the input side of said emitter follower circuit in the transmission line 23 between the plurality of cells And

The feedforward equalizer according to claim 2 is the feedforward equalizer according to claim 1,
The line impedance of the transmission line 23 between the output of the differential circuit 13 and the input of the emitter follower circuit 12 between the plurality of cells 11 is high impedance as compared with a 50Ω termination resistance .

The feedforward equalizer according to claim 3 is the feedforward equalizer according to claim 2,
The pre-tight end resistor 22, characterized in that to connect closer to the input side of the output side or the emitter follower circuit 12 of the differential circuit 13.

According to a method for improving high frequency characteristics of a feedforward equalizer described in claim 4, one unit cell 11 is configured by a combination of an emitter follower circuit 12 and a differential circuit 13, and a plurality of cells are connected in multiple stages according to a delay amount. The sub-tap 3 is connected in parallel to the main tap 2, and the final-stage cell functions as a weighting circuit whose gain can be adjusted by setting a coefficient.
Further comprising the step of connecting a terminating resistor 22 is 25Ω resistor one between ground and between the output side of the differential circuit on the input side of said emitter follower circuit in the transmission line 23 between the plurality of cells Is characterized by.

A method of improving a high frequency characteristic of a feedforward equalizer according to claim 5 is the method of improving a high frequency characteristic using the feedforward equalizer according to claim 4, wherein
The method further includes the step of making the line impedance of the transmission line 23 between the output of the differential circuit 13 and the input of the emitter follower circuit 12 between the plurality of cells 11 higher than that of a 50Ω termination resistor. And

High-frequency characteristics improved method of feedforward equalizer of claim 6, in a method for improving the high frequency characteristics using a feed-forward equalizer of claim 5, the output side of the pre-tight end resistor 22 differential circuit 13 or The present invention is characterized by including a step of connecting to the input side of the emitter follower circuit 12 so as to be connected.

  According to the present invention, it is possible to compensate for the distortion of the input signal by improving the drop of the gain in the high frequency region as compared with the related art.

It is a figure which shows the basic composition of the feedforward equalizer which concerns on this invention. It is a figure which shows the internal structure of the main tap and sub tap in the feedforward equalizer of FIG. It is a figure which shows an example of the circuit structure of each cell of FIG. As a comparative example of the sub-tap of the feedforward equalizer according to the present invention, the line impedance of the transmission line is set to a low impedance of 33.83Ω, and the connection configuration and frequency of the differential circuit and the emitter follower circuit when terminated with a 50Ω termination resistor are used. It is a figure which shows a characteristic. In the sub-tap of the feedforward equalizer according to the present invention, the line impedance of the transmission line is set to a low impedance of 33.83Ω, and the connection configuration of the differential circuit and the emitter follower circuit when the 50Ω terminating resistor is combined into the 25Ω terminating resistor. It is a figure which shows an example of a frequency characteristic. In the sub-tap of the feedforward equalizer according to the present invention, the line impedance of the transmission line is set to a low impedance of 33.83Ω, and the differential circuit and the emitter follower circuit when the 25Ω terminating resistor is brought close to the output side of the differential circuit. It is a figure which shows an example of a connection structure frequency characteristic. In the sub-tap of the feedforward equalizer according to the present invention, the line impedance of the transmission line is set to a low impedance of 33.83Ω, and the differential circuit and the emitter follower circuit when the 25Ω termination resistor is brought close to the input side of the emitter follower circuit. It is a figure which shows an example of a connection structure and a frequency characteristic. In the sub-tap of the feedforward equalizer according to the present invention, the line impedance of the transmission line is set to a high impedance of, for example, 120Ω, and the connection structure of the differential circuit and the emitter follower circuit when the 25Ω termination resistor is brought close to the output side of the differential circuit. It is a figure which shows an example of a frequency characteristic. In the sub-tap of the feedforward equalizer according to the present invention, the line impedance of the transmission line is set to a high impedance of, for example, 120Ω, and the connection configuration of the differential circuit and the emitter follower circuit when the 25Ω terminating resistor is brought close to the input side of the emitter follower. It is a figure which shows an example of a frequency characteristic. It is a figure which shows an example of the frequency characteristic before adjustment as a comparative example of the feedforward equalizer which concerns on this invention. It is a figure which shows an example of the frequency characteristic after adjustment of the feedforward equalizer which concerns on this invention. It is a figure which shows schematic structure of the error rate measuring apparatus with which the feedforward equalizer which concerns on this invention is employ | adopted. It is a flowchart figure which shows the outline of the improvement method of the high frequency characteristic of the feedforward equalizer which concerns on this invention.

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

[About the basic structure of the feed-forward equalizer]
First, a basic configuration of a feed forward equalizer (FFE) that electrically compensates waveform distortion of an input signal caused by dispersion or loss of a transmission medium will be described with reference to FIG.

  As shown in FIG. 1, the feedforward equalizer 1 is roughly configured to include a main tap 2, a sub tap 3, and an adder 4. Note that FIG. 1 illustrates a simple circuit having a 2-tap configuration.

  In the feedforward equalizer 1 of FIG. 1, the amplitude of the main tap 2 is 0.0 dB (x1.00). Further, the sub tap 3 branches / generates a component delayed by ΔT = {1 / (2 * Fpeaking)} with respect to the main tap 2. Note that Fpeaking is a peak frequency set to a frequency to be compensated.

  In this feedforward equalizer 1, when the signal amplified by the input side amplifier 4 is input to the sub-tap 3, the amplitude is attenuated by xdB and then the positive and negative are inverted to obtain a negative component. Then, in the adder 4, the negative component of the sub tap 3 is added to the main tap 2. It should be noted that the amplitude is adjusted by the input side amplifier 4 and the output side amplifier 5 which are arranged in the front stage and the rear stage of the branch portion of the main tap 2 and the sub tap 3.

[About main tap and sub tap configuration]
Next, the configuration of the main tap 2 and the sub tap 3 in the feedforward equalizer 1 of FIG. 1 will be described with reference to FIGS. 2 and 3.

  As shown in FIG. 2, the main tap 2 uses a positive phase signal and a negative phase signal in which the phase of the positive phase signal is inverted as an input differential signal so that the amplitude of the main tap 2 with respect to the sub tap 3 has a proportional relationship. It has a cell 11 for adjusting to.

  As shown in FIG. 2, the sub tap 3 is connected in parallel between the input and the output of the main tap 2, and the cells 11 having the number of stages (6 stages in FIG. 2) corresponding to a predetermined delay amount are close to each other. In this state, the transmission lines 23 are connected in multiple stages (cascade connection).

  Each cell 11 includes an emitter follower circuit (EF) 12 and a differential circuit (CML: Current Mode Logic) 13.

  The emitter follower circuit 12 and the differential circuit 13 are configured by a combination of transistor circuits, as shown in FIG. In the circuit of FIG. 3, the gain is set by adjusting the sizes of the transistors Tr1 and Tr2 of the emitter follower circuit 12 and the transistors Tr3 and Tr4 of the differential circuit 13 and the resistance values of the resistors R5, R6 and R7. Further, by connecting the negative feedback resistors R8 and R9 to the transistors Tr3 and Tr4 of the differential circuit 13 and adjusting the resistance value, the range of the input voltage level that can be used linearly is set.

  The resistors R1, R2, R3 and R4 are terminating resistors. Further, the emitter follower circuit 12 and the differential circuit 13 of the cell 11 at the final stage function as a weighting circuit for determining the amount of weighting of the main tap 2 and the sub tap 3.

[How to improve high frequency characteristics]
Next, a method of improving the high frequency characteristics of the feedforward equalizer 1 of this embodiment will be described with reference to FIGS. 4 to 9, L is the length of the transmission line, W is the width of the transmission line, Z0 is the line impedance of the transmission line, and TD is the delay amount.

  In the feedforward equalizer 1 of the present embodiment, the subtap 3 employs the following methods (1) and (2) for improving the high frequency characteristics in order to improve the drop in the gain in the high frequency region.

(1) The 50Ω terminating resistors on the input side of the emitter follower circuit 12 and the output side of the differential circuit 13 are combined into a 25Ω terminating resistor. For example, the 50Ω terminating resistors 21 on the input side of the emitter follower circuit 12 and the output side of the differential circuit 13 shown in FIG. 4 are combined into a 25Ω terminating resistor 22 as shown in FIG.
(2) The transmission line connecting between the cells 11 is made to have high impedance, and the 25Ω terminating resistor is brought close to the output side of the differential circuit 13 or the input side of the emitter follower circuit 12. For example, the transmission line 23 of FIG. 2 is changed from low impedance (33.83Ω) to high impedance (120Ω), and the 25Ω termination resistor 22 is brought close to the output side of the differential circuit 13 or the input side of the emitter follower circuit 12.

  To further explain, in the feedforward equalizer 1 of the present embodiment, the distance of the transmission line 23 connecting the emitter follower circuit 12 and the differential circuit 13 between the input and output of each cell 11 is, for example, several tens of μm. The 50Ω terminating resistors 21 which are designed to be short and are arranged at both ends of the circuit can be regarded as parallel and can be integrated into the 25Ω terminating resistors 22.

  Here, FIG. 4 is a comparative example of the sub-tap 3 of the present embodiment, in which the transmission line 23 has a low line impedance of 33.83Ω, and the differential circuit 13 and the emitter follower are terminated by the 50Ω termination resistor 21. FIG. 5 is a diagram showing an example of the connection configuration with the circuit 12 and the frequency characteristic. FIG. 5 shows the sub-tap 3 of this embodiment in which the line impedance of the transmission line 23 is a low impedance of 33.83Ω, and the 25Ω terminating resistor 22 is incorporated. FIG. 3 is a diagram showing an example of the connection configuration and frequency characteristics of the differential circuit 13 and the emitter follower circuit 12 in the case.

  In FIGS. 4 and 5, when comparing the cutoff frequency that is −3 dB lower than the gain of 10 MHz, it can be seen that the gain in the high frequency region is extended when the 25Ω termination resistor 22 is combined with the 50Ω termination resistor 21. .

  Next, in FIG. 6, the line impedance of the transmission line 23 is set to a low impedance of, for example, 33.83Ω, and the differential circuit 13 and the emitter follower circuit 12 when the 25Ω termination resistor 22 is brought close to the output side of the differential circuit 13 are shown. FIG. 7 is a diagram showing an example of the connection configuration and frequency characteristics, and FIG. 7 shows the differential circuit 13 in the case where the line impedance of the transmission line 23 is a low impedance of 33.83Ω and the 25Ω termination resistor 22 is brought close to the input side of the emitter follower 12. FIG. 3 is a diagram showing an example of a connection configuration between the emitter follower circuit 12 and the emitter follower circuit 12 and frequency characteristics.

  As shown in FIGS. 6 and 7, when the 25Ω terminating resistor 22 is brought closer to the output side of the differential circuit 13 and the 25Ω terminating resistor 22 is brought closer to the input side of the emitter follower circuit 12, the AC characteristics are compared. For example, when the line impedance of the transmission line 23 is low, for example, 33.83Ω, it can be seen that the change in the AC characteristic due to the arrangement of the 25Ω termination resistor 22 is not so much seen.

  Next, FIG. 8 shows a case where the line impedance of the transmission line 23 is set to a high impedance of, for example, 120Ω, and the 25Ω termination resistor 22 is brought close to the output side of the differential circuit 13 in the sub-tap 3 of the feedforward equalizer 1 of the present embodiment. FIG. 9 is a diagram showing an example of a connection configuration between the differential circuit 13 and the emitter follower circuit 12 and frequency characteristics, and FIG. 9 shows a line impedance of the transmission line 23 in the sub-tap 3 of the feedforward equalizer 1 of the present embodiment, for example, 120Ω FIG. 3 is a diagram showing an example of a connection configuration and frequency characteristics between the differential circuit 13 and the emitter follower circuit 12 when the impedance is set to a high impedance and the 25Ω terminating resistor 22 is brought close to the input side of the emitter follower circuit 12.

  When the line impedance of the transmission line 23 is changed from 33.83Ω to 120Ω, for example, the change in the AC characteristics due to the arrangement of the 25Ω termination resistor 22 is examined, and the higher line impedance of the transmission line 23 increases the gain in the high frequency region. I understand. That is, comparing FIG. 6 with FIG. 8 and FIG. 7 with FIG. 9, respectively, it was found that the gain in the high frequency region was extended when the line impedance of the transmission line 23 was changed from 33.83Ω to 120Ω.

  When the line impedance of the transmission line 23 is high, as shown in FIGS. 8 and 9, the output side of the differential circuit 13 is closer to the output side of the differential circuit 13 than the case where the 25Ω terminating resistor 22 is moved to the input side of the emitter follower circuit 12. It can be seen that the gain in the high frequency region is extended when the gain is increased.

  From the above results, it can be seen that the method of improving the high frequency characteristics of (1) and (2) is most effective when the adjustment shown in FIG. More specifically, before the adjustment of the sub-tap 3 shown in FIG. 4, the cutoff frequency at which -3 dB falls with respect to 10 MHz (low frequency) is about 62.2 GHz, and the gain in the high frequency region is about 30 GHz as a boundary. falling. On the other hand, after the adjustment of the sub-tap 3 shown in FIG. 8, comparing the AC characteristics before adjustment of the sub-tap 3 of FIG. 4, it is possible to extend the cutoff frequency of −3 dB to a high region of about +24.5 GHz by the adjustment. It can be seen that the drop in gain in the high frequency region is improved.

  As a method of making the line impedance of the transmission line 23 high, (A) a method of making the line width of the transmission line 23 as narrow as possible in consideration of the allowable current amount, and (B) a microstrip line to a coplanar line. A method of changing the distance and separating the transmission line 23 from the GND pattern can be considered.

  Next, FIG. 10 is a diagram showing an example of frequency characteristics before adjustment as a comparative example of the feedforward equalizer 1 of the present embodiment, and FIG. 11 is a graph showing frequency characteristics after adjustment of the feedforward equalizer 1 of the present embodiment. It is a figure which shows an example.

  As shown in FIGS. 10 and 11, when comparing the AC characteristics of the sub-tap 3 before and after adjustment with a cut-off frequency of 10 MHz to −3 dB, the one after the adjustment of the sub-tap 3 is more than that before the adjustment of the sub-tap 3. As a result, the gain in the high frequency range was extended to about +4.8 GHz.

  As described above, the method of improving the high frequency characteristics according to (1) and (2) can be adopted for the sub tap 3. Specifically, as shown in FIG. 13, first, the 50Ω terminating resistors on the output side of the differential circuit (CML) 13 and the input side of the emitter follower circuit (EF) 12 in the transmission line 23 between the plurality of cells 11 are connected. Collected into a 25Ω terminating resistor (ST1). Next, the transmission line 23 between the output of the differential circuit 13 and the input of the emitter follower circuit 12 between the plurality of cells 11 is set to high impedance (ST2). Further, the 25Ω terminating resistor is connected close to the output side of the differential circuit 13 or the input side of the emitter follower circuit 12 (ST3). As described above, by adjusting the positional relationship between the terminating resistor and the transmission line, the drop of gain in the high frequency region could be improved.

[Application example]
The feedforward equalizer 1 of the present embodiment can be applied to the error rate measuring device 31 as shown in FIG. As shown in FIG. 12, the error rate measuring device 31 includes a pattern generator 32 for generating a known pattern signal (for example, a PAM signal) input to the measurement object W, and an input of the pattern signal from the pattern generator 32. And an error rate measuring device 33 that receives the pattern signal that is returned from the measurement target W and measures the error rate. Then, the feedforward equalizer 1 according to the present embodiment is connected in front of the error rate measuring device 33 on the receiving side of the error rate measuring device 31. As a result, in the BER test solution corresponding to the transmission of the 56G or 64Gbaud PAM signal, the compensation of the frequency band such as 28GHz or 32GHz can be realized.

  As described above, according to the present embodiment, by adopting the method of improving the high-frequency characteristics according to (1) and (2) for the sub-tap 3, the drop in gain in the tens of GHz band is reduced, and the feedforward equalizer is reduced. The frequency range in which each tap component can be calculated extends to a high range, and a circuit corresponding to compensation in a high frequency range can be realized.

  Further, if this embodiment is applied to the error rate measuring device on the receiving side of the error rate measuring apparatus, it becomes possible to compensate for transmission distortion when receiving an NRZ signal with a high bit rate or a PAM signal with a high baud rate. As a result, it is possible to eliminate the error caused by the transmission distortion in the measurement result and to measure the error rate more accurately.

  Although the best mode of the feedforward equalizer and the method for improving the high frequency characteristic of the feedforward equalizer according to the present invention has been described above, the present invention is not limited to the description and drawings according to this mode. That is, it goes without saying that all other modes, examples, operation techniques, and the like made by those skilled in the art based on this mode are included in the scope of the present invention.

1 Feed Forward Equalizer 2 Main Tap 3 Sub Tap 4 Adder 11 Cell 12 Emitter Follower Circuit (EL)
13 Differential circuit (CML)
21 50Ω termination resistance 22 25Ω termination resistance 23 transmission line 31 error rate measuring device 32 pattern generator 33 error rate measuring device W measurement object

Claims (6)

A combination of the emitter follower circuit (12) and the differential circuit (13) constitutes a unit cell (11), and a sub-tap (3) in which a plurality of cells are connected in multiple stages according to a delay amount is a main tap (2). ) In parallel, and the cell at the final stage functions as a weighting circuit whose gain can be adjusted by setting a coefficient.
The terminating resistor is 25Ω resistor one between the output side of the differential circuit and ground between the input side of said emitter follower circuit (22) is connected in the transmission line (23) between the plurality of cells feed-forward equalizer, characterized in that that. The line impedance of the transmission line (23) between the output of the differential circuit (13) and the input of the emitter follower circuit (12) between the plurality of cells (11) is higher than the 50Ω termination resistance. The feedforward equalizer according to claim 1, wherein Output side or the feed forward equalizer according to claim 2, wherein the connecting Preface to the input side of the emitter follower circuit (12) of the pre-tight end resistance (22) the differential circuit (13). A combination of the emitter follower circuit (12) and the differential circuit (13) constitutes a unit cell (11), and a sub-tap (3) in which a plurality of cells are connected in multiple stages according to a delay amount is a main tap (2). ) Is connected in parallel, and the final-stage cell functions as a weighting circuit whose gain can be adjusted by setting a coefficient.
Connecting a terminating resistor is 25Ω resistance of a single (22) between the ground from the output side of the differential circuit and between the input side of said emitter follower circuit in the transmission line (23) between the plurality of cells A method of improving high frequency characteristics of a feedforward equalizer, characterized by including steps. The line impedance of the transmission line (23) between the output of the differential circuit (13) and the input of the emitter follower circuit (12) between the plurality of cells (11) is higher than the 50Ω termination resistance. 5. The method for improving the high frequency characteristics of the feedforward equalizer according to claim 4, further comprising: Feedforward according to claim 5, characterized in that it comprises the step of connecting Intention to the input side of the output side or the emitter follower circuit (12) of the pre-tight end resistance (22) the differential circuit (13) Method of improving high frequency characteristics of equalizer.

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