Sample-and-hold-based retimer supporting link training

This application claims priority to U.S. Provisional Application 63/045,484, filed Jun. 29, 2020, titled “Sample-and-Hold Based Linear Retimer Supporting Link Training,” which is hereby incorporated by reference in its entirety.

Wired communications systems use link training to enable tuning of the finite impulse response (FIR) filters in the transmitters and receivers. However, nonlinear retimers cannot pass link training information directly from the input to the output. Instead, a digital signal processor detects the transmitter's FIR coefficients from the input signal and reconstructs it in the retimer output signal. The complex signal processing can consume large amounts of power.

A retimer includes an equalizer, a clock recovery circuit, a sample and hold (S/H) circuit, and a linear driver. The equalizer receives an input signal and outputs an equalized signal. The clock recovery circuit receives the equalized signal and outputs a clock signal. The S/H circuit receives the equalized signal and the clock signal and outputs a retimed signal. The linear driver receives the retimed signal and outputs a recovered signal. The S/H circuit is configured to preserve a voltage of the equalized signal in the retimed signal. The retimer can be placed between a transmitter and a channel or after the channel.

In some examples, the S/H circuit is part of a three-tap feedforward equalizer, and the linear driver receives an output of the feedforward equalizer. In some examples, the S/H circuit is a first S/H circuit and the retimed signal is a first retimed signal. The three-tap FFE can include a second S/H circuit, a third S/H circuit, a first amplifier, a second amplifier, a third amplifier, and a summer. The second S/H circuit and the first amplifier receive the first retimed signal. The second S/H circuit outputs a second retimed signal, and the first amplifier outputs a first weighted signal. The third S/H circuit and the second amplifier receive the second retimed signal. The third S/H circuit outputs a third retimed signal, and the second amplifier outputs a second weighted signal. The third amplifier receives the third retimed signal and outputs a third weighted signal. The summer receives the first, second, and third weighted signals and combines them to obtain the FFE output signal.

In some examples, the S/H circuit includes two track and hold (T/H) circuits. One of the T/H circuits can include a preamplifier and a switched emitter follower. In some examples, the preamplifier includes a degeneration capacitor which has a capacitance chosen to extend a bandwidth of the preamplifier. In some examples, the T/H circuit includes a feedforward capacitor coupled between the preamplifier and the switched emitter follower. A capacitance of the feedforward capacitor can be chosen to reduce hold mode feedthrough.

For a detailed description of various examples, reference will now be made to the accompanying drawings in which:

FIG. 1 illustrates an example communication system including a nonlinear retimer.

FIG. 2 illustrates an example communication system including a linear retimer.

FIG. 3 illustrates a block diagram of an example linear retimer with a sample and hold circuit.

FIG. 4 illustrates a block diagram of an example linear retimer with a three-tap feedforward equalizer.

FIG. 5 illustrates an example track and hold amplifier for use in a sample and hold circuit.

The disclosed retimers include a sample and hold (S/H) circuit to retime the received signal, which preserves the link training information while retiming jitter in the signal. Any appropriate S/H circuit may be used. The transmitter finite impulse response (FIR) coefficients are passed directly from the input to the output of the disclosed retimers without any complex digital signal processing. The retimers can be placed before or after the channel. A discrete-time linear feedforward equalizer can be implemented by cascading S/H circuits with weighted additions to reduce both precursor and postcursor inter-symbol-interference.

FIG. 1 illustrates an example communication system 100 including a nonlinear retimer 130 with a D flipflop 140 . Communication system 100 includes a transmitter 110 , the nonlinear retimer 130 , and a receiver 150 . A signal Vin 105 is provided to the transmitter 110 which includes a finite impulse response (FIR) filter 115 . FIR filter 115 adds link training information to signal 120 output by transmitter 110 , as shown in signal diagrams 120 A-B which represent the voltage of signal 120 at a time t 1 and signal 120 at time t 2 , respectively. Nonlinear retimer 130 includes an equalization module 135 and a D flipflop 140 . The D flipflop 140 limits the voltage of signal 120 and cannot pass link training information to signal 145 output from nonlinear retimer 130 .

The signal diagram 145 A illustrates the constant voltage of signal 145 at time t 1 , in contrast to the varying voltage of signal 120 at time t 1 as shown in signal diagram 120 A. The signal diagram 145 B illustrates the constant voltage of signal 145 at time t 2 , in contrast to the varying voltage of signal 120 at time t 2 . As illustrated in the signal diagrams 145 A-B, the signal 145 A at time t 1 is the same voltage as the signal 145 B at time t 2 , masking the link training information injected into signal 120 by the FIR 115 in transmitter 110 . The signal 145 is provided to receiver 150 which outputs a signal Vout 155 . Receiver 150 is unable to recover the link training information in signal 120 from FIR 115 in transmitter 110 . A digital signal processor and a second FIR filter can be included in nonlinear retimer 130 to detect the link training information and reconstruct it at the second FIR filter, as described in U.S. Pat. No. 9,210,008. However, the digital signal processor and a second FIR filter cause nonlinear retimer 130 to consume large amounts of power.

FIG. 2 illustrates an example communication system 200 including a linear retimer 230 with a sample and hold (S/H) circuit 240 . Communication system 200 is substantially similar to communication system 100 shown in FIG. 1 , but includes linear retimer 230 instead of nonlinear retimer 130 . Linear retimer 230 includes S/H circuit 240 as a retiming element, rather than a flipflop such as D flipflop 140 . The S/H circuit 240 is transparent to link training and can pass the transmitter FIR information reliably without the need for complex digital processing, while still retiming the jitter, causing linear retimer 230 to consume less power than nonlinear retimer 130 . In addition because it is linear, retimer 230 can be placed before or after the channel such that the transmitter FIR filter 115 can equalize channels both before and after linear retimer 230 .

The signal 245 output from linear retimer 230 is represented by signal diagrams 245 A-B which illustrate voltage changes in signal 245 A at time t 1 and in signal 245 B at time t 2 , in contrast to the constant voltage of signal 145 output from nonlinear retimer 130 . Receiver 150 receives the signal 245 from linear retimer 230 and outputs signal Vout 255 . As signal diagrams 245 A-B illustrates, receiver 150 is able to retain the voltage changes in signal 120 output from transmitter 110 in the output signal 255 . Although the example shown herein includes a signal with two voltage levels, the S/H circuit 240 linearly passes voltage information, and linear retimer 230 can be extended to three, four, or more voltage level signals as well.

FIG. 3 illustrates a block diagram of an example linear retimer 300 with a sample and hold circuit 320 . Linear retimer 300 includes an equalizer 310 , a clock and data recovery module 315 , the S/H circuit 320 , and a linear driver 370 . Equalizer 310 can be any appropriate equalizer, such as a continuous time linear equalizer. In this example, S/H circuit 320 includes two track and hold (T/H) amplifiers 325 and 350 . T/H amplifier 325 includes a switch 335 and a capacitor 340 , and T/H amplifier 350 includes a switch 360 and a capacitor 365 . In T/H amplifier 325 , switch 335 receives clock signal CK 330 from the clock and data recovery module 315 and is coupled between an output of equalizer 310 and T/H amplifier 350 . Capacitor 340 is coupled to switch 335 and to ground 345 .

In T/H amplifier 350 , switch 360 receives a clock signal CK 355 that is inverted relative to clock signal CK 330 , and is coupled between an output of T/H amplifier 325 and an input of linear driver 370 . Capacitor 365 is coupled to switch 360 and to ground 345 . Linear driver 370 receives the output from track and hold amplifier 350 and outputs the signal Vout 375 . Because the S/H circuit 320 is linear, driver 370 is also linear to retain the transmitter FIR information.

FIG. 4 illustrates a block diagram of an example linear retimer 400 with a three-tap feedforward equalizer (FFE) 450 . Linear retimer 400 is substantially the same as linear retimer 300 shown in FIG. 3 , but includes a three-tap FFE 450 rather than a S/H circuit 320 . Three-tap FFE 450 includes three S/H circuits 420 A-C, three linear amplifiers 425 A-C, and a linear summer 440 . The three S/H circuits 420 A-C each receive a clock signal CK 430 from the clock and data recovery module 415 . S/H circuit 420 A is coupled to an output of equalizer 410 and provides its output to S/H circuit 420 B and to an amplifier 425 A, which multiplies the output from S/H circuit 420 A by a weight α−1.

S/H circuit 420 B provides its output to S/H circuit 420 C and to an amplifier 425 B, which multiplies the output from S/H circuit 420 B by a weight α0. S/H circuit 420 C provides its output to an amplifier 425 C, which multiplies the output from S/H circuit 420 C by a weight α+1. Summer 440 subtracts the outputs of amplifiers 425 A and 425 C from the output of amplifier 425 B, substantially cancelling out precursor and postcursor ISI. The output from summer 440 is provided to a linear driver 470 , which outputs a signal Vout 475 .

FIG. 5 illustrates an example T/H amplifier 500 for use in a S/H circuit such as S/H circuit 320 shown in FIG. 3 . T/H amplifier 500 includes a preamplifier 540 with two switched emitter followers 550 A-B. Preamplifier 540 includes a degeneration capacitor Cs 560 coupled in parallel with the degeneration resistor Rs. Preamplifier 540 exhibits a pole at approximately:

1 ( RL ) ⁢ ( C ⁢ ⁢ 1 ) ,

where C 1 represents an input capacitance of the switched emitter follower 550 A or 550 B, and a pole at approximately:

1 + GmRs 2 Rs ⁢ ⁢ Cs ,

where Gm represents the transconductance of the transistors Q 1 and Q 2 . Preamplifier 540 exhibits a zero at

1 ( Rs ) ⁢ ( Cs ) .

The capacitance of Cs 560 can be chosen such that the zero at

1 ( Rs ) ⁢ ( Cs )

cancels out the pole at

1 ( RL ) ⁢ ( C ⁢ ⁢ 1 ) ,

extending the bandwidth of T/H amplifier 500 . Feedforward capacitors Cf 570 A-B are coupled between preamplifier 540 and switched emitter followers 550 A-B, respectively, and cancel hold mode feedthrough by opposing the effect of the base-emitter capacitance of transistors QEF 1 , 2 respectively.

The term “couple” is used throughout the specification. The term may cover connections, communications, or signal paths that enable a functional relationship consistent with the description of the present disclosure. For example, if device A generates a signal to control device B to perform an action, in a first example device A is coupled to device B, or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B such that device B is controlled by device A via the control signal generated by device A.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.