DFECDR

Decision feedback equalizer (DFE) with clock and data recovery (CDR)

• Library:

• SerDes Toolbox / Datapath Blocks

• 

Description

The DFECDR block adaptively processes a sample-by-sample input signal or analytically processes an impulse response vector input signal to remove distortions at post cursor taps.

The DFE modifies baseband signals to minimize the intersymbol interference (ISI) at the clock sampling times. The DFE samples data at each clock sample time and adjusts the amplitude of the waveform by a correction voltage.

For impulse response processing, the hula-hoop algorithm is used to find the clock sampling locations. The zero-forcing algorithm is then used to determine the N correction factors necessary to have no ISI at the N subsequent sampling locations, where N is the number of DFE taps.

For sample-by-sample processing, the clock recovery is accomplished by a first order phase tracking model. The bang-bang phase detector utilizes the unequalized edge samples and equalized data samples to determine the optimum sampling location. The DFE correction voltage for the N-th tap is adaptively found by finding a voltage that compensates for any correlation between two data samples spaced by N symbol times. This requires a data pattern that is uncorrelated with the channel ISI for correct adaptive behavior.

Ports

Input

WaveIn — Input baseband signal

scalar | vector

Input baseband signal. The input signal can be a sample-by-sample signal specified as a scalar, or an impulse response vector signal.

Data Types: double

Output

WaveOut — Estimated channel output

scalar | vector

Estimated channel output. If the input signal is a sample-by-sample signal specified as a scalar, the output is also scalar. If the input signal is an impulse response vector signal, the output is also a vector.

Data Types: double

Parameters

DFE

Mode — DFE operating mode

Adapt (default) | Off | Fixed

DFE operating mode:

• Off — DFECDR is bypassed and the input waveform remains unchanged.

• Fixed — DFECDR applies the input DFE tap weights specified in Initial tap weights (V) to the input waveform.

• Adapt — The Init subsystem calls to the DFECDR System object™. The DFECDR System object finds the optimum DFE tap values for the best eye height opening for statistical analysis. During time domain simulation, DFECDR uses the adapted values as the starting point and applies them to the input waveform. For more information about the Init subsystem, see Statistical Analysis in SerDes Systems.

Programmatic Use

• Use get_param(gcb,'Mode') to view the current DFECDR Mode.

• Use set_param(gcb,'Mode',value) to set DFECDR to a specific Mode.

Initial tap weights (V) — Initial DFE tap weights

[0 0 0 0] (default) | row vector

Initial DFE tap weights, specified as a row vector in volts. The length of the vector specifies the number of DFE taps. The vector element value specifies the strength of the tap at that element position. Setting a vector element value to zero only initializes the tap.

You can use a valid MATLAB expression to evaluate the Initial tap weights (V) row vector.

Example: set_param(gcb,'TapWeights',"zeros(1,100)") creates a DFE with 100 taps.

Programmatic Use

• Use get_param(gcb,'TapWeights') to view the current value of DFECDR Initial tap weights (V).

• Use set_param(gcb,'TapWeights',value) to set DFECDR to a specific Initial tap weights (V) vector value.

Data Types: double

Adaptive gain — Controls DFE tap weight update rate

9.6e-5 (default) | positive real scalar

Controls DFE tap weight update rate, specified as a unitless positive real scalar. Increasing the value of Adaptive gain leads to a faster convergence of DFE adaptation at the expense of more noise in DFE tap values.

Programmatic Use

• Use get_param(gcb,'EqualizationGain') to view the current DFECDR Adaptive gain value.

• Use set_param(gcb,'EqualizationGain',value) to set DFECDR to a specific value of Adaptive gain.

Data Types: double

Adaptive step size (V) — DFE adaptive step resolution

1e-06 (default) | nonnegative real scalar | nonnegative real-valued row vector

DFE adaptive step resolution, specified as a nonnegative real scalar or a nonnegative real-valued row vector in volts. Specify as a scalar to apply to all the DFE taps or as a vector that has the same length as the Initial tap weights (V).

Adaptive step size (V) specifies the minimum DFE tap change from one time step to the next to mimic hardware limitations. Setting Adaptive step size (V) to 0 yields DFE tap values without any resolution limitation.

Programmatic Use

• Use get_param(gcb,'EqualizationStep') to view the current DFECDR Adaptive step size (V) value.

• Use set_param(gcb,'EqualizationStep',value) to set DFECDR to a specific value of Adaptive step size (V).

Data Types: double

Minimum DFE tap value (V) — Minimum value of adapted taps

-1 (default) | real scalar | real-valued row vector

Minimum value of the adapted taps, specified as a real scalar or a real-valued row vector in volts. Specify as a scalar to apply to all the DFE taps or as a vector that has the same length as the Initial tap weights (V).

Programmatic Use

• Use get_param(gcb,'MinimumTap') to view the current DFECDR Minimum DFE tap value (V) value.

• Use set_param(gcb,'MinimumTap',value) to set DFECDR to a specific value of Minimum DFE tap value (V).

Data Types: double

Maximum DFE tap value (V) — Maximum value of adapted taps

1 (default) | nonnegative real scalar | nonnegative real-valued row vector

Maximum value of the adapted taps, specified as a nonnegative real scalar or a nonnegative real-valued row vector in volts. Specify as a scalar to apply to all the DFE taps or as a vector that has the same length as the Initial tap weights (V).

Programmatic Use

• Use get_param(gcb,'MaximumTap') to view the current DFECDR Maximum DFE tap value (V) value.

• Use set_param(gcb,'MaximumTap',value) to set DFECDR to a specific value of Maximum DFE tap value (V).

Data Types: double

2x tap weights — Multiply DFE tap weights by a factor of two

off (default) | on

Select to multiply the DFE tap weights by a factor of two.

The output of the slicer in the DFECDR block from the SerDes Toolbox™ is [-0.5 0.5]. But some industry applications require the slicer output to be [-1 1]. 2x tap weights allows you to quickly double the DFE tap weights to change the slicer reference.

CDR

Phase offset (symbol time) — Manual clock phase offset

0 (default) | real scalar in the range [−0.5, 0.5]

Manual clock phase offset to move the recovered clock phase, specified as a real scalar in the range [-0.5, 0.5] in the fraction of symbol time. Phase offset (symbol time) is used to manually shift the clock probability distribution function (PDF) for a better bit error rate (BER).

Programmatic Use

• Use get_param(gcb,'PhaseOffset') to view the current DFECDR Phase offset (symbol time) value.

• Use set_param(gcb,'PhaseOffset',value) to set DFECDR to a specific value of Phase offset (symbol time).

Data Types: double

Reference offset (ppm) — Reference clock offset impairment

0 (default) | real scalar in the range [−300, 300]

Reference clock offset impairment, specified as a real scalar in the range [−300, 300] in parts per million (ppm). Reference offset (ppm) is the deviation between transmitter oscillator frequency and receiver oscillator frequency.

Programmatic Use

• Use get_param(gcb,'ReferenceOffset') to view the current DFECDR Reference offset (ppm) value.

• Use set_param(gcb,'ReferenceOffset',value) to set DFECDR to a specific value of Reference offset (ppm).

Data Types: double

Early/late count threshold — Early or late CDR count threshold to trigger phase update

16 (default) | positive real integer ≥5

Early or late CDR count threshold to trigger a phase update, specified as a unitless positive real integer ≥5. Increasing the value of Early/late count threshold provides a more stable output clock phase at the expense of convergence speed. Because the bit decisions are made at the clock phase output, a more stable clock phase has a better bit error rate (BER).

Early/late count threshold also controls the bandwidth of the CDR, which is approximately calculated by using the equation:

$$\text{Bandwidth}=\frac{1}{\text{Symbol time }·\text{ Early/late threshold count }·\text{ Step}}$$

Programmatic Use

• Use get_param(gcb,'Count') to view the current DFECDR Early/late count threshold value.

• Use set_param(gcb,'Count',value) to set DFECDR to a specific value of Early/late count threshold.

Data Types: double

Step (symbol time) — Clock phase resolution

0.0078 (default) | real scalar

Clock phase resolution of the recovered clock, specified as a real scalar in fraction of symbol time. Step (symbol time) is the inverse of the number of phase adjustments in the CDR. If the CDR has 128 steps of phase adjustment, the Step (symbol time) value is 1/128.

Programmatic Use

• Use get_param(gcb,'ClockStep') to view the current DFECDR Step (symbol time) value.

• Use set_param(gcb,'ClockStep',value) to set DFECDR to a specific value of Step (symbol time).

Data Types: double

Sensitivity (V) — Sampling latch metastability voltage

0 (default) | real scalar

Sampling latch metastability voltage, specified as a real scalar in volts. If the data sample voltage lies within the region of (±Sensitivity (V)), there is a 50% probability of bit error.

Programmatic Use

• Use get_param(gcb,'Sensitivity') to view the current DFECDR Sensitivity (V) value.

• Use set_param(gcb,'Sensitivity',value) to set DFECDR to a specific value of Sensitivity (V).

Data Types: double

IBIS-AMI parameters

Mode — Include Mode parameter in IBIS-AMI model

on (default) | off

Select to include Mode as a parameter in the IBIS-AMI file. If you deselect Mode, it is removed from the AMI files, effectively hard-coding Mode to its current value.

Tap weights — Include Tap weights parameter in IBIS-AMI model

on (default) | off

Select to include Tap weights as a parameter in the IBIS-AMI file. If you deselect Tap weights, it is removed from the AMI files, effectively hard-coding Tap weights to its current value.

Phase Offset — Include Phase Offset parameter in IBIS-AMI model

on (default) | off

Select to include Phase Offset as a parameter in the IBIS-AMI file. If you deselect Phase Offset, it is removed from the AMI files, effectively hard-coding Phase Offset to its current value.

Reference offset — Include Reference offset parameter in IBIS-AMI model

on (default) | off

Select to include Reference offset as a parameter in the IBIS-AMI file. If you deselect Reference offset, it is removed from the AMI files, effectively hard-coding Reference offset to its current value.