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DFECDR

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

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  • DFECDR block

Libraries:
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 and optionally second order frequency 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

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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

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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

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DFE

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 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

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

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 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 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

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

Determine the CDR order to enable phase and frequency tracking.

  • 1st order — Only tracks the phase.

  • 2nd order — Tracks both the phase and frequency.

Programmatic Use

Block parameter: CDRMode
Type: character vector
Values: 1st order | 2nd order
Default: 1st order

Clock phase detector option used in the clock data recovery. You can choose between bang-bang (Alexander) or baud-rate type-A (Mueller-Muller).

Programmatic Use

Block parameter: PhaseDetector
Type: character vector
Values: BangBang | BaudRateTypeA
Default: BangBang

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

Programmatic Use

Block parameter: PhaseOffset
Type: character vector
Values: real scalar in the range [-0.5,0.5]
Default: 0

Data Types: double

Reference clock offset impairment, specified as a real scalar in the range [0, 300] in parts per million (ppm). Reference offset (ppm) is the deviation between transmitter oscillator frequency and receiver oscillator frequency.

Programmatic Use

Block parameter: ReferenceOffset
Type: character vector
Values: real scalar in the range [0, 300]
Default: 0

Data Types: double

Early or late CDR count threshold to trigger a phase update, specified as a unitless real positive 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.

Programmatic Use

Block parameter: Count
Type: character vector
Values: real positive integer ≥5
Default: 16

Data Types: double

Clock phase resolution, specified as a real scalar in fraction of symbol time. Step (symbol time) is the inverse of the number of phase adjustments in CDR.

Programmatic Use

Block parameter: Step
Type: character vector
Values: real scalar
Default: 0.0078

Data Types: double

Internal gain for the frequency tracking loop, specified as a nonnegative real scalar.

Dependencies

To enable this parameter, set the CDR Mode to 2nd order.

Programmatic Use

Block parameter: FrequencyStep
Type: character vector
Values: nonnegative real scalar
Default: 0.00048828125

Data Types: double

Once every Frequency tracking update symbols, update the system phase rotator clock with the frequency estimate.

Dependencies

To enable this parameter, set the CDR Mode to 2nd order.

Programmatic Use

Block parameter: FrequencyCount
Type: character vector
Values: nonnegative real scalar
Default: 16

Data Types: double

To help frequency tracking loop lock early in a simulation, the initial Frequency tracking gain starts at a high value of 1/(2*Frequency tracking update). Then the Frequency tracking gain gradually reduces to the specified value by roughly Frequency step ramp symbol times.

Dependencies

To enable this parameter, set the CDR Mode to 2nd order.

Programmatic Use

Block parameter: FrequencyStepRamp
Type: character vector
Values: nonnegative real scalar
Default: 3000

Data Types: double

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

Programmatic Use

Block parameter: Sensitivity
Type: character vector
Values: real scalar
Default: 0

Data Types: double

IBIS-AMI parameters

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.

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.

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.

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.

More About

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Version History

Introduced in R2019a

See Also

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