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ANDYROID BIT SLICER SERIES

After 1 ns delay, pulse appears at tap 2 & is multiplied by C0, generating the 400 mV, 1 ns cursor pulse. 1 ns later, the reaches the final tap, is multiplied by C2, creating the +50 mV, 1 ns 2nd postcursor pulse.ġ6 Tx Pre-emphasis Pulse arrives filter input & is multiplied by C-1, generating a precursor pulse of -50 mV amplitude and 1 ns duration. 400 mV 600 mV 1 ns 1 - 50 mV 3 4 x T T T k C C C C - 1 1 2 4 50 mV 3 - 100 mV S y k 1 ns later, the pulse is at tap 3 tap & gets weighted with C1, generating the first postcursor pulse (-100 mV amplitude, 1 ns duration). 2 After 1 ns delay, pulse appears at tap 2 & is multiplied by C0, generating the 400 mV, 1 ns cursor pulse. Pulse arrives filter input & is multiplied by C-1, generating a precursor pulse of -50 mV amplitude and 1 ns duration. The number of taps depends on the length of the channel relative to the unit interval of the data. D C -1 1 2 S y k x The outputs from the weights are summed to produce the transmitter output.

Negative subscripts compensate “precursor” ISI, positive for “postcursor” ISI. The input signal is sampled between each delay line and multiplied by a weighting factor (Ck). Each line typically has a delay of one unit interval.

ANDYROID BIT SLICER SERIES

The input stream propagates thru a series of delay lines. Equalizedġ4 Pre-Emphasis Circuit DLEs use finite impulse response (FIR) filters. This reduces the maximum swing, but produces an open eye. Often done by attenuating successive bits (“de-emphasis”). Non-Equalized Pre-emphasis adjusts the magnitude of the transmitter output based on prior bit values.

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Isolated bits or rapidly alternating 0s/1s don’t build up to the full swing at the receiver in a lossy channel. The filter can be made of discrete components, integrated into the silicon, or even built into cables or connectors.ĬTLE Closed eye -21.0 dB -16.0 dB PCB Closed eye PCB+CTLE The passive equalizer doubles the usable spectrum.ġ2 Passive Gb/s Non-Equalized Equalized Eye mask : 20 mV x 50 psġ3 Transmit Equalization (a.k.a. Amplification of high frequency components is possible, too. Other types of filtering can compensate for other sources of distortion (e.g. Discrete linear equalizers (Tx pre-emphasis or Rx). Popular types: Continuous time filters (with/without amplification). Interconnects can contain equalizing filters, too. Often done at the transmitter (Tx), receiver (Rx), or both. The equalizer may be placed anywhere in the channel.

Goal: reduce distortion to tolerable levels. The ideal response requires an equalizer that responds as the inverse of the channel: Equalized The ideal response is typically not practical. Losses increase with frequency qS21(f) isn’t linear with frequency: er varies with frequency Result: Amplitude and phase distortion cause “smearing” of pulses (ISI) which closes the “eye” diagram.Ĩ The Ideal Equalizer With an equalizer in the channel: Zero amplitude distortion qS21(f) is a linear function of frequency: constant delay at all frequencies  zero phase distortion Result: distortionless transmission, open data “eye.” The signal at the input to the receiver is a function of the transmitted signal and the channel insertion loss: time domain frequency domain  We describe the channel as a band limited filter:

The signal at the receiver input is: binary 1 binary 0 where ht(t) = transmitter filter hc(t) = channel impulse response n(t) = noise r(t) = received signal * = convolution operationĥ Introduction #2 Let’s simplify things slightly by looking at the transmitter, interconnect channel, and receiver. We use equalization to improve timing margins.Ĥ Introduction Systems with equalization are best viewed as communications links where each block filters the signal. Without scaling, interconnects will take 2/3 of the Gen2 5 GT/s timing budget. For example: The spec for PCIe Gen1 allocates 65 ps to the channel (1/3 of 2.5 GT/s timing budget). Board and silicon timings must continue to scale together. Solution spaces cannot shrink indefinitely. Motivation Signaling speeds continue to increase while interconnects change incrementally. 1 SI Fundamentals Short Course - EqualizationĢ Contents Introduction Channel CharacteristicsĬontinuous Time Linear Equalizers Discrete Time Linear Equalizers Decision Feedback Equalizers Adaptive Equalization Crosstalk Cancellation Summaryģ We use equalization to improve timing margins.

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