Continuous-time linear equalizer (CTLE)

A continuous-time linear equalizer is a small analog amplifier at the receiver's very front that does one thing: amplify high frequencies more than low frequencies, exactly the opposite of what the lossy channel did. Picture a graphic equalizer on a stereo with the treble knob cranked up to compensate for muffled speakers — the CTLE 'boosts the treble' of the bit stream so the smeared edges come back to life before any sampling happens.

Circuit-wise a CTLE is usually a differential pair with a source-degeneration resistor and capacitor that creates a 'peaking' response — flat gain at low frequency, rising gain toward the Nyquist frequency. Its big advantage is that it's cheap, fast, and operates on the raw analog waveform, so it opens the eye before the sampler. Its limit is that boosting high frequency also boosts high-frequency noise and crosstalk, so a CTLE provides the first, broad cleanup and then hands off the residual ISI to a DFE that finishes the job without adding noise.