Jitter injection for on-chip jitter measurement in PI-based CDRs

We propose a technique to estimate the relative jitter between the input data and recovered clock of a 28Gb/s half-rate digital PI-based CDR without using an eye monitor. Instead, we inject square wave jitter with a known amplitude into the CDR, by adding a corresponding signal to the CDR's PI code. By measuring the effect of the injected jitter on the autocorrelation function of the CDR's bang-bang PD output, the RMS relative jitter is estimated with sub-picosecond accuracy, as demonstrated with a test chip fabricated in 28nm CMOS.