Precise Pulse Width Measurement in Write Pre-compensation Test

Bit density on platters in hard disk drives gets so crowded that the signal stream read out of the magnetic media involves inter-symbol interference. In order to reduce the effect write data timing is tweaked according to specific bit patterns. This "write pre-compensation" is a tiny pulse width modification. Device manufacturers test this functionality by generating a special clock stream whose pulse width is very narrow. Time interval analyzers (TIA) can measure this pulse width precisely. However, the drawback of this is the requirement of a wide analog bandwidth and the need to compensate for a DC signal offset. Compared to a TIA approach, the use of a waveform sampler is shown to be simple, less costly and provides an appropriate capability. This article describes that phase increments of the fundamental frequency component can indicate pulse width increments. A data processing based on fast Fourier transform is described. DC shift has no influence in this method. A measurement instrument does not need a wide analog bandwidth.