Jitter - Page 3

- Measuring Jitter

Pit geometry and Jitter can be measured by looking at the HF signal coming from the pickup. The HF (High Frequency) signal coming from the pickup represents the light intensity of the beam reflected back from the disc surface. A higher voltage represents greater light intensity, and a lower voltage represents less light. The signal is rapidly changing between light and dark as the beam passes over the pits. When the beam is over a pit, the light intensity is reduced. When the beam is between pits (over "land"), the light intensity is higher. Data is encoded in the transitions between pits and lands.

The waveform shown below shows "Jitter" as regenerated from an optical disk. "Jitter" values and limits are specified in CD-ROM and DVD published industry standards. Jitter is made up primarily of two phenomena. One is the measurement of the time difference between the data and clock and the other is the measurement of pulse width (pulse cycle measurement for Magnetic Optical drives). The pulse width measurement is used to evaluate disk media.

Because the oscilloscope is triggered by the signal itself, we can see the jitter in the different pit lengths (such as 3T, 4T…11T) separately. The other thing we can see is the "deviation" for each pit length. This is a measure of how different a given pit length appears to be from what it should be. So, for example, the usual tendency is for the shortest pits (the 3T pits) to appear a little shorter than 3T. Jitter and deviation are two aspects of the same thing: jitter is the random variation, and deviation is the average error, in the apparent length.