Measuring Jitter of a Spread Spectrum Clock Generator


Alliance Semiconductor asked Nextest to try to measure the jitter of one of their products, a Spread Spectrum Clock Generator. The purpose of this chip is to jitter the clock so that the product does not generate a lot of energy at a single frequency and put the product outside the limits of the FCC interference standard. I captured the clock with a digital capture pin, did an FFT and then used a pair of masks to test the limits imposed by the spec sheet. The outer mask (RED) is used to make sure that the jitter does not exceed the spec and the inner mask (BLACK) ensures that the jitter meets the low limit for jitter. Alliance decided it would be cheaper to stick with using Spectrum Analyzers instead of buying the Nextest Lightning even though we showed we could do the test in 0.2 seconds instead of the 10 seconds it was taking their spectrum analyzers.