What is the difference between the WFM601M TIMING and HPF jitter reading?
TIMING jitter is a longer term, slower jitter than HPF jitter. There are two separate jitter high pass filter paths in the WFM601/A/E/M, one with a -3 dB cutoff point around 10 Hz, and one path with a selection of cutoff filters.
TIMING measures longer term jitter by averaging out, or smoothing, the reference clock (the indication is extracted ref clock vs. the signal) to follow the slowly jittering signal around, so the jitter reading is really what is left after subtracting any low frequency jitter. The TIMING jitter will generally be higher than HPF jitter, unless the HPF filter is also set at 10 Hz. Even when the HPF is set to 10 Hz, the readings won't be exactly the same at low jitter values because two separate demodulators are used and reading peak jitter excursions is done with two different DACs. At low signal levels, noise becomes a factor.
When the HPF filter is selected to be higher than 10 Hz (1 kHz, 10 kHz, or 100 kHz), the reference clock follows the extracted signal clock more closely, and jitter much below the -3 dB cutoff point will be ignored. This is one way jitter can be characterized; just see which HPF setting reduces the jitter reading and the problem is somewhere above the cutoff point.
The WFM601M includes a jitter display, which is also useful in determining the source of jitter. Since the display is locked to video sync, jitter induced by the vertical interval, or the video itself, can be identified. You can also look at the jitter output on the rear panel with an oscilloscope in FFT mode to see spectral characteristics.
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