Method for reducing analog PLL jitter in video application
The present invention provides a method for reducing analog PLL (Phase-lock loop) jitter in video ADC application. The HSync/CSync is replaced with a faked HSync signal to be inputted to PLL during vertical blank period. Therefore the analog PLL will only see the faked HSync signal of fixed period as a line-lock trigger signal, while no COAST signal is required. Also, the faked HSync is fine-tuned to match with the external HSync/CSync leading edge to minimize PLL jitter.
1. A method for reducing analog PLL (Phase-lock loop) jitter in a video application that uses a PLL, a VSync Separator and an SOG/SOY slicer, the method comprising steps of:
in an Emulate HSync function, accepting a video signal, a COAST signal from the VSync Separator and a PLL_CLK signal from the PLL, and
generating a series of faked HSync signals Fake_HSync to be inputted to the PLL during a COAST period.
2. The method according to claim 1 , wherein the PLL_Clk signal is used to track High/Low width of a Csync signal in the video signal during non-COAST period, so as to generate periodically the Fake_HSyne signal similar to a HSync signal in the video signal to keep stimulating PLL during the COAST period.
3. The method according to claim 2 , wherein the signal Fake_HSync is monitored by a −1T clock window and a +1T clock window (where T means clock period) to see whether the external CSync has leading edge transition;
if no leading edge transition then generate a next Fake_HSync in the same way; and
if the leading edge transition is detected then adjust the next Fake_HSync by +1 clock period or −1 clock period according to which window (−1T or +1T) contains the leading edge transition, and then return to the original period.
4. The method according to claim 1 , wherein a last Fake_HSync is monitored, and when the last Fake_HSync is slower below a predetermined fraction of a period, then generating the next HSync pulse as predicted; when the last Fake_HSync is slower more than the predetermined fraction of a period, then the last Fake_HSync is omitted.
5. The method according to claim 4 , wherein the predetermined fraction of a period is ¼ period.