required accuracy for crystal oscillator

Propability

·OK so I did in my pll on my protoboard and I want to see if driving it with a 80 Mhz crystal oscillator will bring the video back to life. I see there are varying degrees of accuracy from 100 ppm to 25 ppm with the 25ppm being the costliest of course. If the crystal (5Mhz) might have a 10ppm (might be a little higher -just going by memory)· and with the pll set at 16x what is the accuracy of the clock? Is it 16times 10ppm and if so is a 100ppm crystal oscillator good enough for video.

Pete

Mike Green

ppm is Parts Per Million. It doesn't matter what the PLL is set at. 100 ppm of 5,000,000Hz is 500 while 100 ppm of 80,000,000 is 8,000, but it's still the same ratio. You will lose or gain the same amount of time over a period of days.

The accuracy of an external oscillator is both long-term (how close is the frequency to that stamped on the case?) and short-term (how much does the frequency vary with temperature, power supply voltage, aging?) In both cases, video depends on a fairly stable clock over short periods of time (fractions of a second to seconds). Absolute frequency accuracy needs to be on the order of percent and tenths of a percent.

A 100ppm crystal oscillator should be fine for video.

Propability

·Mike, thanks ·for the reply. I revisited Digi-Key and the ppm for the crystal (HC49US) 5 Mhz was listed at +/- 30ppm .

Pete

hinv

Mouser has a 6MHz crystal at +/- 20ppm here:
www.mouser.com/Search/Refine.aspx?FS=TRUE&Ntt=*hc49us*+*6MHz*&Ne=688437+688748&N=5115911+4294622349+4294622948+1323038&Ntx=mode%2bmatchall&Ns=P_SField&OriginalKeyword=hc49us+6MHz&Ntk=Mouser_Wildcards

I am not sure if it is in the right capacitance range though at 18pf.

Propability

Sorry hinv,

I'm out of luck trying to use a crystal (still have the 5 Mhz that came with the protoboard) and the PLL (at 16x) to drive any video since I somehow broke the PLL in the Prop. It does run with the crystal but not with any of the PLL settings. I was just curious as to the needed accuracy the clock had to be since the 100 ppm is about $1.60 versus about $3.80 for a 25ppm crystal oscillator which runs at 80Mhz which will bypass the PLL , so hopefully· I can get the video back without having to remove the Prop chip from the board, which has been populated with a few connectors I don't want to remove.

Pete