Choicing crystal or external oscillator for time critical project on PIC18F

Question

Need to learn how making frequency counter (or any time critical project) using PIC18F that display output over USB.

Having choice of 3 clock sources:

1. ±50ppm 4.0000 MHZ crystal ATP040SM

   4.0000 MHZ 20PF SMD ±50ppm

   http://www.ctscorp.com/components/Datasheets/008-0325-0_A.pdf

2. ±50 ppm 24.0MHZ external oscillator

   XO-54D-24.0MHZ

   ±50 ppm

   http://www.vishay.com/docs/35025/xo-52.pdf

3. ±100 ppm 40.0MHZ external oscillator

   MXO45-40M000 Manufacturer: CTS CORP

   40 Mhz

   ±100 ppm

   http://engineering.dartmouth.edu/courses/engs031/databook/oscillator.pdf

To run internal PLL PIC18F will divide clock input by PLLDIV that can be setting to either 1, 6 or 10 for above options respectively.

I thinking which option providing best stability?

When thinking myself, I feel 3rd option will be best as I reasoning that ±100 ppm 40Mhz clock will be divide by 10 and hence effectively behaving like ±10 ppm 4Mhz clock?

Or is reasoning wrong and ±100 ppm 40Mhz clock after divide by 10 is effectively still ±100 ppm 4Mhz clock?

If you, experienced designing having choice of above 3 - which one you choose and why (no consider PCB space or 5V voltage issue in 3.3v world, but might consider cost only between more expensive ±50 ppm vs less expensive ±100 ppm)

Answer 1

You are interested in the relative error and/or stability, which is unaffected by any division. So, other things being equal, take the lowest ppm clock source for the best accuracy.

If 50 ppm is enough for you, I would choose the crystal for board space and price.

For better accuracy you could consider a better quality (tuned) crystal oscillator. Same physical format as a regular oscillator (14-pin DIP size), but higher price (~ $20).