I've been able to use a 26Mhz and 40Mhz xtal in testing, but for production if 40Mhz is more supported, that's what I would like to design in. I'm considering this XTAL: http://datasheet.octopart.com/FA-128-40 ... 700903.pdf
For other designs, I typically double the capacitance rated on the datasheet for the xtal in the design. However, I imagine that there are higher tolerances required for this design given that it'll be doing RF so I want to make sure I'm picking the right values.
In one of your datasheets (which I can't seen to find, except for a screenshot of the relevant section here in chinese viewtopic.php?t=1924) there were some specified ranges of capacitances and they were asymmetric for the 40Mhz option.
How should I go about picking the right capacitance values? Do I need to measure the the output freq of the xtal and adjust the caps till I'm smack in the middle of 40Mhz? And I would imagine the oscope probe will influence the oscillating frequency. Is there some way to output the 40mhz on an external pin so I can measure it there while making adjustments?
For the XTAL that I posted the datasheet above, I'm planning on using two 20pF caps -- how reasonable is that?
Thnx,
Reza
Crystal Capacitance Tuning
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Re: Crystal Capacitance Tuning
What I would do for a 12pF crystal is exactly what you mentioned. But if you are using a thin, especially 4 layer PCB, you should account for the trace capacitance as well, it can be of the order of 4-6pF in some cases.
Apart from that, you can put out 40MHz clock, or a divided version, on the CLK_OUT pins by programming and count that output. Using 0402 or 0201 capacitors with crystals will be preferable.
Directly probing a crystal will definitely affect the output. Direct probing is only to see if the clock is present, won't trust that any further in an RF application.
Apart from that, you can put out 40MHz clock, or a divided version, on the CLK_OUT pins by programming and count that output. Using 0402 or 0201 capacitors with crystals will be preferable.
Directly probing a crystal will definitely affect the output. Direct probing is only to see if the clock is present, won't trust that any further in an RF application.
- Pratik
Just another hobbyist and consultant
Just another hobbyist and consultant
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