Xtal Caps / Measure Caps

[Scutarius]

Hi

I have this xtal (32.768kHz) and  the datasheet  saids that CL=15.5pF, my question is if it is right by saying that..


CL= (C1*C2)/(C1+C2) + Cs

Cs= 5pF http://www.oscilent.com/spec_pages/PNDescrpt/Load_Cap.htm

I have no idea why C1 and C2 have the same value  most of times so

12.5-5=  (C*C)/(2*C)

That gives C=15pF

So, if all the above is right, my schematic is a 32.768kHz Xtal with two 15pF caps.



http://forum.allaboutcircuits.com/showthread.php?t=9215
http://www.oscilent.com/spec_pages/PNDescrpt/Load_Cap.htm
http://www.eetkorea.com/ARTICLES/2001SEP/2001SEP06_AMD_AN.PDF
http://www.oscilent.com/spec_pages/PNDescrpt/Load_Cap.htm



My second question is how to measure a 0805 cap?, about 1nF or so, my first thought was to desolder the cap and make a filter with a know resistance and due to the Fc know the capacitance, what do you thing? (I need to copy a design I already have must of the values and the schematic)


Thank you!!!

BTW this osc are normally found on watches I am not sure but they usually look that there are no CL, just the xtal

[Scutarius]

wow 81 visits and nobody saids if I am wrong, could somebody take 2 minutes please?

Thanks in advance.

[jahonen]

Yes, your reasoning of the crystal load capacitors is correct. They are double to the required load capacitance (excluding parasitics), as they connect in series from crystal perspective. Usually, it might take some tweaking by using a frequency counter to get the oscillation frequency closest to target.

You can measure SMD capacitors with usual LCR-meter, by using SMD tweezers. Or if that is not available, solder some suitable wires to the cap and then plug it into LCR meter (just be careful not to break the terminations).

Regards,
Janne

[Scutarius]

thank you Jahonen

So I think my multimeter just cant measure small values (not a fluke, no even a good one)

[alm]

So I think my multimeter just cant measure small values (not a fluke, no even a good one)

All multimeters that I know (including Flukes) suck at low capacitance values, that's why dedicated LCR meters (like the AADE kit) are useful. But if you don't own a good DMM, an LCR meter is probably pretty far down your list, since it's a fairly specialized bit of kit.