frequency trimming in crystal for RTC(Real-time clock)

real time clock crystal capacitor

Hi all,

Regards to the 32.768KHz crystall oscillation circuit design for Real-time clock(RTC), some papers use tuning capacitors for frequency trimming such as the red capacitor Cx in the attachment. However, 32.768KHz crystal generally has a very small motional capacitor(Cs=2f~5f farad) and load capacitors C1 and C2 are about 10p~20p.So that the nominal frequency is mostly determined by Cs and Ls, which means if I want to tune frequency from 32.764Khz to 32.768Khz, I have to add about 8p farad as Cx.

Is this trimming technology reasonable or popular for crystal design for RTC? How can I get a precise 32.768Khz nominal frequency for RTC? Thanks in advance!

Regards,
Trashbox

real-time clock crystal capacitor load

Is this trimming technology reasonable or popular for crystal design for RTC?

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If trimming is required, there are no other options, I think. Most designs try to avoid it.

How can I get a precise 32.768Khz nominal frequency for RTC?

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By using a tight tolerance crystal, low drive level and the nominal load capacitance specified by the manufacturer.

P.S.:

if I want to tune frequency from 32.764Khz to 32.768Khz, I have to add about 8p farad as Cx.

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No, you decrease the frequency by adding load capacitance. An adjustment of 122 ppm would be beyond useful limits, I think.

rtc capacitor

Thank FvM very much!

If trimming is required, there are no other options, I think. Most designs try to avoid it.

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Yes. Because it's the first time for me to design RTC and no info about the parasitic packaging capacitors, I have to add options in this test chip.

By using a tight tolerance crystal, low drive level and the nominal load capacitance specified by the manufacturer.

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Yes. I'm afraid even the crystal is very good, my circuit's non-ideal factors such as packaging capacitor will shift the nominal frequency from 32.768KHz.Is this a concern?

No, you decrease the frequency by adding load capacitance. An adjustment of 122 ppm would be beyond useful limits, I think.

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Do you mean 122ppm is two large?

rtc clock capacitance

Standard watch crystals from a catalog distributor have e. g. 10 or 15 ppm tolerance. The variable load capacitance amount is mostly from the RTC chip pin capacitance, which should be specified normally and can be basically considered in the design dimensioning. A systematic correction can be calculated from measuring a prototype. If an individual adjustment is needed for better accuracy, it shouldn't go beyond a few pF, I think.

frequency trimming

FvM said:

Standard watch crystals from a catalog distributor have e. g. 10 or 15 ppm tolerance. The variable load capacitance amount is mostly from the RTC chip pin capacitance, which should be specified normally and can be basically considered in the design dimensioning. A systematic correction can be calculated from measuring a prototype. If an individual adjustment is needed for better accuracy, it shouldn't go beyond a few pF, I think.

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Hi FvM!
Thanks very much for your comments!