Load capacitors for crystal - Page 1
Forum section: electronics / beginners

author: Jonathon_Doran
author_rank: Regular Contributor
timestamp: June 28, 2024, 05:37:53 pm
content: I am double-checking some values on a design before I route it, and came across an oddball. I am trying to use a 16MHz crystal, and JLCPCB has one in its basic parts list! Great. But then I get the datasheet and look up the load capacitance...This is a YXC part, and they list the load capacitance as "10pF, 20pF, or specify".I just want this thing to start up. I'm not sure what to make of this.Any recommendations?

author: Ice-Tea
author_rank: Super Contributor
timestamp: June 28, 2024, 05:50:28 pm
content: Is it not specified in the part number?

author: Jonathon_Doran
author_rank: Regular Contributor
timestamp: June 28, 2024, 06:14:18 pm
content: I have no clue, to be honest. The manufacturer's part number and their datasheet do not match.This is the part that JLCPCB lists: X322516MLB4SIThe description is:16MHz Surface mount crystal oscillator 9pF ±10ppm ±20ppm SMD3225-4P Crystals ROHSI assume the 9pF is the stray capacitance. Maybe I have that wrong.They list the manufacturer as YXC. The link to a datasheet is for a YXC part, but that datasheet names it YSX321SL.

author: Ice-Tea
author_rank: Super Contributor
timestamp: June 28, 2024, 06:32:38 pm
content: So, yes, it's part of the PN. Too bad no decoding is providedhttps://www.yxcxtal.com/product/ysx321sl.html

author: edavid
author_rank: Super Contributor
timestamp: June 28, 2024, 06:35:30 pm
content: The crystal will always start up. The load capacitance just determines the frequency. To get the specified frequency accuracy, you need to use the specified load capacitance. If you aren't building a clock, don't worry about it too much.

author: Gyro
author_rank: Super Contributor
timestamp: June 28, 2024, 06:44:50 pm
content: Quote from: Jonathon_Doran on June 28, 2024, 06:14:18 pmThat sounds like the load capacitance (I can't open the link that Ice-Tea provided). In an ideal world, that would need a pair of 18pF capacitors. In practice you need to subtract the stray capacitance of the PCB traces and IC, so you're more likely to end up with something like 2 x 12pF. As edavid says, the absolute value will affect frequency accuracy.

author: Jonathon_Doran
author_rank: Regular Contributor
timestamp: June 28, 2024, 07:39:02 pm
content: Thanks for the link. The datasheet does not discuss the part numbers. But even then... "S"? Well, the part that I am looking at has "L"... small/large? Are they trying to tell me to use 20pF loads?The 9pF value does not appear in the datasheet at all! And stray capacitance is not shown. They do list "shunt" capacitance (which I read is the capacitance of the plates). Why would I care about the shunt capacitance?Timing is rather important since I want to try to drive some programmable LEDs which rely on timing to control them. The width of pulses indicate a 0/1.Maybe the lesson to be learned is to pick a part that has a datasheet that makes sense and not try to save $$$ with a basic JLCPCB part.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2024, 07:49:33 pm
content: For crystals, "shunt" capacitance normally means the required total external capacitance (stray plus on-purpose) in parallel with the crystal terminals to oscillate at the calibrated frequency.If you want to tweak the frequency, you can use an appropriate (trimmer) variable capacitor.Crystals can be operated in series-resonant or parallel-resonant mode (different oscillator circuits): shunt capacitance is specified for parallel-resonant mode operation.

author: edavid
author_rank: Super Contributor
timestamp: June 28, 2024, 10:08:26 pm
content: Quote from: Jonathon_Doran on June 28, 2024, 07:39:02 pmNot really. No matter what capacitance you use, you can't pull the crystal far enough that the LEDs wouldn't work. As I said, don't worry about it. (I would put in capacitor pads, but leave them unpopulated.)

author: Jonathon_Doran
author_rank: Regular Contributor
timestamp: June 29, 2024, 01:34:20 am
content: Thanks. Not populating the capacitors sounds like not trying. I think 18pF caps will get the job done.

author: Rabbitoose
author_rank: Contributor
timestamp: June 29, 2024, 09:31:19 pm
content: I also noticed this for a time. I guess 9pF is mostly like the load capacitance value and I always use it to calculate the proper capacitor value. I used a agilent 53131A to measure the accuracy and the results were ok.As of why it's not appearing in the datasheet, I believe it's because YXC is lazy on writing a detailed datasheet and just use a simple single version of datasheet for different frequency products of the same series, which have different load capacitances. They might provide the load capacitance value in another way together with the purchase order, and LCSC wrote it on the product page.As I mentioned, it's a guess, I have no solid proof. But there are many unprofessional practices like this in many companies, which I hated very much as a Chinese.