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| RFID Circuit Design |
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ImLeftFooted
Member #3,935
October 2003
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I'm attempting to build the circuit in this RFID chip's white-paper. It left something vague however. {"name":"33164d1252029237-rfid-cystal-oscillator-rfid-crystal.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/1\/8\/1813eecf074911c274e49c8a63576656.png","w":545,"h":505,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/1\/8\/1813eecf074911c274e49c8a63576656"} I can't figure out what these two capacitors by the crystal should be. I've read the data sheet for the crystal from top to bottom and it says nothing to this effect. I've read up on crystals and learned their history, design and production. This was all very interesting but hasn't helped me with the actual question: What kind of capacitors do I need? For reference, here is the white-paper for the RFID chip: |
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someone972
Member #7,719
August 2006
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EDIT: Ignore this post. Incorrect information used to be here. ______________________________________ |
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Bob
Free Market Evangelist
September 2000
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What clock crystal are you using? The data sheet for that should tell you what kind of decoupling caps you need. -- |
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Thomas Fjellstrom
Member #476
June 2000
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IIRC as bob said, it depends on the crystal and what frequency you'll be wanting it to run at. -- |
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GullRaDriel
Member #3,861
September 2003
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Bob's profile said:
Bob I've forgotten that one, and it still make make laugh each time I see it :-) "Code is like shit - it only smells if it is not yours" |
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ImLeftFooted
Member #3,935
October 2003
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I searched for crystals and found one that seemed legitimate. Here is the spec sheet for that: |
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piccolo
Member #3,163
January 2003
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i think that sheets says a min of 10 pF. wow |
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ImLeftFooted
Member #3,935
October 2003
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So does that mean it can be anything as long as it's at least 10 pF? |
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Thomas Fjellstrom
Member #476
June 2000
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Dustin Dettmer said: So does that mean it can be anything as long as it's at least 10 pF? as was said, it depends on what frequency you want the crystal to run at. which decides the frequency the chip itself runs at. you NEED to see what frequency the rfid chip wants and base the calculations for the caps on that. -- |
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ImLeftFooted
Member #3,935
October 2003
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How do I calculate that? The RFID frequency is 125 kHz but I don't think the crystal would run at the same frequency. The RFID spec sheet http://www.nxp.com/acrobat_download/datasheets/HTRC11001T_2.pdf says the crystal can be 6, 8, 12 or 16 mhz. It says the chip can figure out which one it is and be okay with that. So I picked a crystal with 6 8 12 or 16 mhz. But how in the hell do I figure out what the caps need to be? Here is a document that describes how to calculate it. http://www.mpdigest.com/issue/Articles/2008/Mar/Crystek/Default.asp It might as well be written in Chinese because I can't even read it. Okay Given this: Quote:
The total capacitance seen from the crystal looking into the rest of the circuit is called the "load capacitance". When a manufacturer makes a "parallel" crystal, a technician uses a Pierce oscillator with a particular load capacitance (often 18 or 20 pF) while trimming the crystal to oscillate at exactly the frequency written on its package. [1] So I need two 40pf caps? This seems rather large... |
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piccolo
Member #3,163
January 2003
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if you find any nice book on this stuff let me know i am working on the same type of stuff wow |
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Matt Smith
Member #783
November 2000
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you can find 33pf or 47pf caps in many/any discarded circuit board. It's not that critical, so use whatever you have. ceramic ones (the tan flat discs) will work fine, as will SMT ones, if you are lucky enough to have junk with them marked (resistors use the same numbers, so watch out) |
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(20 is the crystal capacitance)
