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ST95HF crystal characteristics

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zvonc
Associate

‎2016-09-26 03:38 AM

Posted on September 26, 2016 at 12:38

Hi,

we are using ST95 HF NFC transceiver with XRCGB27M120F3M00R0 27.12 MHz crystal. So far, we manuufactured about 30 modules and all works well. However, we are planning a bit larger series in a close future (10k+) so we would like to be sure everything will work fine in whole operating range. Currently, we have concerns regarding a crystal specification.

In datasheet (p 59) it is stated that with a crystal of ESR equal to 30 ohms, rated load capacitance should be 6pF, and also that used load capacitors should be in 10 to 20 pF range.

If a crystal ESR is 5 times larger (150 ohms), does that mean that rated load capacitance should also be larger (and how much) in order to keep compatibility with embedded oscillator?

We are using mentioned crystal that has 150 ohms ESR and load capacitance of 6pF. We are expecting about 5 pF of stray capacitance (including pin and PCB stray capacitance), so we are using 6.8pF load capacitors. Could this also be a problem, since they are not in 10 to 20 pF range?

If datasheet had specified oscillator transconductance, that could be also helpful...
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Peter Boringer
Senior

‎2016-09-27 04:40 AM

Posted on September 27, 2016 at 13:40

Dear Tobias,

I guess that your 6.8pF capacitors are suitable and quite close to the 10pF recommended

You can perhaps improve your stray capacitor by keeping trace as short as possible, you will be able then to increase the capacitor close to 10pF.( Stray capacitance can be assumed to be in the range of 2 to 5 pF) the smaller is the best to increase the loading capacitors.

Purpose is to have as close as possible load capacitance equal to crystal Cl

For information large value  of load capacitance will increase frequency stability but may cause oscillator start-up problems.

Perhaps select capacitors with a low temperature coefficient such as NP0 or C0G types.

On the CR95HF board a NDK crystal ref NX2520SA-27.12M is used the ESR max= 60 ohms and load capacitance= 8pF. 2 capacitors of 10pF are connected to ground ref GRM1885C1H100KA01D (C0G) from Murata.

 Regards

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Peter Boringer
Senior

‎2016-09-27 04:40 AM

Posted on September 27, 2016 at 13:40

Dear Tobias,

I guess that your 6.8pF capacitors are suitable and quite close to the 10pF recommended

You can perhaps improve your stray capacitor by keeping trace as short as possible, you will be able then to increase the capacitor close to 10pF.( Stray capacitance can be assumed to be in the range of 2 to 5 pF) the smaller is the best to increase the loading capacitors.

Purpose is to have as close as possible load capacitance equal to crystal Cl

For information large value  of load capacitance will increase frequency stability but may cause oscillator start-up problems.

Perhaps select capacitors with a low temperature coefficient such as NP0 or C0G types.

On the CR95HF board a NDK crystal ref NX2520SA-27.12M is used the ESR max= 60 ohms and load capacitance= 8pF. 2 capacitors of 10pF are connected to ground ref GRM1885C1H100KA01D (C0G) from Murata.

 Regards

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