hi
i want to build a 40MHz crystal oscillator with 74xx devices to use with a spartan II device.
does any body have any idea or schematics.
crystal oscillator
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crystal oscillator
by ali_dehbidi » Wed Jul 15, 2009 9:46 am
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by NickH » Wed Jul 15, 2009 11:31 am
Sorry if this is not the answer to your question, but...
Why not use a 4-pin oscillator module? They are very easy to use, and (though I don't know why) they're often in smaller and more convenient package than a crystal can.
Nick
Why not use a 4-pin oscillator module? They are very easy to use, and (though I don't know why) they're often in smaller and more convenient package than a crystal can.
Nick
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by ali_dehbidi » Thu Jul 16, 2009 4:59 am
thanksfor your reply.
but the point is i want to learn something and know how they build these
but the point is i want to learn something and know how they build these
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by NickH » Thu Jul 16, 2009 11:37 am
Well the trouble is, the correct circuit will vary from one crystal to another. Refer to the crystal manufacturer's datasheet. Also 40MHz is quite a high frequency (which I don't have experience with). But I'll tell you what I know about the standard "parallel resonant" circuit, and maybe with some experimentation you can get it to work...
Start with a CMOS gate such as 74HCU04. Decouple its power supply with a bypass capacitor.
Connect the crystal across the input and output of an inverter, with capacitors "C1, C2" from each terminal to ground. Their value should be specified in the crystal's datasheet; but try 15pF. Also, in parallel with the crystal, put a feedback resistor "Rf" of 1Mohm.
With some crystals you need a resistor "R1" between the output of the inverter and the crystal-capacitor circuit. (The idea is that R1,C1 form a low-pass filter, to reduce clipping and reject overtones.) But for 40MHz I think you don't need R1.
Finally, connect the inverter output to the input of another inverter, and use the second inverter to drive the FPGA: this will amplify and clean up the output signal.
Nick
Start with a CMOS gate such as 74HCU04. Decouple its power supply with a bypass capacitor.
Connect the crystal across the input and output of an inverter, with capacitors "C1, C2" from each terminal to ground. Their value should be specified in the crystal's datasheet; but try 15pF. Also, in parallel with the crystal, put a feedback resistor "Rf" of 1Mohm.
With some crystals you need a resistor "R1" between the output of the inverter and the crystal-capacitor circuit. (The idea is that R1,C1 form a low-pass filter, to reduce clipping and reject overtones.) But for 40MHz I think you don't need R1.
Finally, connect the inverter output to the input of another inverter, and use the second inverter to drive the FPGA: this will amplify and clean up the output signal.
Nick
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by Oneironaut » Sun Jul 19, 2009 8:19 pm
You will have to get VERY lucky to see even 10MHz with any stability whatsoever using a gate/xtal clock.
Brad
Brad
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