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BebopALot
Registered Member

Date Joined Aug 2005
Total Posts : 79

Posted 8/6/2006 2:22 PM (GMT -8)

Hi folks. I am ready to disconnect my Sx-Key and let my SX ride the 5.125 MHz crystal all by itself in my circuit. Problem is, I don't know exactly how to do this. I have the crystal and inverter (if necessary) on my benchtop and am ready to go. Do I first make the oscillator with the 74AC04 (I don't need any help for this step...) or can I drive the SX with the crystal by itself? Anyhow, if someone could kindly give me a step-by-step I would appreciate it.

Thanks!

Bean (Hitt Consulting)
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Date Joined Jul 2004
Total Posts : 5761

Posted 8/6/2006 4:35 PM (GMT -8)

BebopALot,

It depends how close to 5.125 MHz you need to be. The SX will probably work by just putting the xtal onto the OSC1 and OSC2 pins and using OSCXT1 or OSCXT2 on the device line.

If not, you may need some capacitors on each side of the xtal to ground.

That should be in the area of ±100ppM of 5.125 MHz.

Bean.

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BebopALot
Registered Member

Date Joined Aug 2005
Total Posts : 79

Posted 8/9/2006 7:13 PM (GMT -8)

Bean, I basically put a crystal lead in Pin 27 and one in Pin 26. It is not running at all...

Post Edited (BebopALot) : 8/10/2006 3:43:10 AM GMT

Bean (Hitt Consulting)
Forum Moderator

Date Joined Jul 2004
Total Posts : 5761

Posted 8/10/2006 3:04 AM (GMT -8)

Try putting caps on each side of the crystal to ground.

Bean.

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BebopALot
Registered Member

Date Joined Aug 2005
Total Posts : 79

Posted 9/1/2006 6:04 AM (GMT -8)

Bean you comment on how to choose capacitor specs that fall in the area of +-100ppm of 5.125 MHz - lets use .1uf cap. What are the calculations to determine?

Bean (Hitt Consulting)
Forum Moderator

Date Joined Jul 2004
Total Posts : 5761

Posted 9/1/2006 6:41 AM (GMT -8)

0.1uF are WAY too big. Try 10 pF to 100 pF.

Different value caps will offset the crystal frequency.

You would need a calibrated frequency counter to be able to know how much the caps are pulling the frequency.

To calculate it you need all kinds of technical data about the crystal (an the SX too).

You can try finding the smallest and largest cap value that works, then pick a value in the center.

Try different OSCxxx settings on the device line too.

Bean.

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lawlopez
Registered Member

Date Joined Sep 2005
Total Posts : 6

Posted 9/2/2006 5:54 AM (GMT -8)

.1 uf ???

if you are putting .1 uf to ground either side of the crystal
you are doomed.

something like 20 pf would be a better choice.

BebopALot
Registered Member

Date Joined Aug 2005
Total Posts : 79

Posted 9/6/2006 7:39 AM (GMT -8)

Thanks Bean and Mr. LawLopez. Can I use any 20-100 pf caps lying around, or do I need special ones ordered from digikey or mouser.com for example?

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