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Design >> Mixed-Signal Design >> Sourcing appropriate oscillator
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Message started by freqout on Sep 4^th, 2012, 7:21pm

Title: Sourcing appropriate oscillator
Post by freqout on Sep 4^th, 2012, 7:21pm

Hi,

I want to replace the 25Mhz oscillator on my raspberry pi board (labeled X1 on pg3 of the datasheet here).

Here are the specs I was looking for:

3.3V
25MHz

but have several questions surrounding this process:

1) Are there other specs I need to be mindful of?
2) Where is a good place to source this component? I only need one, and was looking at Jameco or Digikey...but digikey ends up being $10 for just one of these: http://www.digikey.com/product-detail/en/636L3C025M00000/CTX697DKR-ND/745486 ...I'm on a tight budget, and $10 doesn't fit.
3) The original oscillator is SMD, but I'd like to use a through hole counterpart and just let it "stand" on the board. It's easier to solder and my iron's not that great. Are there extra considerations I should make when doing this? Is that a bad idea?
4) There seems to be some sort of adhesive preventing the oscillator from being removed from the board even after solder has been removed from the joints. How do I safely liquify the glue so remove the component?

Thanks to anyone for their 2 cents!

Title: Re: Sourcing appropriate oscillator
Post by wave on Sep 5^th, 2012, 2:07pm

Why are you motivated to change this oscillator?
The unit price listed is $2.35 not $10.

The one shown is matched and tuned with C loads and parallel R in the board. You would have to re-match to the IC if you change components.
There are a lot of oscillator alternatives.
BUT, for a novice, it's playing with fire...

8-)

Title: Re: Sourcing appropriate oscillator
Post by freqout on Sep 5^th, 2012, 2:52pm

You're right, it's $2.35 but I was including the $7.00 reeling fee....but I see that if I go with a cut tape packaging then the cost before shipping is back to $2.35.

I want to replace it because I don't see any other options. Sadly, my pi is suffering from an acute case of [urlhttp://www.raspberrypi.org/phpBB3/viewtopic.php?t=5766]dead ethernet and USB[/url]. Symptoms point to the crystal as it provides the clock source for both.

Fixes others have come across are resoldering the joints, and putting it in the fridge. I've tried both. Not sure why the fridge was supposed to help, but I tried it anyway.

It's visibly obvious that I tried to resolder the joints, so I wouldn't qualify for an RMA.

I understand I'm playing with fire...but I don't see any better options...does anyone else?

Title: Re: Sourcing appropriate oscillator
Post by wave on Sep 5^th, 2012, 4:40pm

Freq - Ah, now I understand.
The fire is already burning and you need to put it out.

In that case go for it. Crystal implementations are notorious for startup problems, and clearly this vendor has issues. Whether it's solder joints, or more electrically based - Cap sizes matched to the IC.
You _could_ double check the IC specs and see how to match the crystal to it. Or send it to the Crystal vendor to match with better Caps. Both would take some time & effort. (though cheaper for volume apps)

In this case, removing the Crystal, 2 caps, and 1 resistor, could be replaced by a fully canned oscillator block as you propose. The canned oscl is more robust than crystal, so easier to implement (but more expensive as you note).

Good luck,
Wave

Title: Re: Sourcing appropriate oscillator
Post by freqout on Sep 5^th, 2012, 6:23pm

Yes, the fire's a blazin and right now I feel like a soldering gun slinger. The problem is I don't really know what I'm doing...

I didn't realize an oscillator block replaced all 4 of these components. How do I determine if an oscillator block is an equivalent replacement for all 4 of these? Can I just assume that any 25Mhz @ 3.3V oscillator will take care of the signal conditioning for the frequency, or do I have to look for one that has 1Mohm and 66pF listed in its specs.
(I assume the caps and resistor is for keeping the crystal freq stable.)

Thanks for your help wave