Resonator madness - using two resonators on the same board...can it be done?
DosManDan
Posts: 179
I've spent about 20 hours trying to find the cause of a problem in my design...and I found it...but I don't know what to do to fix it.
I am using two SX-28 chips, one running at 4MHZ (using a blue Murata 4Mhz resonator) and reads a serial line and ouputs characters to an LCD. The other SX-28 runs at 50MHZ (Murata resonator) and runs a timing circuit. I developed the circuit using the PDB, then burned a PCB and wired it up only to find out it didn't work at all. After hours of working with an oscilliscope I decided to wire it all up on a separate prototype board using push connectors. To my surprise, it didn't work there either (ok right about now, half of you are saying duhhh, do that first).
It turns out the cause of the problem is the center ground wire from the oscillators. If they are close together, they mess each other up. If I move the ground wires apart, they work fine. How do I fix the design? The circuit board is small and I can't move the ground wires apart, so that isn't an option.
UPDATE:
OK, I stuck a 220 ohm resistor from the ground on the 4Mhz resistor to the ground on the board and everything works fine. Is this the right way to correct the problem, or is there a more acceptable design change?
Is there something I need to do when using two resonators on a project?
Dan
Post Edited (DosManDan) : 8/21/2008 6:39:03 PM GMT
I am using two SX-28 chips, one running at 4MHZ (using a blue Murata 4Mhz resonator) and reads a serial line and ouputs characters to an LCD. The other SX-28 runs at 50MHZ (Murata resonator) and runs a timing circuit. I developed the circuit using the PDB, then burned a PCB and wired it up only to find out it didn't work at all. After hours of working with an oscilliscope I decided to wire it all up on a separate prototype board using push connectors. To my surprise, it didn't work there either (ok right about now, half of you are saying duhhh, do that first).
It turns out the cause of the problem is the center ground wire from the oscillators. If they are close together, they mess each other up. If I move the ground wires apart, they work fine. How do I fix the design? The circuit board is small and I can't move the ground wires apart, so that isn't an option.
UPDATE:
OK, I stuck a 220 ohm resistor from the ground on the 4Mhz resistor to the ground on the board and everything works fine. Is this the right way to correct the problem, or is there a more acceptable design change?
Is there something I need to do when using two resonators on a project?
Dan
Post Edited (DosManDan) : 8/21/2008 6:39:03 PM GMT
Comments
Just a couple of suggestions...
Have you tried shielding on the proto board yet? (try using shielded wires, then shield the two oscillators, or any mix of the two).
Are you using a ground plane between traces?
- Does everything share a common ground?
- Can you post an image of the section of the PCB where the resonators are located so we can see how the layout looks?
- Where exactly did you install a 220 ohm resistor? Is it from ground to one of the other resonator leads? It isn't clear from your message what the other end is connected to.
- Depending upon the resonator used some people install a 10K resistor across the resonator and I have also seen some add a small cap (around 5pf) across a couple leads as well. That may help.
Robert
1)Everything does share a common ground.
2)I can post the bottom of the board, but not the top.
3)I installed the 220 ohm resistor from the center post of the 4Mhz resonator to ground. The resonator is not supposed to need any capacitors or resistors.
4) I'm not using sheilded wires, as this is a PC board. I did use a copper pour to give as much of a plane as possible.
Thanks again!
Dan
I can't see why attaching a 220ohm resistor from the ground on the resonator to ground would make a difference. Something isn't right and you'll need to double check everything.
Can you tell which SX28 chip isn't working? Do you have access to a scope?
Robert
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-Paul
Yes, I do have an O-scope, and it is the SX-28 running at 4Mhz that is affected.
Velocita, I agree, I need to get the ground under control. I was in a rush and used the autorouter function in the PCB software.
Robot, I'm trying to fix it...or at least understand why it wasn't working.
Thanks again,
Dan
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Chris Savage
Parallax Engineering
I'm not sure about that. The big resiny DIP resonators include capacitors within, but not resistors (like the Muratas). Someone chime up if you it may be otherwise.
A lot of the SX-based schematics I see (and that I build) usually place a 10k or so resistor between OSC1 and OSC2 (across the outside pins of the resonator). This value seems to work for 20MHZ and 50MHZ, and doesn't seem to negatively affect circuits where it might not be required. Guenther has more to say about this if you poke around the forums.
The SX datasheet has resistor and capacitor values that are theoretically required for proper resonator startup. The small surface mount resonators do not include caps, the big dip ones usually do.
This may not be it at all, of course, but it may be worth looking at. Certainly something that could be soldered to the resonator pads on the bottom of your board.
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When the going gets weird, the weird turn pro. -- HST
1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php
I experienced the SAME problem when I used a stand alone prototype board (push wire). If the grounds were closer than about 2 inches, the problem started. When they were farther apart, everything worked fine. Basically, I simply moved the ground wire closer or farther appart.
I'm posting a new image of the bottom of the board so you can see what it looks like now.
Dan
Post Edited (DosManDan) : 8/22/2008 5:54:57 AM GMT
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-Paul
Thanks again,
Dan
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Chris Savage
Parallax Engineering
My appologies for not taking a better picture, but the new design (green) is using the ground plane exactly as shown in your photo. I even used a similar portion of·my board as you have showing in your picture (bottom edge). It looks like you have a row of pins for a servo controller with signal, ground and power lined up in a row.
I've included a magnified photo for you to see. I took the picture before the rosin was cleaned off, so I think it is keeping you from seeing the traces.
Hope this helps,
Dan
Post Edited (DosManDan) : 8/22/2008 5:47:20 PM GMT
I thought the two boards you posted were the same design, but now I see they are radically different. In any event it almost looks as though a few traces and pads are shorted to the ground plane on this photo.
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Chris Savage
Parallax Engineering
I swear all of the joints are fine,
Dan
Post Edited (DosManDan) : 8/22/2008 8:22:03 PM GMT