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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I have started working on LVDS data lines, but I am having trouble getting a good signal. I don't know if it is what actually is happening or just noise. Right now it is 20mhz 300mv LVDS with a 1.5v common mode voltage, but it will go up to 85mhz. I am trying to eliminate long ground leads to the probe (10:1 passive, well compensated), but it still gives me lots of noise. It looks like noise, but my scope (agilent 3000x, 100mhz) tells me it is 20mhz, so I don't know if it is noise or an actual signal coming through.
Any tips?
EDIT: Here are some pics. Here is the very very noisy LVDS line. Here is the input. If I zoomout, and look at the peaks, it shows 25khz. So I think it is working, there is just massive noise. The noise is around 20mhz, which happens to be my clock frequency.
Background: This is from a chip that converts 28 bits of parallel data into 4 LVDS lines + 1 Clock LVDS. A receiver decodes it back into the 28 bits.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
You are spot on Carbon Rod. I do know I have impedance mismatch considering I am using a breakout board and a breadboard, but I was hoping to see something for the sake of testing before having a board fabbed. That document is excellent, I am using the CameraLink interface. So that means right now I have data at 135Mhz, too much for my 100mhz scope. At 85mhz thats going to be 595mhz of data, I guess I will be testing blind! Thanks, G.
Registered Member #531
Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125
You might try a "Z Probe". Easy and cheap to build, almost completely eliminates ground lead exacerbated probe resonance issues. The differential balanced version would be best for LVDS probing but the single ended version should get you where you want to go.
Doug Smith has a construction article on his site -
Howard Johnson also has much useful info on this topic -
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Tips on probing high speed digital signals
