please help me on LVDS problem

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eda_range

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Hello,
Please take a look at the picture.
I have a bad result while testing my LVDS transmitter chip.
Though the Vcm and Vdm of transmitter output is correct, there is great noise signal generated at transmitter output node. It seems as if the noise been amplified a lot, this noise became even larger if frequency increased.
Please kindly tell me where the problem might exist?
Input data and clock frequency is digitally around 1MHz and 2MHz.
Thank you so much for your help!
kind regards!
 

Doesn't look like circuit generated noise rather than measuring artefacts. Also output and input "noise" are probably of equal amplitude considering scale.
 

Im wondering if his problems are due to poor PCB layout and not having the proper terminating resistors.

E
 

Also, check on the power supply. You may have a noisy supply, not have enough decoupling caps or not have enough isolation/ filtering from other circuits on the same supply. The noise peaks seem somewhat periodic... do you have anything else on the system running at that frequency? Are you using switching supply, and if so, is it some multiple of the noise you are seeing?

Also, if you are using ferrite beads (inductor) for supply filter, it can resonate with the decoupling cap.

Good luck.
 

Pls check the GPIO & LVDS IO Power If they are seperated or not.
and also check the CFB's stability.

I guess problem may be from chip inside.

good luck!
 

Thank you for your replies.
The ckt snapshot is too large to be attached here, how could i be able to present full schematic? Or can I send you the schematic by email respectively?
This test result is totally different from simulation, and i verify that the internal CFB loop is very stable and tail current with little variation by simulation.
I am wondering if the design margin is not sufficient.
The terminating resistor is 100Ω between two output ports, which is inside the chip.
How can I commence further examination?
Please help!!
 

It seems the noise of input is amplified by you LVDS.

May be the CMFB is not stable.
 

are you sure you have well matched impedance? your problem may come from having a lot of ringing because of reflections
 

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