I am currently working on a PCB with DDR2 on it. We are bringing out the DDR2 CLK, DQS signals using pogo pins to make some timing measurements. The length of the pins are about 5 cm. The problem is that every time the pogo pins make contact with the test points (of clk and DQS), we see noise on both DQS and CLK. I removed the pogo pins and started experimenting by adding small stubs on the test pads of clock. What I noticed was that by adding stubs of longer length > 1.2cm on Clock, I could introduce noise on DQS and the level on noise is somewhat propotional to length of the stub. Is this because the signals reflect from the stub and cross couple to DQS? I tried the same thing on vias instead of test pads and it is more prevalent. Can some please throw some light on why this happens and how it can be avoided?
The Short Story
This is caused by reflections. The DQS and CLK signals travel down the un-terminated line and, finding an improperly-matched impedance to completely absorb the signals, a portion of the signal will "bounce" back from where it originated. The reflected signals (the ones that bounce back) will be summed with the signals currently being generated which is evidenced as "noise" on your desired signal.
The Slightly Longer Story
Signals that change (especially those at higher frequencies such as that of DDR) are much more influenced by what is in the signal path than static (DC) signals. Signals become more difficult to deal with because everything in the signal path (vias, parallel traces, stub traces, etc) can have some form of resistance, capacitance, or inductance to it which can affect the signal adversely. This is known as complex impedance. Your stub-trace actually looks like some form of complex-impedance in parallel with your DDR, thus changing the signal properties and (from what you're saying) causing reflections. By changing the length of the stub you are essentially changing what the parallel termination looks like and causing it to either absorb more of the signal or reflect more of the signal depending on how well the impedance of the stub matches the impedance of the IC generating the signals.