Assuming your clock source is producing (approximate) square waves with a fundamental frequency of
12.5MHz, an expected rise time might be about
Assuming the signal propagates at the speed of light (in reality, it's less than this, about half or a third), then to travel
10ft it takes
10ns, which indicates that transmission line effects might be important.
I wrote this online transmission line simulator so you can visualize what is happening to the signal inside of the transmission line. The model assumes a constant characteristic impedance (which is pretty good in practice for most things).
tRise = 4ns
tFall = 4ns
tOn = 36ns
tOff = 36ns
tDelay = 10ns
Zeq = 50 ohms
R1 = 0 ohms (source terminator)
R2 = 1 megohms (sink terminator)
- other parameters at their default values
Here are a few snapshots of the results (
green is ideally terminated,
red is poorly terminated, and
blue is the source voltage):
During the first reflection, the poorly terminated signal was 40% higher than the desired line signal, and during the second reflection the signal was about 10% lower than the desired line signal.
Repeating by simulating a shorter cable (approx. 1ft cable, i.e.
tDelay = 1ns):
Notice that the overshoot is much smaller (about 10% for first reflection). This happens even if
Zeq is constant!
tl;dr: You should consider properly terminating your cable when using a 10ft long cable and/or getting a short cable. Also, the measured signal is a product of wave interference, even if the cable's equivalent impedance is near constant. In all cases the properly terminated signal always propagates to the correct level without any distortion, and is only delayed because of the finite signal speed. Only with a properly terminated/short cable can you tell if your clock signal is really bad, or if you're just measuring reflections.