I connected a BK Precision 4017B waveform generator to a Rigol DS1054 Oscilloscope.
BNC ⇽ Co-Axial-Cable ⇾ BNC. 8 MHz square wave generated.
Square wave looks rather distorted to me. See image below.
Also tried a 10 MHz oscillator 4-pin can and also get a distorted square-wave waveform.
What am I doing wrong? I kinda feel the waveform should be more “clean/square”.
That looks perfectly normal for your scenario.
You have a high speed digital signal and the coaxial cable has a lot of capacitance and the scope end is left unterminated.
The B symbol on channel 1 means also that something funny is going on, like you may have bandwidth limiting on that channel on, so it also looks weird because of it. Turn it off.
Also, the 4-pin crystal oscillator module with digital square wave output is not intended to drive 50 ohm BNC cables due to their impedance (50 ohms, and the capacitance). They generally have CMOS outputs that are meant to drive a short piece of PCB track going into some CMOS chip input. As the oscillator setup is unknown, the waveform can be bad if you simply wired some supply to it with short wired and put no bypass cap on the oscillator, and the signal and ground paths to the BNC cable are also unknown, any inductance from even short wiring will also distort the signal.
As it turns out if the scope is only 50 MHz then you can't expect to see a clean square wave. A clean square wave is a composite signal of sine waves with odd harmonics of the fundamental, in theory up to infinity. Your scope can approximately show only 5 first harmonics of the square wave signal.
Also a word about the BNC cable. It likely has standard 50 ohm characteristic impedance. Imagine time is slowed down and a signal like edge of an ideal square wave entering into one end of the cable. The signal starts travelling through the cable at about 0.66 times the speed of light and to the signal source with some voltage the current that enters the cable equals the characteristic impedance, i.e. a 50 ohm load. When the signal has traveled to the scope end of the cable, an unterminated scope looks like 1 Mohm impedance, almost like an open end, so there is an impedance mismatch or discontinuity, and basically at the signal voltage, current that matched the 50 ohms can't flow out of the cable, so the voltage at the scope end shoots up to double, and now the signal edge reflects back towards the source end of the cable and eventually hits the signal source. If the signal source is e.g. the CMOS output chip, it will mix in with the transmitted signal again and you see all kinds of weird waveforms that are just the steady state of square wave signals ping-ponging back and forth in the cable. In the case of the signal generator, it has a source impedance of 50 ohms, so it will dampen the reflections hitting back towards it.
I did the experiment suggested by @gbarry and switched to a 100kHz square-wave and got something that looks like a clean square-wave. See oscilloscope screenshot.
All the analysis and advice that has been added to this thread makes a lot of sense. Very helpful.
