This discussion with images has gone a ways to show how proper instrumentation should be done and all the things that can go so very wrong. A single mis-step in the instrumentation set up and the results will easily mis-lead resulting in a very wrong conclusion of circuit or instrument behavior.
Stuff that can go wrong and it becomes increasingly difficult as the rise times shrink and transmission distances grow.
*Poor coax performance. Common RG58 is pretty poor for any serious RF work. This why semi-rigid coax, Gore, Huber-Suhner and similar coax cables with known specifications should be used for nanosecond-picosecond pulse work. Beyond losses as frequency goes up, coax cables often have dispersion which compounds the problems.
*Impedance match for all component in the system. Deviations cause ripples and reflections in the response.
*Don't trust the scope image alone, know what to expect before setting up the instrumentation and if those results do not appear figure out why.
*No matter how much automation, quality-state of the art instrumentation or what not understand every tiny aspect of the test set up. As frequencies goes up, levels goes down and related are pushed to extremes all it takes is one tiny item wrong and BIG errors happen.
*Keep in mind what one is trying to achieve with the measurement as over kill is not always needed to get the desired result. There is a balance here. One does not need a picosecond pulser with matching instrumentation to test an audio amplifier.
*Then there are probes.. which are an entire universe of good and bad, some times REALLY BAD.
Bernice
This is a surprisingly interesting thread. A square wave being both hard to generate and measure at high dv/dt and frequency.
On the generator side - what do you have to spend to get a high-quality square wave. There are a million Fluke, HP, Agilent, etc on eBay. I am about to buy a 1Ghz scope and need an AWG and just a plain Function Gen (for general purpose uses like injecting noise into a circuit). Trying to learn how to understand the sources of the errors to make full use of the new scope. I have spent too much time chasing my tail on a design only to learn that my measurement was the problem, not the circuit.