Forget the 25MHz screenshots, they show garbage if your scope only has 70MHz bandwidth (which isn't enough for a 25MHz square wave).
Good point, thanks.
Here is SDS1072CML frequency response.
Here is your SDG1025 square
Square wave is series of sinewaves 1. in this example 25MHz fundamental freq. Then harmonics, 3,5,7,9,11,13...etc (levels depending risetime (and other things in real signal shape))
Look freq response even 5th is attenuated over 3dB and 7th attenuated over 6dB
It explain well your square wave images including also my knowledge about SDG1025.
25MHz square fundamental f is 25MHz, then next harmonic is 75MHz, then next 125MHz and next 175MHz and next 225. (if we have enough fast risetime it continue lot of more.)
With SDG1025 risetime SDS1072CML is just barely enough for this SDG square but it do not show SDG square shape exactly (it can see just there in top after rising endge an bottom after dalling edge. Now here come also SDG1025 risetime to limiting factor... so we do not need very wide BW oscilloscope. (also SDG1025 own filters... there is same rules about produced signal harmonics levels.... )
Some may wonder why I need 1GHz oscilloscope for 100Hz square wave (But really not for SDG1025 any waveform). Of course, depends risetime and how far forward we need go with harmonics and so that they are not attenuated too much.
Pure sinewave is easy, it is only fundamental freq and it is all. 100MHz sinewave need 100MHz oscilloscope (or some over if level accuracy is needed and if scope do not have flat response for stated freq band.)
Every other signal than pure ideal sinewave have harmonics and if we need inspect this some signal using oscilloscope its BW need include all important harmonic frequencies with enough accurate level.