Where to place a few pF to make the BW shape better (1204X-E)?
What is better shape?
If want better shape then filter need follow ADC sampling frequency and so on.
Here same experimental with 1GHs ADC sampling frequency.
Then if have BW limited more steep with this 500MSa/s then can ask is it ok anymore for this next.
As previous, also this is produced using freq hopping sinewave
(yes there is also rf generator harmonics what also produce aliasing but they are mostly buried behind others in display) sweep so that it leave enough spacing for also get alias peaks visible when freq rise and aliases start fold back from fNyquist.
Of course often we use oscilloscope with usual passive probes. This is also filter.
Then, example, if we look, say example square waves. Harmonics levels are not at all same as fundamental freq. (3th level 1/3, 5th: 1/5, 7th 1/7, 9th 1/9 etc... IF square is ideal square with 0 risetime, what is is never).
So in practice this is not at all as bad situation what it first may feel example if look these images where input freq is constant level over whole sweep freq range. But important is that users undestand these basics in theory and also his scope and his probes etc. Enough knowledge and scope do not fool user so easy.
Also when look these images, there vertical scale is LOG. Normally when we use time domain, vertical scale is linear. Why I draw there 26dB range. If example normal time domain display we have 1Vrms sinewave on the screen. When it drop 26dB there is 50mV on display. (5%)
If we have 10M square and think its harmonics example 11th. It is 110MHz but it is dropped say example over 20dB... if it have very fast edges it may easy have exaple 31th harmonic 310MHz. But it is perhaps -30dB down from fundamental level. If Sampling freq is 500M (as previous image) it is aliasing, its alias is 190MHz but if we are in time domain display with linear vertical and our fundamental is example 6div height, how is level of this harmonics. Can you see its effect. If front end attenuate 310MHz example 5dB then this harmonic alone is around 0.1div or so. In practice, no visible effect in this square wave shape.
But, with FFT we really need care about alias frequencies. There these can mess lot of.
Just example. Some RF generator 3rd harmonic can be example -30dBc.
If some one is looking 124.9MHz carrier on FFT and example case he is using 500MSa/s.
Say example he look 0dBm carrier. 3rd harmonic -30dBc (374.7MHz)
Now he can see 124.9MHz 0dBm peak and he can also see 125.3MHz peak level -36dBm. (If scope attenuate 6dB 375MHz.) And then user wonder why there is fundamental carrier 124.9MHz and after then 400kHz higher also signal. Think these with different frequencies and different waveforms with different harmonics. In some cases this situation with many aliases can be really confusing.
Keep care and not input over fN frequencies (including harmonics and nonharmonics)