I want to come back to my question: if none of the signals in the electronic design, either the source, the intermediate signals in the signal path or the output signal has a frequency above the limit, then the higher frequencies can only come from interference, noise or higher harmonics.
Interference: Can this really go up to 100 MHz with a noticeable impact? Don't believe so.
Noise: Can this really go up to 100 MHz with a noticeable impact? Don't believe so.
Higher harmonics: Let's say we have a 100 MHz square wave. With a square wave at 100 MHz, the fundamental frequency is at 100 MHz, the 3rd harmonic is at 300 MHz, the 5th harmonic is at 500 Mhz. Doesn't the low-pass filter cut-out frequencies above 300 MHz? I believe it does!
If not, it is a design failure I would say, if a 100 MHz scope does not even handle a 100 MHz square wave (DS1104Z with 100 MHz BW, 250 MS/s per channel).
You really don't have a clue, do you? Or are you deliberately trolling this forum?
And let's put it to the test.
Your perfect square wave (where can you buy these btw?) has a third harmonic with 1/3 of the amplitude of the fundamental.
So.
First a measurement with 0dBm out of the generator to the scope (250 MSa/s). 100 MHz 0 dBm png attachment.
Then a 300 MHz 0 dBm to the scope (to check the attenuation at 300 MHz): 300 MHz 0 dBm png attachment.
You see that:
- due to aliasing the scope "sees" a 50 MHz signal
- this signal is down
only 20 log (237/120) = 5.9 dBThe 3rd harmonic of a perfect square is 1/3 of the amplitude of the fundamental so the actual amplitude that the scope will show is 120/3 = 40 mV.
So the scope will show a waveform consisting of a 237 mV 100 MHz signal + a 40 mV 50 MHz signal.
To show you how that looks I added 237 mV 10 MHz and a 40 mV 5 MHz signal together (I can only generate up to 30 MHz with my siggen), and look what the waveform is:
There is your perfect square wave @ 100 MHz....
Higher harmonics will also show, but probably not significant:
- the 5th harmonic is at 500 MHz.. being 2xFs the amplitude here is zero (checked it).
- 7th harmonic is way down, not relevant any more.
This scope is really marvelous, especially at this price point. No competition. But you
must understand aliasing and leakage. Know what you're doing.
Oh, and if you are not sure about aliasing, just use only one channel. With 1 GSa/s aliasing is probably not a problem. And switching between 1 GSa/s en 250 MSa/s will give you an indication if aliasing is an issue. If the waveform changes drastically, it is! (at 250 MSa/s).
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