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| Oscilloscopes and harmonics |
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| sentry:
Hi, I have some questions regarding scopes and harmonics. So I understand that any repetitive waveform is basically the sum of sinusoid waves of appropriate amplitude and frequency. Does that mean that when we display any signal on a scope what we essentially are looking at is the scopes decomposition of the signal into the sum of the fundamental waveform and the harmonics up until the scopes bandwidth limit? If I want to display a 20MHz square wave with my 50MHz Rigol 1054z scope, what I will essentially see was only the fundamental waveform and not any of the harmonics because the harmonics all have frequencies above my 50MHz limit? So a 20MHz square wave will not look like a square wave on my scope, but will look like a sine wave. Is that correct? |
| capt bullshot:
This is correct, to some extent. The oscilloscope won't cut the harmonics off at its maximum frequency, it will have a rather smooth decay in amplitude of the harmonics. So with your exampe (20MHz square on a 50MHz scope) the third harmonic at 60MHz won't be cut off, but will be still there with a significant amount, the fifth (100MHz) will be there quite attenuated etc. The result won't be neither a square wave nor a sine wave, but something "in between". |
| RoGeorge:
In theory, yes. In practice, keep in mind that the frequency response of any oscilloscope does not look like a square ideal low pass filter. However, if you care to do a simulation, then compare the results with what you really see on the screen of a Rigol DS1054Z, prepare to be surprised. The band of a "50MHz" Rigol DS1054Z is in fact much larger, about 120...150MHz. Now, if you enable the "BW Limit" (20MHz), the waveform on the screen will start to look like the simulated waveform. Otherwise, the 20MHz square wave will look more like a square than like a sinewave on a DS1054Z. Offtopic: if you use Riglol tool to unlock the DS1054Z, you will see that the oscilloscope's band does not change when you unlock it to be a DS1104Z. I don't know why it was advertised as a 50MHz oscilloscope when, in fact, DS1054 is an 100MHz one. I guess that was just a marketing decision. |
| tggzzz:
You are on the right track. If you want to confirm your understanding in a way that will help you with real-world situations, see some measurements at https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ |
| Old Printer:
--- Quote from: RoGeorge on August 15, 2018, 07:13:49 am --- Offtopic: if you use Riglol tool to unlock the DS1054Z, you will see that the oscilloscope's band does not change when you unlock it to be a DS1104Z. I don't know why it was advertised as a 50MHz oscilloscope when, in fact, DS1054 is an 100MHz one. I guess that was just a marketing decision. --- End quote --- Does this mean that if you order a new 1054z & a 1104z the only difference will be the label? |
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