Bandwidth and maximum frequency

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nutron
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Bandwidth and maximum frequency

Post by nutron » Mon Jul 08, 2019 12:02 am

Hi.
i am looking at the 3205D scope and in its specs it says that it has a bandwidth of 100MHz.

When you guys say 100MHz bandwidth , does that mean that one can capture a 100MHz square wave mostly accurately with a 3dB amplitude drop or does one have to de-rate the maximum possible frequency one can accurately capture by 3 ?

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Re: Bandwidth and maximum frequency

Post by Martyn » Mon Jul 08, 2019 5:10 am

Bandwidth is for a sine wave with 3dB drop.
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Re: Bandwidth and maximum frequency

Post by nutron » Mon Jul 08, 2019 1:02 pm

so how far can one push the frequency of other non sine wave forms before they start losing their shape. lets take a square wave as example.

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Re: Bandwidth and maximum frequency

Post by Martyn » Mon Jul 08, 2019 5:16 pm

If you consider that a square wave consists of the sum of sine waves of odd harmonics at diminishing levels, you need at least the fundamental, third and fifth harmonics to generate a basic square wave appearance.

So divide the bandwidth by 5, and a 100MHz bandwidth will give a reasonable representation of a 20MHz square wave.
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Re: Bandwidth and maximum frequency

Post by nutron » Sun Jul 14, 2019 9:10 pm

Please help me to understand something here , i am fairly new to picking a scope by spec.

What i know about sampling analog signals to digital is that the maximum frequency that can be sampled is half of the sampling frequency when the waveform frequency is constant , like a static sine wave (Nyquist) .
I also know that sampling analog signals right at the Nyquist frequency is not a good idea either.
So i assume you rate the maximum bandwidth of your products bellow the Nyquist frequency.

When i look at this website : 3000 series specs

Let's pick a 100Mhz bandwidth version of this series.
Looking at the "Oscilloscope - Horizontal" section it says that using the scope with one channel the sampling rate is 1Gs/s and with two channels it is 500Ms/s etc.. .

So at 1 Gs/s the Nyquist frequency is 500MHz and at 500Ms/s the Nyquist frequency is 250MHz etc... .

Does that mean this for the different channel configurations of the 100MHz bandwidth version?
1 channel : 1Gs/s , Nyquist freq. 500MHz ,max bandwidth 100MHz ,5 samples per period at 100MHz signal
2 channels : 500Ms/s , Nyquist freq. 250MHz ,max bandwidth 50MHz ,5 samples per period at 50MHz signal
4 channels : 250Ms/s , Nyquist freq. 125MHz ,max bandwidth 25MHz ,5 samples per period at 25MHz signal

If that is the case , would that mean that the 200MHz version of this scope will be less accurate at the maximum bandwidth frequencies of each channel configuration , since the sampling rate is the same for all versions of the 3000 series?
Like this...

1 channel : 1Gs/s , Nyquist freq. 500MHz ,max bandwidth 200MHz ,2.5 samples per period at 200MHz signal
2 channels : 500Ms/s , Nyquist freq. 250MHz ,max bandwidth 100MHz ,2.5 samples per period at 100MHz signal
4 channels : 250Ms/s , Nyquist freq. 125MHz ,max bandwidth 50MHz ,2.5 samples per period at 25MHz signal

Please correct me if i am wrong , I am not certain if am right with all of this. As i said i am new to picking scopes by specs and i would really appreciate some help to understand the specs of a scope.

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Re: Bandwidth and maximum frequency

Post by nutron » Sun Jul 28, 2019 11:40 pm

Bump.

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Re: Bandwidth and maximum frequency

Post by Martyn » Mon Jul 29, 2019 6:44 am

Looking at specifications and comparing them between manufacturers can be misleading, it is better to consider the intended use for the scope, so if you can provide details on this we can advise which models from our range would be suitable.

If you still require a complete answer on Bandwidth, Sampling Theory, and Nyquist you can email your questions to support@picotech.com and one of the team will provide an in depth answer for you.
Martyn
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