Author Topic: When does a square wave become a sine wave?  (Read 9908 times)

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Offline miguelvp

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Re: When does a square wave become a sine wave?
« Reply #25 on: May 21, 2015, 02:16:27 am »
Or use a coax cable with proper termination on the scope end, don't use the probes to measure the signal generator.
 

Offline Paul Moir

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Re: When does a square wave become a sine wave?
« Reply #26 on: May 21, 2015, 07:13:41 am »
Also note that with an unhacked scope with 50MHz bandwidth, you'll not be able to see enough harmonics on the 20MHz square wave for it to look particularly square.  1st is 20MHz, 3rd is 60, 5th is 100MHz, etc.  So you'll only see the first, some of the 3rd and maybe a hint of the 5th.

But yeah I'll bet 1x probing is the biggest problem here.  1x sucks...
 

Offline grumpydoc

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Re: When does a square wave become a sine wave?
« Reply #27 on: May 21, 2015, 02:28:22 pm »
Also note that with an unhacked scope with 50MHz bandwidth, you'll not be able to see enough harmonics on the 20MHz square wave for it to look particularly square.  1st is 20MHz, 3rd is 60, 5th is 100MHz, etc.  So you'll only see the first, some of the 3rd and maybe a hint of the 5th.

But yeah I'll bet 1x probing is the biggest problem here.  1x sucks...

Indeed.

There are two discussion threads on this.

As well as the measurement issues there is also the fact that the the Wavetek 145 has a rise time of ~20ns so it probably can't manage a very "square" 20MHz square wave
 

Online hamster_nz

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Re: When does a square wave become a sine wave?
« Reply #28 on: May 21, 2015, 06:37:49 pm »
I did some playing around last night with my 100 MHz scope, 150 MHz probes, along with 4 MHz, 20 MHz and 40 MHz square waves. And as expected at 40 MHz they didn't look like a square wave at all. But one big difference from the OP's screen-grabs was that it looked nothing like a pure sine.

So I guess that the making a 100 MHz scope hardware into a 50 MHz scope is achieved through a brick wall 50MHz DSP filter, hence the OP's 'pure' looking sine? If that is the case, then that is a reasonable explanation for his traces...

This could be verified by watching the waveform as the frequency is swept. Between 15 MHz and 18 MHz the waveform should change dramatically as the 3rd harmonic gets nuked by the filter, and between 9.5 MHz and 10.5 Mhz when the 5th harmonic gets cut.

Between 10.5MHz and 15MHz the waveform should stay pretty much the same shape.
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