Author Topic: There are two oscillograms and two respective FFT plots of wide known IC...  (Read 1053 times)

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Offline CiscERsangTopic starter

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Hello everyone,
There are two oscillograms and two respective FFT plots of wide known IC. https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf
page 11.
Where,
-Left, the signal shape looks good, however FFT plot is quite dirty.
-Right, the signal shape looks worse, however FFT plot is clearer.

To me that looks like they confused FFTs or so. Didn't they?

What do you think about?

thank you




« Last Edit: May 16, 2019, 08:18:44 am by CiscERsang »
 

Offline palpurul

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That's what you'd expect.

When the edges are shaper (faster rise/fall time), you got more freuqunecy content in your signal. As the edges slows down (slower rise/fall time), you expect frequency content to be less.

That's why the one on the left looks more "dirtier" than the one on the right because it's got more frequency content (sharper edges).

 
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Offline CiscERsangTopic starter

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That's what you'd expect.

When the edges are shaper (faster rise/fall time), you got more freuqunecy content in your signal. As the edges slows down (slower rise/fall time), you expect frequency content to be less.

That's why the one on the left looks more "dirtier" than the one on the right because it's got more frequency content (sharper edges).

So, why do they provide that? To show that ICs that belong to the left are better? Or?
 

Offline palpurul

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That's what you'd expect.

When the edges are shaper (faster rise/fall time), you got more freuqunecy content in your signal. As the edges slows down (slower rise/fall time), you expect frequency content to be less.

That's why the one on the left looks more "dirtier" than the one on the right because it's got more frequency content (sharper edges).

So, why do they provide that? To show that ICs that belong to the left are better? Or?


Different models have different rise/fall time characteristics I think that's why they include two different FFT plots for different models of the trancievers.

Faster rise time doesn't neccesarily mean better, it all depends on your requirements. If your application can tolerate slower rise time you should choose the ones with slower rise time because they radiate less and high frequency stuff is less relevant, making the design less complicated.
 
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Offline dzseki

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the two ICs are for different purpose, as stated in the datasheet the MAX483 is slew rate limited, that is if you don’t need fast communication will not emitt much electronic noise (through the wires) as it has its spectra limited like the FFT diagram shows. MAX485 on the other hand will handle faster bitrate, but this is only possible with using faster edge transitions which may be emitted to the surrounding circuits. It is the designer’s duty to select which one suits the application more.
HP 1720A scope with HP 1120A probe, EMG 12563 pulse generator, EMG 1257 function generator, EMG 1172B signal generator, MEV TR-1660C bench multimeter
 
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Offline CiscERsangTopic starter

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Ok, I've got that. Thanks to all.
 


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