Why is ripple from carrier frequency suppressed with higher pole in Sullen-Key low pass filter?

Thread Starter

Gpand

Joined Dec 11, 2023
82
Hi, how come the ripples from carrier frequency get suppressed when you use the Sulley-key low pass filter with more poles? (see below from actual circuit). I know more poles (or orderof filters) mean it is closer to the idea vertical response but why ripples decrease when closer to vertical frequency response like in...?

butterworth.jpg

output filter 1.jpg

output filter 1.jpg

output filter 2.jpg
 

crutschow

Joined Mar 14, 2008
34,452
For the same corner frequency, the higher order filter attenuates the high frequencies above the corner more.
Does that not seem apparent from the rolloff graph?
 

Thread Starter

Gpand

Joined Dec 11, 2023
82
For the same corner frequency, the higher order filter attenuates the high frequencies above the corner more.
Does that not seem apparent from the rolloff graph?
How does higher order filter exactly attenuate the high frequencies?

And is there any standalone Sulley Key module around where for example I can connect it to the output of the ISO122P to get rid of the ripples. So if I can get hold of 5th order (or 5 pole Sullen Key) then even the better?
 

Ian0

Joined Aug 7, 2020
9,817
A fourth order filter is two cascaded 2nd order filters.
Each 2nd order filter attenuates the high frequencies a small amount.

Let's see if you might be able to write "Sallen and Key" correctly in future posts.
 

Thread Starter

Gpand

Joined Dec 11, 2023
82
It rolls off faster to a higher degree of attenuation.
What do you mean "exactly"?
Note more poles means more abrupt cutoff. What has this got to do with suppressing the ripples which is just riding on the frequency. I want to understand exactly how more abrupt cutoff can attenuate ripples that are higher frequency. For example. The ripples in the following is higher frequency. How can more ideal vertical response attenuate it?

noises above 100hz.JPG
 

Thread Starter

Gpand

Joined Dec 11, 2023
82
because there is more attenuation at the higher frequency.
Please show references or illustrations how there is more attenuation at the higher frequency because I'd like to understand exactly the details how it works. Thanks.
 

Thread Starter

Gpand

Joined Dec 11, 2023
82
Ok. Thanks.

Anyway. How are ripples produced from a carrier frequency? Do all frequency have ripples? I think light doesn't but only electricity. In what kind of circuit can this occur?
 

BobTPH

Joined Jun 5, 2013
8,967
If a single filter reduces a particular frequency by half, what happens when you put its output through a second identical filter?
 

Thread Starter

Gpand

Joined Dec 11, 2023
82
It will reduce it too.

Anyway. is there a finished product that uses the maximum number of Sullen-Key poles so that I can just input the signal and get the output without having to build any pcb from scratch (without any good experience in this)?
 

crutschow

Joined Mar 14, 2008
34,452
Anyway. How are ripples produced from a carrier frequency? Do all frequency have ripples? I think light doesn't but only electricity. In what kind of circuit can this occur?
Frequency has nothing to do with the ripples.
This ripple occurs in this particular type of isolation amp which modulates the input to transfer the signal across a capacitor coupling for isolation.
From the date sheet: "The internal oscillator forces the current source to switch at 500 kHz.", so the ripple is at that modulation frequency (and as can be seen from the ripple in the Figure 12 oscilloscope plot).
Thus the 2-pole, 50kHz filter provides about 40dB of attenuation to the 500kHz ripple.
Anyway. is there a finished product that uses the maximum number of Sullen-Key poles so that I can just input the signal and get the output without having to build any pcb from scratch (without any good experience in this)?
So what will the amp be built on, as I assume it won't be just dangling in the air?
You put the filter on the same board as the amp.
And from the amp data sheet, you don't need more than a 2-pole Sallen-Key filter to reduce the ripple below the amp's noise level.
 
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