Oscilloscope input filters

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Crt Valentincic
Posts: 67
Joined: Wed May 28, 2014 12:15 pm

Oscilloscope input filters

Post by Crt Valentincic » Thu Jun 19, 2014 10:18 am

Originally posted by Andrey Sobol:
Hi guys,
How can I calculate coefficients for input filter from basic ones a0,a1,b0,b1 IIR or FIR filter?

Crt Valentincic
Posts: 67
Joined: Wed May 28, 2014 12:15 pm

Re: Oscilloscope input filters

Post by Crt Valentincic » Thu Jun 19, 2014 1:37 pm

More detailed information about input filter can be found here: http://wiki.redpitaya.com/index.php?tit ... ifications

SeniorAndre
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Joined: Mon Jun 16, 2014 8:36 am
Location: Russia, Nighniy Novgorod
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Re: Oscilloscope input filters

Post by SeniorAndre » Fri Jun 20, 2014 6:52 am

Wow, thanks!!!
I drawed the diagram also but I can`t build the transfer function, thank you.
Andrey.

SeniorAndre
Posts: 5
Joined: Mon Jun 16, 2014 8:36 am
Location: Russia, Nighniy Novgorod
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Re: Oscilloscope input filters

Post by SeniorAndre » Fri Jun 20, 2014 9:22 am

Hi,
I took coefficients from the file 'Test / acquire / fpga_osc.c"
Then simulate them in the Scilab

Code: Select all

//fpga_osc
//Equalization  filter 
//LV - Enableb

z=poly(0,'z');
aa=hex2dec('7D93'); //18
bb=hex2dec('437C7'); //25
pp=hex2dec('0'); //25
kk=hex2dec('ffffff'); //25
//kk=-1;
a=1-(aa/2^25);
b=1-(bb/2^28);
p=pp/2^16;
k=kk/2^24;
h=k*(z-b)/(z^4*(z-p)*(z-a));
[hp,fr]=frmag(h,256);
plot(fr,hp);
Rezult is unreal, see attacments.
Image
Where is error here?

PS. Add attachment don`t work.....

Crt Valentincic
Posts: 67
Joined: Wed May 28, 2014 12:15 pm

Re: Oscilloscope input filters

Post by Crt Valentincic » Mon Jun 23, 2014 11:13 am

We believe the response you attached is correct. The applied filter coefficients introduce a pole and a zero at kHz range (we call that equalization filter). The smoothing filter is here disabled (pp=0).
You can better visualize that by replacing the plot function with semilogx. The equalization filter is used to cancel out extra poles and zeros introduced by probes or capacitive dividers.
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