can't get a bandstop filter to work

Forum for discussing PicoScope version 6 (non-automotive version)
Post Reply
johnbicht
User
User
Posts: 5
Joined: Wed Jul 18, 2018 5:20 pm

can't get a bandstop filter to work

Post by johnbicht »

20180618-0003.psdata
(19.84 KiB) Downloaded 330 times
I am finding it impossible to get a bandstop filter to work. I have 120Hz unfiltered output from a DC motor drive, that I very much would like to eliminate to clarify the current/torque driving the motor. Windows 7, 2204A, Picoscope 6.13.6 . I used the wizard multiple times. The setting is basically "bandstop(A,100, 140)" voltage setting -.05V +.05V. I started off using a much narrower band and widened it to the above setting to hopefully get some results. I can't see the difference between the raw data and the filtered. I have repeatedly checked the 120 Hz using Picoscope.

This is a sample of what I am trying to filter.

bennog
Advanced User
Advanced User
Posts: 206
Joined: Mon Nov 26, 2012 9:16 am
Location: Netherlands

Re: can't get a bandstop filter to work

Post by bennog »

Because your signal is slow you can use a lowpass filter
Lowpass(A,1)
Lowpass(A,1)
12 bits resolution enhangement
12 bits resolution enhangement

johnbicht
User
User
Posts: 5
Joined: Wed Jul 18, 2018 5:20 pm

Re: can't get a bandstop filter to work

Post by johnbicht »

Thank you, but why can't a bandstop be got to work?

bennog
Advanced User
Advanced User
Posts: 206
Joined: Mon Nov 26, 2012 9:16 am
Location: Netherlands

Re: can't get a bandstop filter to work

Post by bennog »

I think because the sample rate is too low.
you are sampling at 750 Hz, but not sure if this is root cause.

Someone from picoscope can probably tell you more about this.

Benno

johnbicht
User
User
Posts: 5
Joined: Wed Jul 18, 2018 5:20 pm

Re: can't get a bandstop filter to work

Post by johnbicht »

I just tried a run with sample rate 20ks/S. Bandstop still didn't work.

I appreciate both of your time and thoughts.

johnbicht
User
User
Posts: 5
Joined: Wed Jul 18, 2018 5:20 pm

Re: can't get a bandstop filter to work

Post by johnbicht »

Trying a low pass filter, I discover that these filters are very shallow sloped. My guess is that I need to very much broaden the bandstop parameters.

Gerry
PICO STAFF
PICO STAFF
Posts: 1145
Joined: Mon Aug 11, 2014 11:14 am

Re: can't get a bandstop filter to work

Post by Gerry »

Hi johnbicht,

The Math Channel filters have a 20dB per decade roll-off so you are not going to be able to differentiate between a signal and unwanted distortion/noise if they are too close. For example, if you have a signal at 100Hz and you're attempting to reduce induced interference modulating your signal at 10Hz and noise beyond 1kHz corrupting the signal then you will be able to reduce them by 20dB.

In your case it appears that you have signal components extending up to 2.5Hz or almost 8Hz (depending upon whether the almost 8Hz component is a desired part of the signal), while the 120-ish Hz signal that you want to eliminate is more than a decade above the almost 8Hz component signal. So I'm not quite sure why you would want a bandstop filter (unless you want to also keep what looks like noise at frequencies above 1kHz).

So if you want to remove the 120Hz signal then you would get a reduction of about 20dB using a lowpass(A,12) for only a slight reduction in the signal. You could also use the lowpass filter in the input channel options (if it is available for your PicoScope) which would give you the same result (as shown in the attached psdata file).
12Hz lowpass.psdata
(18.64 KiB) Downloaded 325 times
If you also want to remove the almost 8Hz component as well then you would need to drop the cut-off further as in the first reply, but then you would loose the component at 2.5Hz (or most of it).

Regards,

Gerry
Gerry
Technical Specialist

Post Reply