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anti alias filter

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anti alias filter

Postby Guest » Mon Aug 03, 2009 7:23 pm

I just purchased a 4224.

I am quite happy with it. I am using it for conducted emission evaluation of a product.

My question is whether or not there is a anti alias filter in hardware on this unit for the max sample rate. For lower rates the data can be down sampled but at max there must be hardware filtering.
Can anyone comment on whether the filter is there and adequate to the task of 12 digitizing?

Also how about PICO changing the 1 Meg csv point limit on saving files.
I am importing into Octave and would like to get the whole record.


Thanks
Guest
 

Postby Pico Stuart » Tue Aug 04, 2009 2:23 pm

There is no anti-aliasing filter in the hardware, however there is filtering available in the software.
Filter details

The lowpass filtering algorithm is chosen according to the relationship between the selected cut-off frequency (fC) and the sampling rate (fS), as shown in the following description.

fC / fS

0.0 to 0.1
Filter type:- Moving average
Description
A moving average filter is used for low cut-off frequencies. The length of the filter is adjusted to achieve the selected cut-off frequency, which is is defined as the first minimum in the frequency response. There is significant signal leakage above the cut-off frequency. This filter changes a vertical edge into a linear slope.

0.1 to < 0.5
Filter type:- FIR
Description
A finite impulse response filter is used for medium to high cut-off frequencies. This has a monotonic roll-off above the cut-off frequency and therefore suffers from less leakage than the moving average filter.

You can force PicoScope to use one or other of the filter types by adjusting the Samples control in the Capture Setup toolbar to make the ratio fC/fS fall into one of the two ranges described. As the description shows, the cut-off frequency must be below half the sampling frequency.

Kindest regards,
Pico Stuart
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Postby Guest » Tue Aug 04, 2009 3:12 pm

Greetings

I'm a little sorry to hear there is no anti alias filter as it is not possible to eliminate artifacts from frequencies above nyquist.
Software cannot do it, once alias data is present.


However i am considering building my own with a some video op amps and a differential input stage.

Looks like i have a new project.

I still like the product and recommend it.
How about giving option to download full size csv files, maybe with a warning screen if it is large.

Thanks
Guest
 

Postby ziko » Fri Aug 07, 2009 11:45 am

Hi as you pointed out the .csv files are limited to 1 million samples, however you can save as a Matlab (binary) format which is not limited.

Kind regards
Ziko

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ziko
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Postby Erik » Mon Aug 10, 2009 7:35 am

That there is no anti-aliasing filter is not entirely true.
The analogue bandwidth of the device is less than your maximum sampling rate. So if you are sampling flat out on two channels the sampling rate is 80 MS/s and the -3 dB bandwidth is 20 MHz. However if you are sampling slower than 40 MS/s there is no anti-aliasing filter. Maximum sampling rate on four channels on this product is 20 MS/s.
Erik
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Postby Guest » Wed Aug 12, 2009 7:25 pm

Can anyone say the DB down or gain for the device at the nyquist rate of 40 MSPS ??

Thanx
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