fir notch filter design.(using DFD tool kit)

[SunilRaj]

Hi

We used standard narrow band lowpass FIR filter design vis to desin a filter which can filter out any signal between 0.1 hz to 5hz. This does generate a filter design but is of a very high order, hence takes a very long time to process the raw data. Our raw data is of about 5000 samples and probably need to have higher samples when acqiring in the lower band frequencies,(say 0.1 hz or 0.5 hz). 

Presently the need is to design a filter with folllowing characteristics:

1. Phase linearity to be maintained in the pass band.

2. sharp cut off freq band (can be the best possible to optimize speed of computation).

3. speed should be optimized.(require fast)

4. signal is mostly sign waves with noise riding on it.

5. High sampling rate is necessary to maintain the shape of the sign wave since we are more concerned with the phase of the input signal.

I have attached the snap of our design, (we used "DFD Narrowband Filter Design.vi"), which already uses the cascading technique.

thanks and regards,

sunil

[.avi file]

Greetings!
   Thanks for the email. Please let me know if you are seeing any specific issue designing your VI with the toolkit. Are you looking for VI with that design specification you listed?

1) FIR filters provide phase linearity and the same goes with computation speed.
2) The link below provides good information on DFD
  http://www.ni.com/pdf/manuals/371353a.pdf

Thanks and hope this helps
Best regards
Avi Harjani

[SunilRaj]

Hi,

Thanks for the reply.

The vi is designed but we are having an issue of speed of processing.

This filter takes a long time to process.

We are looking for the following:

Since our signal can range from 0.1Hz to 5Hz our sampling freq needs to be higher, about 8000 samples/s, this is necessary since the shape needs to be maintained for accurate phase measurements (the signal is always sinewave and with noise on it).However you may suggest this sampling rate based on your experience.

Since the phase in the pass band needs to be linear we have to select FIR and since the filter roll off needed is very sharp as provided in the attachment, the resulting design has a large number of taps(coefficients), this makes the processing slow.

--What would be the appropriate sampling frequency for such a filter design.

--Is it that such filters are slow, if so how can they be put in the front end filtering of signals acquired from a daq (PCI-6229)analog channel in a continuous acquisition mode.

we would be thankful for a quick response.

PLS refer to the attachment for filter characteristics.

thanks & regards,

sunil

[LittleQ]

Basically, you can't use digital filter to replace analog filter if the input signal contains lots of noise.

So in your case, I suggest that you use a scc-LP(lowpass filter signal conditioning) before the daq and then acquire the singal using a low sampling rate. For example, you can use a 25Hz scc-lp and then a sampling rate larger than 50 (25X2)Hz.

Since you have a lower sampling rate, then your digital filter have a wider passband and a lower order. 

[johnsold]

If your signal is one sine wave of stable frequency (during the measurement time), perhaps fitting a sine wave to the data and extracting the frequency and phase from the model would be faster than trying to do a difficult filtering job. Filters with sharp cutoff and well-behaved phase tend to be mutually exclusive or very high order, as you have learned.

Lynn