Lowpass filter in modulation

hello
i'm using LABVIEW to implement LSB-SSB modulation, this requires a VSB signal passed through a lowpass filter, the problem i couldn't get my required spectrum, i wonder if the problem is in the filter or elsewhere, i hope you can help me in that , i only need to know how the filter in labview works , thanks
here is the right attachment (the previous one was wrong sorry
Solved!
Go to Solution.
Attachments:
ssb1.png ‏71 KB

The realities of real filters!
1. It is not possible to implement an ideal filter in the real world. *
2. All filters, hardware or software, have a transient when the signal is first applied (or changed).
3. The narrower the bandwidth or the sharper the cutoff, the more significant the transient.
4. All software filters have errors due to the finite size of the numbers used in the calculations.
5. Many filters and all spectral measurements are affected by the finite size of the data set provided as inputs. The mathematics of the Fourier transform assume the signal behaves the same for all time from -infiinty to +infinity. Since people are unwilling to wait infinite time to get results, errors occur.
How did I design the filter in the VI I posted?
- I selected a filter type -elliptic- based on my knowledge that elliptic filters can provide a very deep null close to the cutoff frequency. I used the default passband ripple of 1 dB because the passband ripple is somewhat irrelevant in this situation and 1 dB is a small change for an audio signal. Then I experimented with the filter order until I got what I thought seemed like a reasonable upper sideband suppression. If I had detailed specifications for a filter, I wold have looked up the characteristics in a filter handbook to see if the selected parameters met all the requirements.
* You can sometimes approximate one by calculating the Fourier transform, setting all the undesired frequency bin values to zero, and then doing an inverse transform. However, if there is any spectral leakage, this technique does not work. In any case it is limited by the same numeric concerns in item 4 above.
Lynn

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Active 2nd order analog lowpass filter in a feedthrough module SCC-FT01.

I would like to build an active 2nd order lowpass filter with cutoff frequency of 4000Hz in a feedthrough module SCC-FT01 for antialiasing purposes. Since I am not an electronic expert, I would like some advice on which components I can use (and how I can dimension them) and on how I can connect them on the module board.

Caccia,
I would recommend that you build an external box with your filter in if you require hardware filtering. The feedthrough panel FT01 is just a piece of wire really with connections on each end, so there isn't room to build a filter in there.
Unfortunetly I can't really recommend what components to use, but there are lots of websites and books out there that you could use to help you, none of them specific to NI but that shouldn't matter.
After a quick search on Google I found the following website which seems to have a lot of information.
http://my.integritynet.com.au/purdic/lcfilters.htm
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Regards,
Peter H
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SCXI 1125 lowpass filter

Hello I am using a SCXI 1000 chase with two SCXI 1125 cards. My question is how can I configure a software filter that works like the one implemented in the SCXI 1125 card. More exactly i what to build a 4 Hz cuttoff lowpass filter. Thank you.
Attachments:
filter.jpg ‏139 KB

Which module are you using?
I suppose you could always just run your signal through the Filter express VI (in the Express » Signal Analysis palette) or one of the VIs on the Signal Processing » Filters palette.
Kyle B | Product Support Engineer | ni.com/support

Does Strain task in DAQmx include lowpass filter?

I am using a load cell with a 2.232 mv/V with an excitation voltage of 10V. When I put a scope to the output, there is a lot of high frequency noise which causes me to have to use an amplifier with a lowpass filter built in.
I would like to be able to accomplish this same task without having to purchase the amplifier (I have 3 load cells per machine, 6 machines so cost does add up). I am currently using a PCIe-6320 for some encoder measurements on this same application and did find where I can use the analog inputs to measure Strain using DAQmx.
My question is does the Strain measurement in DAQmx have a built in lowpass filter to clean up the signal? If not, is there a solution to this that does not require a Toolkit or additional module for signal processing? We have the basic LabView 2009 SP1.

aeastet wrote:
I would also look at why you are getting so much noise. How do you have your channels hooked up to the system?
Yes.
Using a strain guage module the bridge is built into the terminal block and the hardware does the mv/V thing so there must be some place the noise is entering the system, long, non-shielded, non-twisted pairs, bad ground...
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NI SCXI 1104 Lowpass filter

I have a NI SCXI-1104 and in its datasheet says that it has on each channel a three-pole lowpass filter with a 2 Hz cutoff frequency to reject 60 Hz noise. I need to know: What kind of filter this device uses? What happen with the data over the cutoff frequency? In my case I need to analyze the frequency between 1 to 8 Hz and reject the others.

This board is for the Excel add-0n called Measure.
Well it's three-pole low pass filter and frequencies above 2 hz are going to be greatly attenuated. This module is intended for thermoucouple (dc) inputs and would not be suitable for any sort of frequency analysis, in my opinion

Lowpass filter in communication

hello
i'm using LABVIEW to implement LSB-SSB modulation, this requires a VSB signal passed through a lowpass filter, the problem i couldn't get my required spectrum, i wonder if the problem is in the filter or elsewhere, i hope you can help me in that , i only need to know how the filter in labview works , thanks
Attachments:
ssb.png ‏89 KB

Have you tried the detailed help on the Filter express VI? (Ctrl H --> Detailed help)
David H.
National Instruments

How to configure the lowpass filter integrated to the serie M DAQ (6280)

It's written that for the NI 62xx serie M DAQs, a lowpass filter can be programated (to prevent noise or aliasing), but I can't find where I can configure it ...
I use LV 7.1, DaqMX 7.3 and MAX 3.1.1.3003.
Thanks for your help !
Mathieu

Bonjour,
Effectivement, la valeur du filtre de la carte NI-6280 est 40kHz.
En simulant la carte, j'ai pu faire le test avec le VI joint.
Si ce VI ne fonctionne pas, quelle est le code d'erreur obtenu et à quelle fonction apparaît-il ?
Salutations,
Isabelle
Ingénieur d'applications
National Instruments France
Attachments:
filtre 6280.vi ‏73 KB

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My btns do not work, I think becuase I added this:
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You might want to find out what purpose the filter(s) serve(s) before destroying (disabling/removing) it/them. For example, URLScan is there to prevent potentially harmful requests from being processed by IIS, and is configurable.

Lowpass filter help for thermocouple reading

Hello
I have an old legacy E-Series AI-16SE 12bit 500KHz PCI card.
I am attempting to setup a temperature reading test program. I will have the E-Series connected to a SCB-68, which will have five T type thermocouples connected. I realize this is not the best test setup but it is an attempt at a quick setup on a bench so hopefully any noise interferance will be minimal.
I will be using LV 8.2 professional development.
I was wondering if I setup a software based lowpass filter if that would be helpful in this situation?
If so can someone show an example the has a cutoff freq for 2Hz? I am unfamilular with all the settings of the lowpass filter VI and some guidance would be much appreciated.
Thanks in advance
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Tim,
Rob's idea is probably what you should try first. It is in effect a low pass filter with about a 10 Hz corner frequency.
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Noisy Analogue Signal and Lowpass Filter Questions

Hi everyone,
I am getting analogue signals from a strain gauge that are a bit more noisy than I would like... Normally, I jump right in an smooth the signal using a lowpass filter of whatever order gave me the result that was acceptable, without overkill and smootig the signal to nothing... However, I am settingup a new DAQ device and would like to a little more informed about what and how am doing this...
So, here are my questions:
1. How do I determine the filter order? I usually use a 2nd order filter for smoothing.. I understand the comncepts of roll-offs and phase delay as they relate to the order numbers... On what basis should I decide which order filter to use for this specific signal?
2. I am still thinking that using a lowpass filter is the best approach... I assume the lower cutoff freq is in Hz (i.e. 10Hz) and is NOT a normalised freq [i.e. cutoff freq / (sample rate/2)]?.. The default value is 0.125, which seems too low for a Hz value? Also, on what basis should I decide what cutoff freq to use for this specific signal?
3. I need to synch the filtered strain gauge signal with other, unfiltered signals... But as the filter will add a phase delay it will affect signal synchronisation... I heard somewhere that a 4th order filter is zero-phase, but I can't see how that could be right as the filter only operates in one direction... Any suggestions for what I could do to maximise the timing alignment between the filtered and unfiltered signals?
All comments welcome!
Regards,
Jack

Jack,
I was the Instrumentation Engineer at a university for 24 years and worked with several execise physiologists, biologists, and neuroscientists on similar projects. My group was the "lab/tech staff" you have little of. There were 3.5 (engineer, 1.5 electronics technicians, and machinist) of us for the entire university.
OK. The strain gauge signals probably have no information above a few tens of Hz (hummingbirds) or a few Hz for most larger species. EMG is usually filtered in the amplifiers to slightly larger bandwidths (few hundred Hz) unless you are trying to see the individual action potentials. Does the dynamometer have specifications on the bandwidth or sampling rates used?
Since your bandwidths are realtively low, I suggest that you sample all the analog inputs (and the trigger outputs, if the boards have the capability) from the same clock. You can later decimate or average data on channels where less data is needed. Record the trigger signals on an unused analog input if you have one available. It can be very handy to have the information about when the triggers actually occurred in the same dataset as the response data.
Delay equalization is a complex topic. Suppose that you are sampling all your channels at 1 kS/s. Suppose you have a 10 Hz low pass filter, 2 pole Butterworth, on a strain gauge channel and a 50 Hz filter, 4 pole, elliptical, on an EMG channel and you want to synchronize the filtered data with some unfiltered channels. The two filters have very different phase and delay responses over the 0-500 Hz frequency range. To equalize you would need to determine the maximum delay at any frequency in the range for both filters. Then you would need to design THREE all-pass filters. One would have a phase response which compensates the 10 Hz filter. Another would compensate the 50 Hz filter. These would need to be designed so that the total delays through the low pass plus all-pass filters is the same. Then you need a third all pass filter with that same total delay to compensate the unfiltered data. This is may be a non-trivial job for an expert, depending on the low pass filters.
What kind of noise are you seeing and how is it affecting your measurements? Can you post some data?
Lynn

Filter module NI9205 cRIO

Looking for a low pass filter(~5kHz) module that would interface to the NI9205 when measuring a differential signal.
The signal is a squarewave signal (500Hz) that has 25kHz of noise riding on it..
Is it nessary to add a RC to both the plus(+) and minus(-) inputs minimize the degradation of the amplifier’s CMRR ?
See attached..
Thanks.
Attachments:
Diff_AmpFilter.png ‏134 KB

Hi B_Lind,
Since the source impedance is so low you shouldn't have any concerns as far as settling time and ghosting. The low source impedance will make the time constant of the low-pass filter very small and will help prevent ghosting. Please see the article shown below for more information on ghosting and how/why it occurs.
http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF?OpenDocument
As a side note, please keep in mind when implementing the capacitors for a low-pass filter that the 9205 has an internal 100pF capacitor connected in parallel before the ADC as shown in the diagram in the article above.
Regards,
James D.
Applications Engineer
National Instruments

Is there a low-pass filter that is compatible with the SC-2040?

I am presently using external anti-aliasing filters with my SC-2040, SSH card. Is there a low-pass filter module that is compatible?

Greetings,
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SCXI-1140
SCXI-1142
Good luck with your application.
Spencer S.

Tried the solutions posted for this "SubVI filter not executable. Undefined error" but not solving the problem

Hello,
I am trying to use a lowpass filter for my application but getting the error "SubVI filter not executable. Undefined error".I tried working with the solutions at this post
http://forums.ni.com/t5/LabVIEW/SubVI-filter-not-executable-Undefined-error/m-p/1996701#M657751 and results with
1. Create a blank vi, put a VI filter in the block diagram (this should not be functional as it is not wired), copy it and paste it in my other project. I believe if the filter is the first vi of a blank project, it is funcitonal and then you may copy it wherever you need to.
Result:
2.Tried to open the filter panel of the Filter but showing the same error as shown inthe below image
Don't why the solutions worked for others are not working.Can someone suggest a solution for this.
thanks in advance.

Hi PatanGova,
Very strange error. Actually, there are no required inputs on this express VI, so it should be able to run even without wires. It is not correct as you said: "this should not be functional as it is not wired". Nothing will happen, true, but the run arrow should not be broken.
For some reason, the VI with the Filter express VI fails to compile. I tried it on my computer (LV 2013) and get no error when I add it to a new VI. As was suggested in the linked forum thread, you need to open the front panel of the Filter VI and run it to see the explicit error.
If you do, do you also get the error:
"This VI claims to be part of a library that does not include the VI. The VI might have been deleted from the library. Add the VI to the library again or select File - Disconnect from the library."?
If you do, it seems that you may have a project library somewhere that has taken ownership of the express VI. The ownership prevents you from running or editing the express VI. To free the VI, you need to do one of two things. Either, you need to load the library into memory, or you need to disconnect the VI from the library. Either way, you need to find the project library that has taken the ownership of the VI. As soon as it is released, it should run for you. You release it by selecting File»Disconnect from Library on the VI.
The workaround described in your link (and also here: http://forums.ni.com/t5/LabVIEW/compatibility-problem-labview-2009-2010-2011-quot-This-VI-claims/td-... tells that you can make a copy of the VI and thereby create another instance that is free. This solution may work for you as well, but if not, you may not have created the copy properly. Maybe your copy of the VI also became part of some project library that took ownership. In either way, you should rather find the project library to release the VI before trying the workaround to copy it.
If project library ownership is not the problem, maybe the problem is connected to LV 2011, as most forum posters report that it only happens here.

How to design a stable IIR-Filter for FPGA?

Hi, I´m new to Labview for FPGA and currently trying to design a lowpass-filter (for a digital mixer, more or less). It should work with a sample-frequency of 200kHz, a passband edge frequency of 3Hz and a stopband edge frequency of 8Hz. An FIR-filter would have a too large order to be implemented, an IIR one seems more practicable. For the design of the filter I'm using the digital filter design toolkit, refering to its manual and the casestudy with analyzing, modeling, simulating and generating a fixed-point filter. Now, I have problems to get a stable filter. While the manual states to adjust the coefficents by trial and error, the designed filter never matches the floating-point reference. Does anyone have experience in creating such filters? Or a hint, how to optimize the coefficient finding process?
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LabVIEW FPGA 8.20 does include a Butterworth filter function (1st, 2nd and 4th order) in the FPGA palette. You can configure it directly using a dialog on your FPGA diagram. The cutoff may not be quite as sharp as you want but it could be an option for you to consider. Once you have configured the filter VI, you have the option of opening up the subVI with the code that is generated based on your filter parameters, which would allow you to further configure the code directly.
I have attached the configuration dialog with the settings you specified.
Message Edited by Christian L on 11-16-2006 09:27 AM
Christian Loew, CLA
Principal Systems Engineer, National Instruments
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Any attached Code is provided As Is. It has not been tested or validated as a product, for use in a deployed application or system,
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Attachments:
filterdialog.JPG ‏44 KB

How can I filter thermocouple noise using a TC-2095 and SCXI-1102c?

Hello all,
I currently have four TC-2095s connected to SCXI-1102c cards inside a SCXI-1001 chassis. These thermocouples are floating and I have been getting a fair amount of noise (3-phase 400 Hz and even 60 Hz noise) when we power on some equipment such as blowers and fans. Using this current set up, how can I filter the noise? Or do would I need to set up a new hardware configuration (i.e. SCXI-1102 cards)?
Solved!
Go to Solution.

Hi RALoo,
The 1102C has a lowpass filter with a cutoff at 10kHz, so you will not be able to filter in hardware with your current setup.
Your first option would be to use a software filter. LabVIEW has several functions which accomplish this. More information on this can be found here: http://zone.ni.com/reference/en-XX/help/371361J-01/lvanls/filter_vis/
The second option would be to change to SCXI 1102 cards as you suggested. The lowpass filter of the 1102 has a selectable cutoff frequency at 2 Hz, which will stop the 400 and 60Hz noise from affecting your readings.
Patrick W.
Applications Engineer
National Instruments

Lowpass filter in modulation

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