Why this LPF doesn´t work?!

[8144-9199]

I´m using the VI bellow and the low pass filter doesn´t work. When I increase the message frequency, but maintain the cut off frequency a constant, the frequency measured by Tone Measurements is the same that I selected on message frequency. Why?!

[JLS]

Hello,

I can't actually run your program to see what's wrong because you are writing and reading to a serial device it appears.  What I can tell you is that I think you'll find the filter shipping examples very useful to you.  Try taking a look at the Express Filter.vi example (click the Search tab, type the keyword filter, and then double click filter in the double-click keywords list in the Example Finder, launched from Help -> Find Examples...).  You'll find other examples there as well, and I think you'll find what you need there.

Best Regards,

JLS

[8144-9199]

JLS,

For to simulate this vi you need a null cable (cross-over cable). I´m using RS-232 (conector DB-9 pins) interface and I´ve jumped Tx with Rx.

Thanxs for help me...

[johnsold]

I modified your VI so that no serial port is necessary for simulation. When I do that and enter resonable values for the frequencies, everything works fine. If your cutoff frequency is of the same order or less than the message frequency you will get incorrect data because you are filtering out the signal you are trying to measure. Most communications systems use carrier frequencies one or more orders of magnitude greater than the modulating frequency. The filter cutoff is set to a value well above the highest modulating frequency.

If you are still having troubles, let us know what frequencies and amplitudes you are trying to use.

Lynn

[8144-9199]

Johnsold,

My LV version is 7.0 and I can´t run your program. Do you have LV 7.0 ?! Can you rewrite this program for me?! A carrier frequency 10 times greater than message frequency is a reasonable value. I can´t select the same message frequency in LPF because he is not ideal, so I´m selecting a few hertz above that (+1Hz). My message frequency is 1Hz, carrier frequency 10Hz, cutoff frequency 2Hz. Therefore for to test the LPF is necessary that I change this 3 parameters, and respect that the carrier frequency let be greater, at least, 10 times than message frequency. Right?!

Thanxs...

[johnsold]

Yes, you are on the right track. The other issue is that all filters have a transient response when the input signal changes from 0 (for t < 0) to the signal you want to filter (the modulated signal). With a 2 Hz filter the transient lasts more than half a second. (I did not calculate it, I just looked at the graph and guessed). If your number of samples in the simulators is increased to 10000 (10 seconds equivalent) or your frequencies are scaled up by a factor of ten or more you can see the effects. With the default values of my VI you can see a transient of about 0.1 seconds.

Here is a LV 7.0 version.

Lynn

[johnsold]

Yes, you are on the right track. The other issue is that all filters have a transient response when the input signal changes from 0 (for t < 0) to the signal you want to filter (the modulated signal). With a 2 Hz filter the transient lasts more than half a second. (I did not calculate it, I just looked at the graph and guessed). If your number of samples in the simulators is increased to 10000 (10 seconds equivalent) or your frequencies are scaled up by a factor of ten or more you can see the effects. With the default values of my VI you can see a transient of about 0.1 seconds.

Here is a LV 7.0 version.

Lynn