freq

[labros]

Hi

I'm trying to design a vi in order to measure the frequency response of a transformer (two coils, the one inside the other) by generating a sinus wave of varying frequency and feeding with that wave the primary coil. I want to measure the voltage response of the transformer through the amplitude response of the secondary coil. This is done through the input channel. There seems to be a problem, with this kind of design. I think that it has to do with the second Function Generator inside the loop. The problem is that, plotting the amplitude values i get with respect to the corresponding frequencies, there are some very high amplitude values taken. Noticing the graph while running the vi, i see that these high values are related to a "noise wave" occuring in the input channel and thisway confusing the vi. It's as if this noise occurs, every time the triggering takes place.

Does anyone notice something wrong?

Are there any suggestions for different frequency sweep methods?

Thanks in advance

[stilly32]

Hi,

You might consider using one of the tone measurement VI's (ie the "Tone Measurements" express vi) to measure the amplitude of the tone that gets through - that way if you had any high frequency noise it would not be measured.

Also, when you start/stop your AO, you're introducing a step function - can your transformer handle that or is it going to cause some ringing?

It would be helpful to know more about the signal you are reading back - do you have screenshots of it? Specifically the "noise wave" would be nice to see - is this just a high frequency component riding on a carrier wave?

Hope this helps, please post back if you have additional questions.

Andrew S

National Instruments  

Message Edited by stilly32 on 05-08-2007 03:16 PM

[labros]

Hello again,

I attach a screenshot of the noisy wave.

Also, using an oscillator and a scope, i found out that  by feeding the transformer with "bursts" of signals, this has as a result abnormal amplitude responses.

Is there a way, to output a "more lasting" sine wave, in order for the device to settle? After all, the purpose of my vi is to create continuously (as much as it can be done of course) the wave through the output and use "bursts" of incoming sets of data, in order for my input channel to "catch the wave" and calculate it's amplitude.

(please check again the vi, there are some slight changes)

Thanks

[johnsold]

Your waveform does not appear to be noisy.

The large ramps look like an inductor being driven by a constant. The peak occurs either when the drive is removed or the core saturates. Then the four or five cyles after that are the decaying oscillation resulting from the inductance of the windings and stray capacitances. The excitation is at about 20 kHz and the self resonsance is at about 400 kHz.

Are you loading the transformer? If the secondary does not have a closed circuit, the only currents which flow are magnetizing currents and capacitive currents. Without a load, you are measuring some combination of the primary inductance and stray capacitances but not any transformer behavior.

You are still sending out short bursts. Look at the attachment. I pulled pieces out of your program to make this. Each frquency is present for 1 to 5 milliseconds. Is this long enough to get steady state measurements?

Your diagram could be cleaned up a bit. Left to right wiring and wires under VIs make it hard to see what is going on.

Lynn

[stilly32]

You can change the frequency of your generation on the fly - without stopping and starting the task over and over again. Check out the "Cont Gen Voltage Wfm-Int Clk-Variable Rate.vi" example from Example finder. This will allow you to change the frequency of your sine wave without going to a DC component and maintaining a phase continuous signal. So you could set the frequency, wait, read the response, and repeat.

Would this work for you?

Andrew S