FM Demodulation

[fasterbyelan]

Hi,

Having recorded a 3150Hz test tone on my turntable/record player, I wish to de-modulate the resulting FM signal using LabView. This will enable me to examine the wow and flutter present possibly displaying such on a Polar plot.   I am however at a loss how to achieve this. Can anyone help please?

I have been able to upload the test tone (.wav file) and view the frequency content via the Auto Power Spectrum.vi, so know that the modulation is within +/- 10Hz.

Karl

[NI-MarkN]

Hi Karl,

Did you have a look at the "MT Demodulate FM.vi" VI in LabVIEW? It might be exactly what you are looking for. You can find it by using quick drop (ctrl+space) on your block diagram or going to Functions Palette - RF Communications - Modulation - Analog - Demodulation category.

Let me know if that helps.

[fasterbyelan]

Mark,

I assume that the vi you mention is part of the Modulation Toolkit and therefore not available in the normal LabView install.  I cannot justify the cost of the Toolkit and hence am looking for an alternative method of achieving my aim.

Karl

[NI-MarkN]

I see, apologies for that. I just assumed you had the toolkit, just needed some guidance using it.

Did you have a look at this post: http://forums.ni.com/t5/LabVIEW/Frequency-Demodulation-Help/td-p/1658534 ?

It looks like a similar problem and the reply has a brief description of how you should approach FM modulation without the RF toolkit there.

[johnsold]

Consider an alternative: Heterodyne the FM signal with a fixed frequency signal at ~3165 Hz. One of the outputs will at the difference frequency, which would be from 5 to 25 Hz.

The process involves multiplying and low pass filtering.

Lynn

[Henrik_Volkers]

here is a quick untestet phase demodulation,

for more help please post that wfrm 🙂

[Henrik_Volkers]

If you captured that wfrm(.wav) with a soundcard, it migth be that the sampling frequency has a tolerance ... that would show up as a constant drift in the demodulation ....

So if you know the number of periodes for one turn of your table, you can compensate this effect.

(Simply by changing the Modulationsfrequenz)

low cutoff freq should be somewhere in the 100Hz .....

[ARahman786]

Sir I used this VI but I have a little phase problem here. Sir can you help me with180 phase shift

[Henrik_Volkers]

Swap SIN / COS multiplication 🙂

as written 7(!) years ago...  untested 😄

[harrimansat]

Hi, it works, thanks!