Demodulating AM bitstream

[CobhamDC Blackburn]

Hi,

I am attempting to grab and demodulate a simple AM modulated transmitted code (433Mhz @ ~26Kbps) using a PXI 5660.

Using the AM demodulation example code I added some filters and I have managed to capture the bitstream as shown in the diagram below in the Analog waveform graph window, but I don't know how to convert this to a simple digital bitstream that I can analyse, i.e of the form 11010001100 etc...

I'm hoping this is pretty easy when you know how? Please help!

Cheers

Matt

[rpursley8]

I would first pick a threshold value between the low and high (for the data example, 0.5 should work).  Then I would search for the first edge to cross this threshold and that would be my reference point.  Since the data is coming in at 26 Kbps, that translates to a period of ~38.5 us.  I would then break up the data into blocks of points ~38.5 us wide based on the position of the first edge (go backwards and forwards in this step size).  I don't know what sampling rate your data has so I can't tell you how many points there will be in each block.  Hopefully, each block of data can easily be interpreted as a high or low (0 or 1), assuming the period is correct.

[tbob]

In the function palette Waveform - Digital Waveform - Digital Conversion, there is a function called Digital Waveform to Binary.  If your waveform is analog, you need to use the Analog to Digital.vi first to convert to a digital waveform.  Then call the Digital Waveform to Binary vi.  This should give you a bit stream in the form of a boolean array.  You need to choose the data type (U8, U16,U32) from the function's drop down menu.  I haven't tried it, but I think this should work.

[rpursley8]

Tbob,

Thanks for pointing out these functions.  I have never used them before.  They seem to be a good way of cleaning up the digital data, but in the end you have the same number of points as when you started.  None of the functions I found reduces the data down to one value per bit.

[tbob]

Randall:

Actually I never used these functions.  I just noticed that they were there one day while searching for something else.  I thought they would produce one value of binary data per bit of waveform data.  Seems like that is what they were written for.  I'll try it out when I have time.  Meanwhile, your previous suggestion should do the trick.

[CobhamDC Blackburn]

Hi Guys,

Thank you very much to you both, your posts have prooved very useful.

Cheers

Matt