NI PXI-5105 FSK Demodulation

[plasmaphase]

I just want to say right away that you've been a huge help and I really do appreciate it.  I'll take a look at the above suggestions/questions and get back to you.  Thanks again.

[plasmaphase]

Here's a couple screenshots of an FFT of the FSK waveform:

The upper waveform is filtered, the lower is not.

[plasmaphase]

Does the bit pattern really matter if I'm not trying to sync, or are you saying I should be trying to sync?  I am sending a repeating pattern of 0x55, 0xAA, 0x55, 0xAA.... at a baud rate of 115200. My mark and space frequencies are 1MHz and 2MHz. 

[MikhailK]

Hi,

If you have a sync sequence defined then you should enable sync. You can then look at sync found index output to determine if you sync sequence was found in the acquired data.

sync found index returns the symbol index within the input complex waveform where the peak correlation to the sync sequence was found. If no sync sequence is specified in the synchronization parameters cluster, this parameter returns the offset from the start of the input complex waveform to the first complete symbol.

What is the current FSK error that you are getting? From the spectrum that you posted, your carrier frequency is around 1.5MHz and FSK deviation is 5MHz, which places the peaks at 10MHz and 20MHz. Are these the parameters you are setting in your FSK generation VI.

For you reference, I ran a simple FSK send-receive test using NI VSG and NI VSA and I was able to obtain FSK errors less than 1.5%. I used a pseudorandom bit sequence (PN sequence order=9) as my input bit spream.

I am attaching the VIs and the results (spectrum, eye-diagram, etc.) I obtained. You can use this as a reference for in your application.

[plasmaphase]

Alright, I think I got it to work.  I had to change my carrier frequency to 1.5MHz, and my bandwidth to 1.5MHz (marker and space frequencies are 1MHz and 2MHz).  Do you have any suggestions as to how to convert this binary stream back to my original data as a byte stream?  Shown below is my decoded stream with the sync byte enabled:

Also, is this decoding a continuous process, or am I going to drop data?  Thanks again for all your help!

[MikhailK]

Hi,

You can use the Map Symbols to Bits function to recover your message. Have a look at the MT FSK Mod-Demod Example on how to accomplish this.