fsk 315Mhz demodulation and decoding

[bryanthomas]

Hello,
I have a ni 5660 and am trying to decode the following Communication.

Modulation: FSK
carrier: 315Mhz
deviation: +-32khz
mark: 315M+32k
space: 315M-32k
baudrate = 10k

I have attached the datagram in a jpeg, it consists of 2 charge pulses, a few sync pulse and then the actual data. I do not need to recover the charge pulses.

the goal here is to demodulate the wave and then decode it into its proper bits and eventually get the bit widths

I have also attached the vi that I am working with. Any advice or guidance would be greatly appreaciated.

Thank you,
Bryan

Message Edited by bryanthomas on 08-01-2006 10:46 AM

[bryanthomas]

More information:

I have the Modulation toolkit and labview 8.1

thanks,
BT

[50ohmTerminator]

Hello,
I am trying to demodulate a 315Mhz FSK signal that has bits encoded in bi-phase encoding scheme. Bi-phase coding consists of a level change at the start of every bit. If the data bit is logic ‘0’, there is a level change halfway through the bit. If the data bit is logic 1, there is no transition in the bit, see Figure4.jpg and Figure5.jpg.

Working with labview 8.0, ni-5660, and the modulation toolkit I have managed to put together the vi attached (fskdecoder.vi). However I cannont consistantly decode it into the same bits. Currently I am only trying to demodulate the FSK into its bits and not worrying about the conversion from the bi-phase encoding to actual data bits. I have a transmitter simulator card that is directly connected to the ni-5660 and can output an identical packet consistantly.

I have also attached datagram.jpg which shows the format of the message that comes from the transmitter. The message consists of 3 pulses that precondition the data slicer on the reciever and then 4 bi-phase sync bits, which is really 8 FSK bits, then  followd by 64 more bi-phase bits and then a bi-phase stop bit. so a total of 69 recoverable bi-phase bits.

Signal Attributes:
315Mhz +/-32khz FSK modulation
10khz baudrate
200khz bandwidth

I am having trouble getting the same bits back from the FSK demodulation VI. Do I have the sync parameters correct? Do I have the filter parameters correct? What is meant by 'SYMBOL' in the documentation

thank you,
Bryan Thomas

Hey Bryan,

I posted your email because it makes the questions clearer. I agree that we should first try to get all the bits. Then we can decode them from biphase to get the data. I did take a look at your VI. I think you will need to take a look at the filter characteristics of your transmitter and set the filter for the receiver accordingly. Take a look at the example program at

C:\Program Files\National Instruments\LabVIEW 8.0\examples\Modulation\simulation examples\Interactive\MT FSK Transceiver.vi

It is quite helpful and gives you some idea on how to set up the filter characteristics. A symbol is the set of bits each frequency represents for FSK modulation. For example in 4 FSK 00, 01, 10 and 11 are symbols. In 2 FSK 0 is a symbol and 1 is a symbol.

Hope this helps!

Regards
Malay Duggar
Applications Engineer
National Instruments

[bryanthomas]

Thanks for the response.

How do I go about choosing the proper filter parameters? I know the transmitters bandwidth and carrier frequency, what else do I need to determine?

thanks,

Bryan

[50ohmTerminator]

Hey Bryan,

The filter characteristics for acquiring the data depends on the filter characteristics used when transmitting it. So the manual for your transmitter might tell you what filter they are using. Once you put these as the inputs to the 'MT Generate Filter Coefficients.vi' it will generate the matching and pulse shaping coefficients that you can use for to demodulate the signal.

Let me know if you have any more questions.

Regards
Malay Duggar
Applications Engineer
National Instruments

[bryanthomas]

Thanks Again Malay,

Other than a simple bandpass filter in hardware there is no filtering going on from the transmitter.

thanks,

Bryan

[50ohmTerminator]

Hey Bryan,

I made a small example VI that shows you how to set up the demodulation. Once you have the bits you can use some logic to convert it biphase. If you are sure that the transmitter does not have a pulse shaping filter in front of it then you can choose the none option. This example should get you going. If you still have questions, please let me know.  You could also post (on this board) which transmitter you are using,

Have a good weekend!

Regards
Malay Duggar
Applications Engineer
National Instruments

[bryanthomas]

Malay,

Thanks for taking the time to make the VI.

Best Regards,

Bryan Thomas

[xingbofeng]

hello

      I am trying to transmit a bit stream before  modulating. But I am not getting the original bit stream after  demodulating. i use the MT RFSG PSK.VI and MT RFSA PSK.VI,i try set sync  parameters and filter parameters same as the transmiter.i use 5671 and 5660 .