Best demodulation method

[Altera_Forum]

Hi,  

I have to transmit data modulated with 250Hz carrier and bit time of 12ms (3 period of carrier). Obviously i need to demodulate that message on receiver side.   The problem is that on the transmission channel 60Hz and 180Hz signals are present. So that two frequencies could be added to the transmitted signal @ 250Hz.  Now i don't know witch kind of modulation/demodulation method is the best suitable for my purpose and if i use a classical one (ask or bpsk) is there a technique to filter out undesired frequencies?   Thanks let me know.

[Altera_Forum]

--- Quote Start ---  Hi,  I have to transmit data modulated with 250Hz carrier and bit time of 12ms (3 period of carrier). Obviously i need to demodulate that message on receiver side.   The problem is that on the transmission channel 60Hz and 180Hz signals are present. So that two frequencies could be added to the transmitted signal @ 250Hz.  Now i don't know witch kind of modulation/demodulation method is the best suitable for my purpose and if i use a classical one (ask or bpsk) is there a technique to filter out undesired frequencies?   Thanks let me know.  --- Quote End ---    your question is not right.You should know your tx signal(modulation, bandwidth, frequency centre) and then demodulation is reverse of that plus some rx front end complex work to lock to clock and/or carrier.

[Altera_Forum]

i'm looking for the most suitable modulation/demodulation method in order to transmit a message with 250Hz carrier and receive it correctly in presence of noise @60Hz and @180Hz. 

Thanks

[Altera_Forum]

--- Quote Start ---  i'm looking for the most suitable modulation/demodulation method in order to transmit a message with 250Hz carrier and receive it correctly in presence of noise @60Hz and @180Hz. Thanks  --- Quote End ---     you can choose any of FSK,ASK,BPSK,QPSK depending on complexity you accept. You start with bits (0,1), convert them to symbols (+1,-1 etc depending on mapping then scale up for DAC). You can choose your carrier at 250H (too low and I assume it is just some practice). As to noise @ specific frequency you need to explain that since random noise is everywhere so you need to tell what is nature of that noise or is it just interferer.

[Altera_Forum]

--- Quote Start ---   As to noise @ specific frequency you need to explain that since random noise is everywhere so you need to tell what is nature of that noise or is it just interferer.  --- Quote End ---   Yes it is an interferer, "noise" is a wrong term.  Actually on channel where i have to transmit my 250Hz messagge 60 an 180Hz signals are just present.  The amplitude of 250Hz signal should be half or less of the 60 and 180Hz signals. What about cdma or similiar? Let me know. Thanks.

[Altera_Forum]

--- Quote Start ---  Yes it is an interferer, "noise" is a wrong term.  Actually on channel where i have to transmit my 250Hz messagge 60 an 180Hz signals are just present.  The amplitude of 250Hz signal should be half or less of the 60 and 180Hz signals. What about cdma or similiar? Let me know. Thanks.  --- Quote End ---    I suggest you go for the simplest modulation of ASK(or OOK). you can multiply the symbol stream with 250Hz carrier before sending it to DAC.  Your bandwidth will be your symbol rate spread on either side of carrier and so you have very narrow space indeed. For example if symbol rate is 200Hz your bandwidth becomes 200Hz i.e. 100Hz on either side of 250Hz carrierand gets mixed with interferer. What is your data bandwidth (or symbol rate) and what do you actually want to achieve at system level.  cdma ? no way, you don't need that complexity unless you want practice.