I would like to modulate and demodulate a binary stream of data at 128kbps baud rate using BFSK or N-FSK with carrier freq. of around 10 MHz. I have the following questions?
1) What is the theoretical bandwidth I require to achieve the 128kbps data rate?
2) Are there any ICs available to do this? if not what are the simplest solutions/work around to achieve this?
3) Is the RX side affected by the change in the phase of the received signal?
4) Can anyone kindly suggest books or online resource to implement such system.
I am not worried about the PA on Tx side nor the AGC on Rx side at this stage. It is mostly the implementation of the FSK modulation and demodulation part that I am struggling with. Digital communication is not my strong point in electronics at all so any help would be highly appreciated. Thanks in advance!
Dear@Arash,
welcome
- In frequency shift keying you have to shift the carrier frequency by a frequency deviation df such that you have binary frequencies fm= fc+df and fs=fc-df.
Fm will represent the mark which is logic one and fs represent the logic zero.
So the number of transitions from the space to mark and vice verse will be equal to the bit rate fb= 128 kb/s.
According to the theory of frequency modulation the band width the binary FSK
can be expressed by
BW= 2 df +fb, so df is a parameter of the FSK
You can adjust it according to the bandwidth of your transmission channel.
If you have excess power you can reduce df to even equal to fb/2.
In this case the total bandwidth will be BW=2fb
As for the modulation and demodulation you can use the proper phase locked loop pll. In the transmitter you will use the voltage controlled oscillator and for the demodulator you can use the whole pll.
You can refer to the book of the electronic communications techniques by Young.
Best wsihes
Hi Arash Pouryazdan ,
1) the theoretical bandwidth depends not only on the symbol frequency (and so on the bit rate) but also on other things like: the particular FSK waveform you want to use (i.e. CPFSK, MSK, GMSK, ...) , the shaping filter, etc
For example, if you use a CPFSK waveform with rectangular window, the bandwidth is roughly a little bit more the symbol frequency.
2) I don't know, probably yes. We have implemented it as digital electronics on FPGA devices from scratch.
3) The received signal will be affected by timing, frequency and phase errors that you have to recover if you want to demodulate the signal...it's the hard part of a demodulator design!
4) It depends on your target platform, if you want to implement only the PCB and use ICs or you want to implement the waveform modulator from scratch (like we do). You can use FPGA technology if you have digital design skills or with SDR paradigm if you have programming skills...there are a lot of ways to Rome :)
A very practical book is the one from RICE if you want to go inside the digital communication processing.
Hope this helps!
Angelo
Hi Arash Pouryazdan
As a complement to the previous answer, you may have a go to the following links which feature FSK Tx/Rx implementations based on GNU Radio (with a tutorial-like flow):
https://nccgroup.github.io/RFTM/
https://wiki.gnuradio.org/index.php/Simulation_example:_FSK
https://wirelesspi.com/fsk-demodulation-in-gnu-radio/
You will have to familiarize yourself with GNU Radio (software-defined radio -SDR framework), which I think is a good starting point for you.
Typically, in SDR you write code (e.g., C, Python) that is executed in a general purpose PC, which is interfaced (e.g., via GigE, USB, PCIe) with an SDR board featuring signal conversion ICs (ADC, DAC), together with up and down RF converters. The SDR board is used to transmit and receive the RF signals. GNU Radio has a big collection of developed DSP functions and running examples in a user friendly graphical environment (GNU Radio companion). Passing to an FPGA implementation requires more skills and time.
Good luck!
Nikos
Dear@Arash,
welcome
- In frequency shift keying you have to shift the carrier frequency by a frequency deviation df such that you have binary frequencies fm= fc+df and fs=fc-df.
Fm will represent the mark which is logic one and fs represent the logic zero.
So the number of transitions from the space to mark and vice verse will be equal to the bit rate fb= 128 kb/s.
According to the theory of frequency modulation the band width the binary FSK
can be expressed by
BW= 2 df +fb, so df is a parameter of the FSK
You can adjust it according to the bandwidth of your transmission channel.
If you have excess power you can reduce df to even equal to fb/2.
In this case the total bandwidth will be BW=2fb
As for the modulation and demodulation you can use the proper phase locked loop pll. In the transmitter you will use the voltage controlled oscillator and for the demodulator you can use the whole pll.
You can refer to the book of the electronic communications techniques by Young.
Best wsihes
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