How to implement FSK modulation?

[Alexandros81]

Hi!

I have an FTDI cable that outputs TTL signal when connected to PC. I would like to use that binary signal to do
FSK modulation. When binary 1 a 15KHz sinewave as an output from modulator and when binary 0 5Khz sinewave from modulator.

There should be continuity between 5KHz and 15KHz signals.

Does anyone can point me to the right direction meaning to suggest theoretical background, circuit diagram for modulator, IC's etc.

I would like also to simulate that.

Thanks

 

[sonic012]

How to do FSK is a pretty general question. My opinion you should back out of your TTL solution right off the bat. Implement the FSK modulation in DSP and then use a D/A converter. It's low frequency, and it's dead easy.

Either way, the signal processing steps are the same. By continuity I assume you mean smooth transitions between FSK values (not abrupt step transitions). So you take your binary signal and you low pass filter it and then you use that to drive an NCO. Then you convert that to analog through a D/A with an appropriate reconstruction filter. That's it.

 

[Alexandros81]

How to do FSK is a pretty general question. My opinion you should back out of your TTL solution right off the bat. Implement the FSK modulation in DSP and then use a D/A converter. It's low frequency, and it's dead easy.

Either way, the signal processing steps are the same. By continuity I assume you mean smooth transitions between FSK values (not abrupt step transitions). So you take your binary signal and you low pass filter it and then you use that to drive an NCO. Then you convert that to analog through a D/A with an appropriate reconstruction filter. That's it.

Can you point me to the right direction for selecting a DSP chip? Can you suggest one? Is it a dsPIC30F by microchip a DSP chip? Also
can you suggest a D/A converter?
By NCO you mean "numerically controlled oscillator"?

- - - Updated - - -

By doing a quick search I found that microchip has some microcontrollers that have an NCO.
http://www.microchip.com/pagehandler/en-us/family/8bit/peripherals/coreindependent/nco-prdpge.html

Also I found the following VCO implementation:
http://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en560172

DAC:
http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP4725

 

[Alexandros81]

By doing a quick search I found that microchip has some microcontrollers that have an NCO.
**broken link removed**

Also I found the following VCO implementation:
https://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en560172

DAC:
https://www.microchip.com/wwwproducts/Devices.aspx?product=MCP4725

 

[FvM]

The shown NCO generates pulses respectively square waves, a sine generator has to be programmed in software. It won't need specific hardware, except for a DAC to output the sine. 15 kHz can be possibly made with a pwm pin and a low-pass filter without a DAC. So if you go for a microcontroller solution, everything is about writing respective software.

 

[Alexandros81]

Ok thanks. So to understand there are two ways to do FSK
1. NCO with a DAC
2. PWM pin with low pass filter

In the long run I am aiming in designing a 35 MHz transmitter (RC toys frequency). So what I am thinking is to mix the FSK signal with
a higher frequency say 455KHz and then do another mixing with 35MHz. Maybe use some filtering. Finally boost the signal if needed with an amp and transmit via an antenna. Any thoughts, tips, ideas are welcome.

 

[FvM]

An usual modulation scheme uses FSK modulation of the carrier rather than mixing a FSK baseband signal which would generate an AM signal unless you have a SSB mixer.

These days a FSK transmitter is a single chip device from Hope RF, TI, ADI, Siliconlabs or others, may be even integrated with a microcontroller.