Wednesday, August 22, 2012

DIGITAL MODES: Part 2 (FSK)


Let me stop and explain "symbols" for a moment. In ASK a symbol might just be a dot or dash in Morse code. Other digital modes use multiple audio tones for each symbol being transmitted. This is usually explained in documentation by appending a number to the mode acronym. MFSK16 for example uses 16 tones for each symbol.   Not that's the only implementation of MFSK, there is also MFSK8 and so on. 

So if ASK is Amplitude Shift Keying you probably already guessed that FSK is Frequency Shift Keying. In FSK, the center frequency of the carrier wave alternates between multiple values. A common application of this would be BFSK (Binary Frequency Shift Keying.) In BFSK only two frequencies are used. In this scheme, the 0 and the 1 are called the space frequency and the mark frequency respectively. More here.

AFSK (Audio Frequency Shift Keying) is another early FSK-type digital mode. In this scheme the data is represented by changes in the frequency of an audio tone. AFSK differs from regular frequency-shift keying in performing the modulation at baseband frequencies. In radio applications, the AFSK-modulated signal normally is being used to modulate an RF carrier (using a conventional technique, such as AM or FM) for transmission.AFSK differs from BFSK in that AFSK is modulating baseband frequencies, BFSK is modulating a carrier wave. It can send and receive data up to1,200 baud.  It is also used in the the Emergency Alert System and by NOAA

The problem with MFSK is that the acronym stands for two different types of MFSK, Minimum Frequency Shift Keying and Multiple Frequency Shift Keying. Multiple Frequency Shift Keying is just FSK using more than two frequencies. The most common application of it is Dual-Tone Multi-Frequency (DTMF) also known as touch tone dialing. The other MFSK, (Minimum Frequency Shift Keying) is also known asMSK is (Minimum Shift Keying) yet another application of FSK. MSK is a more efficient form of FSK. MSK encodes each bit as a half sinusoid and thus the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. More here.

GMSK (Gaussian Minimum Shift Keying) is based on MSK.Thsi is a form of continuous-phase frequency-shift keying modulation. the data stream is first shaped with a Gaussian Filter.  A Gaussian filter modifies an input signal by convolution with a Gaussian function (named after Carl Friedrich Gauss) these apply an exponential function to a general quadratic function, in this application the result looks like a bellcurve because it reduces sideband power. More here.   If you think that's complicated try wrapping your head around PDGMSK (Pulse Driven Gaussian Minimum Shift Keying) More here.  This modification to GMSK sort of hurts my brain.

Even more complex is CPFSK, Continuous Phase Frequency Shift Keying.  In this application of FSK the carrier phase abruptly resets to zero at the start of every "symbol." In normal FSK at the end of each "symbol" the phase of the carrier wave changes because it's signals are generated by separate oscillators that aren't phase-synchronized. CPFSK changes its frequency continuously but the phase is a constant. It maintains a constant amplitude signal and has very narrow bandwidth requirements. DCPFSK stands for Differential Continuous Phase Frequency Shift Keying and please don't ask me to explain it. I stopped reading when I encountered the word "phase trellis." More here and here. I'll get into more phase related shifting in part 3.


VOCABULARY TERMS
  • FSK - Frequency Shift Keying
  • BFSK- Binary Frequency Shift Keying
  • DTMF - Dual-Tone Multi-Frequency
  • AFSK - Audio Frequency Shift Keying
  • MSK - Minimum Shift Keying
  • MFSK - Multiple Frequency Shift Keying
  • PDGMSK - Pulse Driven Gaussian Minimum Shift Keying
  • DCPFSK -  Differential Continuous Phase Frequency Shift Keying
  • GMSK - Gaussian Minimum Shift Keying
  • CPFSK- Continuous-phase frequency-shift keying