GFSK Differences & Advantages over FSK Modulation
Original Post: Can anyone explain GFSK ?? (SIG
Forum) Date: 2000-05-03
The Bluetooth radio chip uses GFSK Modulation. It
is natural to assume that GFSK is quite like FSK Modulation, however the
differences between the 2 are substantial in implementation and results
(output)
Question 1
What are the physical Differences between an FSK &
GFSK Modulator, and how do their results vary?
Answer 1
An FSK Modulator is much the same as a GFSK Modulator
,but GFSK uses a Gaussian filter as well. In a GFSK modulator everything is
the same as a FSK modulator except that before the baseband pulses
(-1, 1) go into the FSK modulator, it is passed through a gaussian filter to
make the pulse smoother so to limit its spectral width.
Question 2
What is Gaussian Filtering?
Answer 2
Gaussian filtering is one of the very standard ways for
reducing the spectral width, it is called Pulse Shaping. If we use -1 for
fc-fd and 1 for fc+fd, once when we jump from -1 to 1 or 1 to -1, the
modulated waveform changes rapidly, which introduces large out-of-band spectrum.
If we change the pulse going from -1 to 1 as -1, -.98, -.93 ..... .96, .99,
1, and we use this smoother pulse to modulate the carrier, the out-of-band
spectrum will be reduced.
Question 3
So, why is GFSK implemented in Bluetooth
Radios , and not FSK , is it cheaper?
Answer 3
The spectral width for FSK is unlimited, comparatively,
there is a limitation on GFSK. fc "climbs slowly" to fd in
GFSK, However, in the case of FSK, fc "jumps sharply" to fd, which
greatly decreases spectral efficiency.
GFSK is not implemented in the Bluetooth radio unit for
cheaper chips (since you need to put an extra pulse-shaping filter in front
of the modulator), but for spectral efficiency.
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