Frequency shift keying (FSK) in detail
In this article, I will explain Frequency shift keying (FSK) in along with the waveform and frequency spectrum in detail. This article is easy to understand, clear and with all the information. Suitable for engineering and diploma students.
In a frequency shift keying of the digital information signal affects the frequency of a continuous time signal by switching the carrier frequency. The digital high signal is assigned a different fixed frequency a specific frequency and the digital low signal. Both characteristic frequencies are arranged symmetrically to the carrier frequency. The data stream, the bit sequence, provides the keying modulator for frequencies between the leads and the output to the FSK signal.
In the simplest variant of the binary FSK, two fixed frequencies are used. For each of the 4 FSK symbol frequency transmits two bits at a time, wherein one frequency is assigned to the dibit 00, 01, 10 or 11th The frequency shift is like the analog FM in the amplitude constant and insensitive to interference. The characteristic frequencies can be determined by counting the zero crossings at known intervals. Therefore, an error-free recovery of the information is possible even with severe disturbances of the amplitude of the received signal.
Examples
The oldest application is the wireless telegraphy. When faxing, fax is switched between two frequencies. Other applications are found in modem technology, the wireless multi-Rated, With DECT cordless phones and GSM, the Global System for Mobile Communications. In the long-and short-wave region using radio teletype.
Hard keying
The generation of FSK can be done using hard and soft keying. In a very simple method to directly on the digital information between two independent oscillators. The result is a discontinuous phase FSK. The Fourier analysis of such an FSK signal shows an undesirably high bandwidth requirements.
Continuous Phase Frequency Shift Keying - CPFSK
The following figure shows an FSK signal with continuous phase profile. The frequency-determining signal is generated either by switching on or off components in the carrier oscillator or the use of a voltage controlled oscillator, a VCO Voltage Controlled Oscillator. The bandwidth requirements for CPFSK and hard keying is lower because the only oscillator when switching relative to itself is synchronous and does not cause phase shifts.
The hard keying in the line chart shows the two characteristic frequencies. They are also known as mark and space frequency. Cord or power is often for the higher frequency. Theoretically, a superposition of two ASK signals produced by the on-off keying method, a result equal to looking result. The modulation or frequency information (pixel frequency) is the symmetrical 100 Hz square wave signal here. The two carrier frequencies are 1500 Hz and 2500 Hz A corresponding ASK-frequency spectrum by Fourier amplitude shift is shown.
As the following figure shows the Fourier spectrum of the CPFSK signal can be obtained from the superposition of the two spectra. The amplitudes of the spectral lines in the FSK signal to be greater at the characteristic frequency or carrier lines by superimposing. Mark and space are far enough apart so is missing in the spectrum of their arithmetic mean, which is equal to the center frequency of the unmodulated carrier.
The frequency deviation of the FSK DELTA.F is the distance to the center frequency of the characteristic frequencies. As with the analog frequency modulation modulation index n can be specified for the frequency shift. The ratio of the overall stroke · DELTA.F 2 based on the data (baud) rate, as well as the modulation index determines the product of the full stroke, and the time duration Ts of a symbol.
In the frequency range shown above applied, the modulation index is calculated as n = 5 The frequency is 500 Hz in the modulation frequency of 100 Hz, which is here equal to the frequency point, the step duration is 5 ms. The signal thus has a bit rate of 200 baud (bps).
If the distance of the characteristic frequencies decreases at the same bit rate as the modulation index decreases. The appearance of the spectrum changes as the side lines now grouped around a distinct line around the center frequency. The spectrum resembles that of an ASK signal. The minimum bandwidth of the FSK signal recorded from virtually all of the side lines of an amplitude of 10%. With soft keying can be further reduced and the bandwidth here.
The telegraph and telephone service uses the narrowband with a modulation index n = 1.2. With soft keying the same filters can be used as the ASK. The bandwidth of the FSK keying with soft is thereby calculated as the ASK by the equation B = 2.1, 6 · f p. Both G-Fourier spectra are shown offset from each other in the small graph for comparison.
Minimum Shift Keying - MSK
By further reducing bandwidth, more channels can be accommodated in the transmission band. At a modulation index n = 0.5 is called Minimum Shift Keying MSK. The method is known under the name FFSK or Fast Frequency Shift Keying. The following figure shows the timing diagram of MSK. The signal frequency is 100 Hz, which two characteristic frequencies 1950 Hz and 2050 Hz, the bit change is always from high to low in this phase position f = 90 °, and from low to high at f = 0 ° instead. Under certain conditions, could the MSK as phase shift keying with multiples of f = 90 ° view with equal frequency for high and low.
Gaussian Minimum Shift Keying - GMSK
Minimum Shift Keying is done with hard keying, remain in the output signal is still some side lines of higher order exist that may cause interference to neighboring channel end. Cycles through the digital signal prior to modulation of the baseband low pass filter, the steep signal flanks are smoothed. The modulated output signal, the higher frequency side lines are then suppressed. The step response of the filter then shows a bell-shaped amplitude profile and thus corresponds to a Gaussian function. The modulation method is known as Gaussian Minimum Shift Keying or GMSK. Gaussian filters are high-or low-pass filter whose step response does not overshoot occurs and the transition from the passage in the stop band is not very steep. This condition is met, a passive RC Pass 1 But ok, not steep enough. The method is for use in GSM.