What is FSK Modulation What does FSK modulation mean?

The principle of demodulation of the signal.

There are also two kinds of demodulation of FSK signals: incoherent and coherent, and FSK signals can be regarded as being transmitted alternately with two frequency sources, so the receiver of FSK consists of two ASK receivers connected in parallel.

1) Coherent demodulation.

Coherent demodulation is the use of multiplier, input a reference signal coherent with the carrier frequency multiplication and carrier frequency, through low-pass filtering, filter out the high-frequency signal, that is, the original signal, FSK after band-pass filtering, can be regarded as two ASK signals, the principle of coherent detector composition is as follows:

FSK coherent demodulation structure.

The picture above is an easy-to-implement FSK coherent demodulator, and an optimal FSK coherent demodulator is shown below

FSK optimal demodulation structure.

As can be seen from the figure, the waveforms of two known signals S1 (t) and S2 (t) should be generated at the receiving end, multiplied by the input waveform respectively, and then sent to the integrator, integrated in a certain time, at the time of t = tb, the integration result is sampled, and the judgment is compared in the comparator, and then output, the performance of the whole coherent demodulator is greatly affected by the carrier phase-locked loop and bit synchronization performance, and in the case of high speed, the integral, sampling and cleaning circuit is difficult to achieve, Therefore, the structure of the first coherent demodulator is usually used.

2) Incoherent demodulation.

Due to the relative difficulty of extracting coherent carriers from FSK signals, the incoherent demodulation method is mostly used in practical engineering applications, and the signal-to-noise ratio required for incoherent demodulation is only 1 2dB higher than that of coherent demodulation under the condition of the same bit error rate. There are many types of incoherent demodulation, such as demodulation method based on adaptive filtering, differential detection algorithm, AFC loop demodulation method, zero-crossing detection method, envelope detection method, etc.

Demodulation method based on adaptive filtering.

In adaptive demodulation, the more common method is to use the single-frequency adaptive notch in the adaptive filter for demodulation, which can provide easy-to-control bandwidth and extremely deep zero point, and has the ability to adaptively track the frequency and phase of the carrier, which is equivalent to an adaptive filter with a reset, and adjusts the amplitude and phase of the single-frequency sine wave at the same time with two real weights to track the amplitude and phase of the original input signal and eliminate interference. The following is the structure of the adaptive filter:

Adaptive filtering structure.

When the single-frequency tracking technology in adaptive filtering is applied to demodulation, it has the functions of both demodulation and phase locking, and the appropriate step size can achieve a small transmission delay and obtain excellent demodulation performance. Binary FSK signals have two modulation frequencies, and the energy is mainly concentrated in these two frequencies. Two adaptive filters (SFTs) are used to track the two frequencies.

FSK modulation (2FSK is binary digital frequency modulation), which uses the frequency of the carrier to transmit digital information, that is, the frequency of the carrier is controlled by the transmitted digital information.

The 2FSK signal is a modulated waveform with the symbol "0" corresponding to the carrier frequency F1 and the symbol "1" corresponding to the carrier frequency F2 (a different carrier frequency from F1), and the change between F1 and F2 is instantaneous.

When transmitting a "0" signal, a carrier with frequency f1 is transmitted; When transmitting a "1" signal, a carrier wave with frequency f2 is transmitted. It can be seen that FSK uses carriers of different frequencies to deliver digital messages.

FSK (Frequency-Shift Keying) - Frequency shift keying uses the frequency change of the carrier wave to transmit digital information. It is a digital modulation technology that uses the discrete value characteristics of baseband digital signals to key the carrier frequency to transmit information. FSK (frequency-shift keying) is an earlier modulation method used in information transmission, and its main advantages are:

It is easy to implement, and the anti-noise and anti-attenuation performance is better. It has been widely used in medium and low-speed data transmission. The most common is a dual-frequency FSK system that carries binary 1s and 0s with two frequencies.

There are two classifications of technically FSKs, incoherent and coherent FSKs. In the incoherent FSK, the transfer between instantaneous frequencies is two discrete values named Mark and spatial frequencies, respectively. On the other hand, in coherent frequency shift keying or binary FSK, there is no discontinuity in the output signal.

In the digital era, computer communication is transmitted on data lines (** wires, network cables, optical fibers or wireless media), which is carried out by FSK modulation signals, that is, the binary data is converted into FSK signal transmission, and in turn the received FSK signal is demodulated into binary data, and it is converted into a binary language represented by high and low levels, which is a language that the computer can directly recognize.

Answer: There are three ways to achieve FSK modulation

1) Adopt linbo3

waveguide modulators;

2) Adopt acousto-optic modulator;

3) Modulation in the diode laser skin disassembler.

In binary frequency shift keying, the frequency of a carrier signal with constant amplitude switches as the input stream changes (called treble and bass, representing the binary 1s and 0s).

The easiest way to generate an FSK signal is to switch between two independent oscillators depending on whether the input data bit is 0 or 1. The waveform produced by this method is discontinuous in phase at the moment of switching, so this FSK signal is called a discontinuous FSK signal.

Due to the spectrum spread caused by phase discontinuity, this FSK modulation method is more commonly used in traditional communication equipment. With the continuous development of digital processing technology, successive phase FSK modulation technology is increasingly adopted.

At present, the most commonly used method of generating FSK signals is to first generate the FSK baseband signal, and use the baseband signal to modulate the frequency of a single carrier oscillator.

The power spectral density function of a phase-continuous FSK signal eventually fades to the negative fourth power of the frequency offset. If the phase is discontinuous, the power spectral density function fades by the negative power of the frequency offset.

In the integrated experimental system of communication principles, the modulation scheme of FSK is as follows:

FSK signal: S(t)=cos( 0T+2 Fi·t).

In the baseband signal of the communication channel FSK mode, the FH frequency is used for the transmission signal, and the FL frequency is used for the empty signal. In FSK mode, Hamming error-correcting codec technology is not used. The data sources provided by the modulator are:

1. External data input: can come from synchronous data interface, asynchronous data interface and M sequence;

2. All 1 code: the sending frequency (high) when the number can be tested;

3. All 0 codes: the sending frequency (low) when the empty number can be tested;

1 code: 0101 alternate pattern for general testing;

5. Special code sequence: code sequence with period 7 to facilitate observation by conventional oscilloscope;

6. M sequence: used to test the channel performance;

FSK is an earlier modulation method used in information transmission, and its main advantages are: it is easy to implement, and the anti-noise and anti-attenuation performance is better. It has been widely used in medium and low-speed data transmission.

FSK is the use of digital signals to modulate the frequency of the carrier. If a binary modulated signal is used, it is called 2FSK; Multi-modulated signals are called MFSKs.

l Modulation method: 2FSK can be regarded as the sum of two ASK modulated signals with different carrier frequencies.

l Demodulation method: coherent method and incoherent method.

l Type: Binary Frequency Shift Keying (2FSK), Multi-Component Frequency Shift Keying (MFSK).

In addition to the above three basic modulation methods, with the development of large-capacity and long-distance digital communication technology, some new problems have emerged, mainly the bandwidth limitation of the channel and the influence of nonlinearity on the transmitted signal. In this case, the traditional digital modulation mode can no longer meet the needs of the application, and a new digital modulation mode is required to reduce the influence of the channel on the transmitted signal, so as to obtain a higher transmission rate under the condition of limited bandwidth resources. The research of these technologies mainly focuses on the full saving of spectrum and the efficient use of frequency bands.

Multi-system modulation is an effective way to improve spectrum utilization, and the constant envelope technology can adapt to the nonlinearity of the channel and maintain a small spectrum occupancy.

Technologies that extend from traditional digital modulation techniques include Minimum Frequency Shift Keying (MSK), Gaussian Filter Minimum Frequency Shift Keying (GMSK), Quadrature Amplitude Modulation (QAM), Orthogonal Frequency Division Multiplexing (OFDM), and so on.