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This Java applet allows you to experience the basics of FSK/MSK modulation and demodulation visually.
It’s interesting to look at the spectrogram and try changing the various parameters. You can understand the background to the characteristics of the signal.

To run the applet, click the picture. *Java Runtime Environment version 5.0 or higher is required to run the applet. Download:http://www.java.com/en/download/index.jsp

An outline of digital frequency modulation and demodulation

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With digital frequency modulation, the modulating signal is a digital signal, and it’s given the name FSK (frequency shift keying).

FSK radio offers stable communication that is resistant to noise and since it can be achieved at relatively low cost, it’s often used for specified low-power radio. FSK is digital modulation, but the theory is similar to analog frequency modulation, and the modulating signal is digital instead of analog. In addition, in a normal radio use environment, the receiving wave undergoes amplitude and phase fluctuations due to multipath factors, but the frequency modulation system is resistant to related errors.

Besides the common binary FSK, FSK includes multi-level modulation such as quaternary FSK. In addition, of the types of FSK, MSK (Minimum Shift Keying) which has a modulation index of 0.5 is particularly resistant to noise and can be used for long distance communication. 

The theory of digital frequency modulation and demodulation

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◆FSK modulation and demodulation
FSK modulation assigns different frequencies to each information signal status. On the receiving side it is passed through a circuit that determines differences in the frequency of the received signal and obtains the original information signal.
FSK uses a modulator to shift the frequency of the carrier wave proportionally to the level of the digital signal, which is the information signal. However, a digital signal has only two statuses, 0 and 1, and so a high frequency F1 is assigned to signal 1 and a low frequency F0 is assigned to signal 0 in relation to the center frequency Fc of the carrier wave.

The figure below is the system for switching the transmitter according to the modulating signal level. If the switching timing in the synchronization and modulating signal of the two oscillators is not good, the continuity of the phase between bits cannot be maintained as shown in the figure, resulting in an unnecessary spectrum that is not in fact used. The out-of-band unnecessary spectrum interferes with adjacent channels, and this spectrum is called a spurious emission. Spurious emission are undesirable from the point of view of effective utilization of radio waves and they are regulated by the Radio Act.

In many cases, FSK modulation uses a VCO (Voltage Controlled Oscillator). The VCO changes the frequency proportionally to the voltage of the modulating signal so that the phase between bits is continuous. CPFSK (Continuous Phase FSK) is characterized by its low levels of unwanted emissions (spurious emissions) which cause various adverse effects.

◇Modulation index
The relational expression for FSK modulation index m is as follows.

Modulation index: m, Symbol rate: Sr, Symbol cycle: T, Frequency variation: Df, Carrier frequency: Fc
Frequency 0: F0, Frequency 1: F1, Deviation (maximum frequency shift): Δf

The modulation index of specified low-power radios is normally about 0.8, but with MSK demodulation with a modulation index of 0.5, synchronous detection is possible making it advantageous from the point of view of error rate.
The MSK spectrum is not very wide and is concentrated around the carrier frequency. For this reason the frequency band of the circuit filter can be narrow, with good signal to noise.

◇FSK constellation?
FSK changes the frequency in relation to the signal status so it cannot normally be expressed as a constellation. However, if the signal point is moved to the circumference and the phase is made changeable as shown in the figure below, the frequency becomes higher with counterclockwise rotation and lower with clockwise rotation, enabling the frequency changes of the FSK modulated waveform to be expressed as an image.

Click the image: Frequency modulated wave spectrum applet

◆MSK modulation and demodulation
MSK changes the phase 90 degrees (π/2) in relation to the center frequency Fc of the carrier wave with each symbol, so that it can be shown as in the figure below without being expressed as a constellation.
MSK modulation using I and Q signals uses changes when the signal point is moved to the circumference of the constellation.

The changes and phase shift for the MSK information signal are as follows.

◆Sub-carrier MSK
There are various method modulation methods for wireless, but sub-carrier MSK modulation is widely used for specified low-power radio.
As shown in the figure below, FM modulation and demodulation is frequently used in sub-carrier MSK radio components. FM modulation and demodulation has been used for a long time and the modulation circuits are well-established. They are cheap and have a low error rate.

MSK modulation is one type of FSK modulation, but it occupies even less bandwidth than FSK modulation so the bandwidth is interrupted by very little noise. The fact that noise is low means that it has a high carrier-to-noise ratio and good receiver sensitivity which gives it a long range with stable communication. The modulation index of sub-carrier MSK modulation is MSK modulation, so it’s m = 0.5.

The bit rate for sub-carrier MSK for specified low-power radio is typically 1,200 bps or 2,400 bps. Although you might think this speed is extremely low, by a rough calculation 150 bytes of data can be sent in one second which is sufficient for many fields such as telecontrol and telemetry. If the data transmission rate is low, naturally the error rate is lower too. This is because, if the transmission rate is low, the transmission band can be narrow which means that there is less noise to impact demodulation. This also becomes clear from the relational expression for modulation signal frequency and required bandwidth explained in the section on frequency modulation.

However, this only relates to the MSK modulation and MSK demodulation units in the block diagram above, and not to the bandwidth of the FM modulation and demodulation units. For the frequency modulation and demodulation units, the signal of the MSK unit signal is the base band. Naturally, the radio waves are FM radio waves, and the bandwidth and modulation index of these radio waves are specified separately in relation to the MSK input signal.

Many people will tend to associate MSK modulation with GMSK mobile phones (GSM) in Europe and elsewhere, and in fact, the MSK aspect is the same. However the bandwidth required for MSK is not necessarily narrow, and with the aim of using radio effectively, GMSK applies a Gaussian filter to the square wave modulation data before performing MSK modulation. However, as a result, the orthogonality of MSK is lost, resulting in many errors. In addition, GMSK performs highly precise digital modulation of the base band signal such as quadrature modulation and then the signal is up-converted to radio frequency. There are also GMSK modulation radios for amateur radio. But types which control VCO with the base band have problems with modulation accuracy and don’t perform synchronous detection, so the benefits of MSK are lost.

The figure below shows the waveform of sub-carrier MSK
The center frequency of the carrier is 1,500 Hz and the deviation is ±300 Hz. Therefore, the data 1 (mark) frequency is 1,200 Hz and data 0 frequency (space) is 1,800 Hz. The bit phase is offset by 90 degrees in relation to the respective carrier frequency.

FSK/MSK modulation and demodulation applet

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A FSK/MSK modulation and demodulation applet
Click the “Explain” button at the bottom right of the applet for how to use it.

Click the image: FSK/MSK modulation and demodulation applet

Sub-carrier MSK modulation and demodulation applet

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A sub-carrier MSK modulation and demodulation Java applet
Click the “Explain” button at the bottom right of the applet for how to use it.

Click the image: Sub-carrier MSK modulation and demodulation (wireless frequency modulation) applet

MSK/FSK modulation and demodulation applet using an IQ signal

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A Java applet for FSK/MSK modulation and demodulation using an IQ signal
Click the “Explain” button at the bottom right of the applet for how to use it.

Click the image: MSK/FSK modulation and demodulation applet using an IQ signal