Frequency modulation, full name of "frequency modulation".The modulation method that makes the instantaneous frequency of the carrier wave change according to the change law of the signal to be transmitted.[1]It is a modulation method that makes the instantaneous frequency of the modulated wave change with the modulated signal.The circuit to realize this modulation method is called frequency modulator, which is widely used in FM broadcasting, TV audio, microwave communication, phase lock circuit, frequency sweeper, etc.The basic requirements of the frequency modulator are large frequency shift, good frequency modulation characteristics and small parasitic amplitude modulation.[2]The radio wave generated by the frequency modulation method is called the frequency modulation wave. Its basic feature is that the amplitude of oscillation of the carrier wave remains unchanged, and the oscillation frequency changes with the modulation signal.Frequency modulation (FM) means that the frequency of the high-frequency carrier is not a constant, but a modulation mode that changes with the modulation signal in a certain range, and its amplitude is a constant.Correspondingly, amplitude modulation means that the frequency of the carrier frequency is constant, and its amplitude varies with the modulation signal.
Change the carrier frequency according to the modulation signalmodulation mode It's called FM.The magnitude of the modulated frequency change is determined by the size of the modulation signal, and the change period is determined by the frequency of the modulation signal.The amplitude of the modulated wave remains unchanged.The waveform of FM wave is like a spring that is compressed unevenly. FM wave is represented by the English letter FM.
A modulation in which the instantaneous frequency of the carrier wave varies according to the modulation signal, but the amplitude does not change.The carrier wave becomes FM wave after frequency modulation.Using FM wave to transmit signal can avoid the influence of amplitude interference and improve communication quality.Widely used in communicationsFM stereo broadcastingAnd TV.Generally, jamming signals are always superimposed onsignalChange its amplitude.thereforeFrequency modulated waveAlthough the amplitude will change after being interfered, the amplitude change of the signal can be cut off at the receiving end with a limiter, so the anti-interference performance of FM wave is excellent.For example, receive with radioFM radio, basically no noise can be heard.
We usually use FM to refer to general FM broadcasting (76-108MHz, 87-108MHz in ChinaJapan76-90MHz). In fact, FM is also a modulation modeshortwaveThe range of 27-30MHz is used as the band for amateur radio, space, and satellite communication applications, and also uses frequency modulation (FM).FM radio is FM radio.
FM is the radio function.As an additional function of MP3.Of course, if you pay attention to this function, there are also products that do well.For specific models, different products have their own segmentation for FM. Whether the selected channels can be saved, how many channels can be saved, stereo and ordinary channels can be set by themselves or by the machine.
classification
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Primary frequency modulation
Primary frequency regulation refers to the ability inherent in the frequency characteristics of the speed regulating system of the generator set to automatically adjust the frequency with the change of frequency.It is characterized by fast frequency adjustment speed, but the adjustment amount varies with different generator sets, and the adjustment amount is limited, which is difficult for the dispatcher on duty to control.
Secondary frequency modulation
Secondary frequency modulation refers to the frequency modulation mode adopted when the primary frequency modulation cannot recover to the specified range when the load or generation output of the power system changes greatly.Secondary frequency modulation is divided into manual frequency modulation and automatic frequency modulation:
Manual frequency regulation: in the frequency regulation plant, the operator adjusts the output of the generator according to the change of the system frequency to keep the frequency within the specified range. Manual frequency regulation is characterized by slow speed. When the adjustment range is large, it often fails to meet the requirements of frequency quality. At the same time, the operator on duty operates frequently and has high labor intensity.
Automatic frequency modulation: This is the frequency modulation mode adopted by the modern power system. Automatic frequency modulation is to automatically increase or decrease the generator output with the change of system frequency through the automatic device installed in the power plant and the dispatching center, so as to keep the system frequency within a small range. Automatic frequency modulation isPower system dispatching automationIt has integrated functions of frequency modulation, inter system tie line exchange power control, and economic dispatching.
Difference between FM and AM
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(1) FM is more anti-interference than AM:
The influence of various external interference, processing industry and sky electricity interference on the modulated wave is mainly manifested as parasitic amplitude modulation and noise.FM system can eliminate parasitic AM caused by interference by limiting amplitude.In the AM system, the amplitude of the amplitude modulated signal is variable, so amplitude limiting cannot be used, and it is difficult to eliminate external interference.
In addition, the greater the signal to noise ratio, the stronger the anti-interference ability.The signal to noise ratio of the demodulated signal is related to the modulation coefficient. The larger the modulation coefficient, the greater the signal to noise ratio.Because the FM coefficient is far greater than the AM coefficient, the signal to noise ratio of FM wave is high and the interference noise in FM broadcasting is small.
(2) Frequency bandwidth of FM wave over AM wave:
The bandwidth is related to the modulation coefficient, that is, the modulation coefficient is large and the bandwidth is wide.In frequency modulation, the frequency modulation coefficient is usually greater than 1, while the amplitude modulation coefficient is less than 1, so the frequency band width of the frequency modulation wave is much larger than that of the amplitude modulation wave.
(3) The power utilization ratio of FM system is greater than that of AM system:
In the total transmission power, the side frequency power is the effective power to transmit the modulated signal, and the side frequency power is related to the modulation coefficient. The modulation coefficient is large, and the side frequency power is large.Due to frequency modulation coefficient mfGreater than amplitude modulation coefficient maTherefore, the power utilization ratio of FM is higher than that of AM.
FM synthesis technology
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Frequency modulation (FM) is one of the most effective synthesis technologies in electronic music synthesis technology. It was first developed by the United StatesStanford UniversityJohn.Dr. John Chowning proposed.In the 1960s, Zhuo Ning began to taste different types of trills at Stanford University. He found that when the frequency of the modulated signal increased beyond a certain point, the trill effect disappeared in the modulated sound, and a new and more complex sound replaced it.
Today, Zhuo Ning was just finishingradioThe most commonly used FM technology in radio transmission (i.e. FM broadcasting).However, Zhuo Ning's accidental discovery has given this traditional FM technology a new role in sound synthesis.When Zhuo Ning understood the basic principle of FM modulation, he immediately began to study the technology of FM theoretical synthesis, and became the first person to use FM technology to produce music in 1966.
Fundamentals
If you want to transmit a signal, its amplitude limit cannot be greater than, and the carrier is,
frequency modulation
frequency modulation
fcIs the carrier center frequency, in Hz.A is any amplitude.
The transmission signal will be
frequency modulation
Here, f (t)=fc+f Δ xm (t)
In the formula,f(t)It's an oscillatorinstantaneous frequency，fΔyesfrequency deviation, representing relative in one directionfcMaximum frequency deviation of, here we assumexm(t)The limited amplitude is limited to ± 1.
audio frequencyThe change of signal is often periodic, one of which is easiest to understand audiomodulation technique The example of is violin and string rubbing, which vibrates rapidly on the string through fingers and wrists, making the string length change rapidly, thus ultimately affecting the softness of violin sound.And "FMradio waves”In the same way, "FM synthesis theory" also has two elements: pronunciation (carrier) and modulation.The pronunciation or carrier body is the frequency oscillator that actually produces sound;The modulator, or modulator, is responsible for adjusting the sound generated by changing the carrier wave.Carrier frequency, modulator frequency and modulation value are important factors affecting FM synthesis theory.
The most basic FM instrument includes two sinusoidal oscillators, one is a stable carrier frequency fc (Carrier Frequncy) oscillator;One is the modulation frequency fm (Modulation Frequency) oscillator.The carrier frequency is added to the output of the modulation oscillator.The carrier oscillator is a simple sine wave frequency with fc frequency. When the modulator occurs, the signal from the modulation oscillator, that is, the sine wave with fm frequency, drives the frequency of the carrier oscillator up or down. For example, a 250Hz sine wave modulation wave modulates a 1000Hz sine wave carrier, which means the 1000Hz frequency generated by the carrier wave,The modulation generated by the influence shall be accepted 250 times per second.Both the system body and the carrier body are periodic or quasi periodic oscillators with frequency, amplitude and waveform.
In frequency modulation technology, the amplitude of the modulator also plays a key role in frequency modulation. The amplitude of the modulator affects the depth of change after carrier frequency modulation. If the amplitude of the modulation signal is 0, no modulation will occur.Therefore, just as in amplitude modulation (AM), the frequency of the modulator has an impact on the amplitude of the carrier, in frequency modulation (FM), the frequency change of the carrier is also affected by the amplitude change of the modulator.
Therefore, in the process of frequency modulation, we can find that:
one
The frequency of the modulator affects the speed change of the frequency of the carrier.
two
The amplitude of the modulator affects the depth change of the carrier frequency.
three
The waveform (or voice) of the modulator affects the waveform change of the carrier frequency.
four
The amplitude of the carrier body remains unchanged during frequency modulation.
Calculation of frequency spectrum in FM
In simple frequency modulation, both oscillators only use sine wave.However, because frequency modulation technology can produce a very rich spectrum, which makes it unnecessary for composers to complete FM synthesis with waveform with too complex spectrum.In fact, if a waveform with very rich spectrum components is used as a modulator to modulate another sound (carrier), the modulated spectrum will be extremely complex, so that it sounds very rough and harsh.
There are some frequency spectrums on either side of the carrier frequency, and the spacing distance is consistent with the modulated frequency.These upper and lower side frequencies are combined in pairs according to the overtone number of the modulation frequency (fm).In mathematical language, a simple FM spectrum displays the frequency fc ± kfm. k as an integer, which can be assumed to be any value greater than or equal to 0, and the carrier component is displayed by k=0.
The energy distribution in the spectrum composition is partially affected by the amount of frequency deviation.This deviation (abbreviated as d) is generated by the modulation oscillator.When d=0, no modulation occurs.Increasing the deviation index will generate side frequency, which will gain more energy, but at the expense of the energy of the carrier frequency.The greater the deviation, the wider the energy distributed between the side frequencies, which will lead to greater side frequencies with amplitude changes.Therefore, deviation can play the role of controlling the side frequency of FM signal spectrum.
If the input carrier is 1000Hz and the modulator is 250Hz, then according to the FM spectrum allocation calculation principle, the final output frequency value after frequency modulation should be as shown in Figure 13-7.The amplitude of each spectral component is determined by the deviation index and the modulation frequency.
The effect of frequency modulation is sometimes similar to that of additive synthesis. The essential difference between the two is that additive synthesis adds harmonic divider to the basic waveform, layer by layer. The basic waveform and harmonic divider coexist, while the waveform added by FM synthesis completely modulates its basic waveform to produce another very complex waveform.Therefore, frequency modulation technology and additive synthesis technology are totally different.
Compound frequency modulation
Composite frequency modulation includes two or more carrier body oscillators and two or more modulation body oscillators, which can generate more side frequencies and increase the computational complexity.There are many possibilities for the combination of composite frequency modulation, and each combination will bring a unique frequency combination effect.
In general, there are at least five basic combinations of composite frequency modulation.
1、 Multiple carrier combinations with independent modulators - Additive carriers with one modulator
This combination includes two or more simple FM instruments working at the same time, and the effect obtained is the sum of the output of each FM instrument (in the legend symbol abbreviation, al represents the amplitude of carrier 1, fl represents the frequency of carrier 1, d1 represents the frequency offset of modulator 1, that is, the amplitude of modulator 1, f1 represents the frequency of modulator 1,A represents the carrier amplitude, and other graphic abbreviations are also identified according to this).
2、 Additive carriers with one modulator
The effect obtained is the sum of each carrier output.
3、 Single carrier with parallel modulators
4、 Single carrier with serial modulators
5、 Self modulating carrier
The so-called self modulated carrier wave is to modulate its own frequency with the output of the signal oscillator.The output signal of the oscillator is multiplied by a feedback factor (expressed in fb), and a frequency value (fm) is added before being re input to its own frequency input.The feedback factor (expressed in fb) can be regarded as a modulation index here.
Since the self modulation processing technology always works in a 1:1 frequency ratio, it always generates sawtooth waveform.The amplitude of harmonic components varies in proportion to the value of the feedback factor (fb).
