調頻
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調頻(FM,可看作调幅PM的一种特殊形式)是一种将信息加在载波瞬时频率上的调制方式。(与此不同的调制方式是调幅,这种调制方式是将信息加在载波的幅度上。) 在模拟的应用中,载波的瞬时频率跟随调制信号的幅度连续变化。在数字应用领域，载波的瞬时频率则 根据数据序列的值作离散跳变,即所谓的频键控。
调频技术通常运用在甚高频段VHF无线电上的高保真无线电音乐和语音的传送(见调频广播)。普通的(模拟)电视的音频信号也是通过调频传递。窄带形式的调频广播(N-FM)限于商业上的声音通讯和业余无线电领域，广播中使用的调频技术则一般称为宽带调频(W-FM)。
调频技术还用于大多数的模拟VCR，包括家庭视频系统VHS，用于记录视频信号的亮度(黑和白)信息，不过是在中频段使用。调频是用于录取视频磁带时唯一不造成大的信号走样的调制技术，因为视频信 息的所包含的频谱范围很广，从几个赫兹到几十兆赫，同均衡器工作时很难将噪声信息保持在-60分贝以下。调频也时磁带处于饱和状态，其到降噪的作用，同时接收端的调频捕获效应基本消除了print-through和pre-echo等现象。如果在信号上加上一个连续的pilot-tone，就像在V2000以及许多Hi-band 格式上作的那样，机械jitter可以得到有效的控制，从而有助于timebase correction。
调频技术还应用在音频的合成上，即所谓的调频合成，在早期的数字合成器上应用很普遍，并成为几代个人电脑声卡的标准特徽。
目录 |
[编辑] Applications in radio
Edwin Armstrong presented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935.
Wideband FM (W-FM) requires a wider bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio" (although for many years the BBC insisted on calling it "VHF radio", which is quite logical, since commercial FM broadcasting uses a well-known part of the VHF band; in certain countries, expressions referencing the more familiar wavelength notion are still used in place of the somewhat mysterious modulation technique name).
FM receivers inherently exhibit a phenomenon called capture, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically, however, frequency drift or lack of selectivity may cause one station or signal to be suddenly overtaken by another on an adjacent channel. Frequency drift typically constituted a problem on very old or inexpensive receivers, while inadequate selectivity may plague any tuner.
An FM signal can also be used to carry a stereo signal: see FM stereo. However, this is done by using multiplexing and demultiplexing before and after the FM process, and is not part of FM proper. The rest of this article ignores the stereo multiplexing and demultiplexing process used in "stereo FM", and concentrates on the FM modulation and demodulation process, which is identical in stereo and mono processes.
This is a very important part of the frequency since it provides a way to continue with the correct signal on the output side.
[编辑] Theory
若欲傳送信號為
其振幅限制不可大於一
載波為
fc為載波中心頻率，單位為赫茲。A是任意振幅。傳送信號將會是
![x_c(t) = A \cos \left( 2 \pi \int_{0}^{t} f(\tau)\, d \tau \right) = A \cos \left( 2 \pi \int_{0}^{t} \left[ f_c + f_\Delta x_m(\tau) \right] \, d \tau \right)](../../../math/2/5/e/25efb1773d36757c43fa3c96d896ce23.png)
- 在此，f(t) = fc + fΔxm(t)
公式中，f(t)是振荡器的瞬时频率，fΔ是frequency deviation, 代表在一个方向上相对fc的最大频率偏离，在此我们假定xm(t)是有限的幅值限于±1之间。
Although it may seem that this limits the frequencies in use to fc ± fΔ, this neglects the distinction between instantaneous frequency and spectral frequency. The frequency spectrum of an actual FM signal has components extending out to infinite frequency, although they become negligibly small beyond a point.
For a simplified case, the harmonic distribution of a sine wave signal modulated by another sine wave signal can be represented with Bessel functions - this provides a basis for a mathematical understanding of frequency modulation in the frequency domain.
A rule of thumb, Carson's rule states that nearly all the power of a frequency modulated signal lies within a bandwidth of
where fΔ is the peak deviation of the instantaneous frequency f(t) from the center carrier frequency fc (assuming xm(t) is in the range ±1) and fm is the highest modulating frequency of xm(t).
Note that frequency modulation can be regarded as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal.
Frequency-shift keying refers to the simple case of frequency modulation by a simple signal with only discrete states, such as in Morse code or radio-teletype applications.
Manchester encoding may be regarded as a simple version of frequency shift keying, where the high and low frequencies are respectively double and the same as the bit rate, and the bit transitions are synchronous with carrier transitions.
When used in supervisory signaling in telephony, the term frequency-change signaling has been used to describe frequency modulation.
The phrase frequency-modulated, an adjective, should have a hyphen when used attributively.
[编辑] Modulation index
As with other modulation indices, in FM this quantity indicates by how much the modulated variable varies around its unmodulated level. For FM, it relates to the variations in the frequency of the carrier signal:
With a tone-modulated FM wave, if the modulation frequency is held constant and the modulation index is increased, the (non-negligible) bandwidth of the FM signal increases, but the spacing between spectra stays the same.
If the frequency deviation is held constant and the modulation index increased, the bandwidth stays roughly the same, but the spacing between spectra decreases.
[编辑] 参看
- 調幅
- Carson bandwidth rule (Estimate of RF bandwidth required for an FM signal)
- Frequency modulation synthesis (FM as an audio synthesis method)
- Modulation index
- Modulation, for a list of other modulation techniques
- History of radio
