频率调制
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調頻（frequency modulation，缩写：FM）是一种以载波的瞬时频率变化来表示信息的调制方式。（与此相对应的调幅方式是通过载波幅度的变化来表示信息，而其频率却保持不变。）在模拟应用中，载波的频率跟随输入信号的幅度直接成等比例变化。在数字应用领域，载波的频率则根据数据序列的值作离散跳变，即所谓的频率键控。
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（参见调频广播）。普通的（模拟）电视的音频信号也是通过调频方式传递。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。调频方式也使磁带处于饱和状态，起到降噪的作用，同时接收端的调频捕获效应基本消除了透印和前回声等现象。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] 无线电中的应用
Edwin Armstrong于1935年11月6日在无线电工程师学会纽约分部发表了一篇名为《一个通过频率调制系统降低无线电信号干扰的方法》的文章，第一次描述了调频无线电。
宽带调频（W-FM）和调幅相比，在同样的调制信号作用下，宽带调频需要更宽的带宽。但是这也使信号具有更强的抗噪声和干扰能力。调频还具有较强的抗简单信号振幅衰减能力（simple signal amplitude fading phenomena）。因此，调频被选做高频、高保真无线电传输的调制标准。
调频接收机固有的一个现象叫做“捕获”，即调谐器能够清晰地接收到两个同频率广播电台中的较强者。然而，随之而来的问题是：频率漂移或选择性差可能会导致一个电台或信号突然被另一个毗邻频道的压制。频率漂移只是对非常老式的或廉价的接收机来说是个问题，而选择性差则可能给所有调谐器都带来了困扰。
调频信号也能用于搭载立体声信号，参见调频立体声。然而，这是在频率调制过程之前和之后，通过使用多路复用技术和去多路复用技术来完成的，而不是作为频率调制过程的一部分。本文余下的部分将忽略调频立体声中使用的立体声多路复用和去多路复用过程，而是集中在调频调制和解调过程，这对立体声和单声道处理过程都是一样的。
[编辑] 原理
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)](/images/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] 参见
- 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
