正交调制器

zrynancy

　　现在在混频阶段很多地方采用正交调制，而不在使用超外差的方式，请教各位一下正交调制的优缺点。　　多谢

zhaobolove

我听说：
  1. 提高抗干扰。
  2. 增强保密性。
  3. 提高数据传输。
  4. 分担数据处理能力。
。。。

lijunping

我是这样理解的:
     非线性调制只能用相干解调,非线性转化成线性后得到的就是IQ形式.这是用IQ调制的原因.
     至于好处嘛,可以看看关于复函数的知识, IQ混频器可以很好的消除镜象频率.

    再看看,好象楼主搞错了吧,超外差也是有用到正交调制的啊.

   至于楼上所说的4点,没听说过，望高人指点.

zrynancy

不太清楚IQ调制器的知识，请楼上在具体说说其调制的好处，I/Q两路不平衡的话，岂不是会引入很多的噪声。

lijunping

Thus far we have discussed what IQ data is, but not why it is used. Since magnitude and phase data seem more intuitive, it would seem that we should use polar magnitude and phase data instead of Cartesian I and Q data. However, practical hardware design concerns make I and Q data the clear cut choice in this matter.

It is difficult to precisely vary the phase of a high frequency carrier sine wave in a hardware circuit according to an input message signal. A hardware signal modulator that manipulated the magnitude and phase of a carrier sine wave would therefore be expensive and difficult to design and build, and as it turns out not as flexible as a circuit that uses I and Q waveforms. To understand how we can avoid having to manipulate the phase of an RF carrier directly, we first return to trigonometry.



Figure 9. Mathematical Background of IQ Modulation

With the trig identity shown in line 1 above, we can multiply both sides of the equation by A and substitute 2πfct in place of alpha and Φ in place of beta to arrive at the equation shown in line 2. By then substituting I in place of A Cos (Φ) and Q in place of A Sin (Φ), we can represent a sine wave with the equation shown on line 3. One final observation to be made is to remember that a sine wave and a cosine wave of the same frequency are exactly the same, except for a 90 degree phase offset between them. The implications of this are very important. What this essentially means is that we can control the amplitude, frequency and phase of a modulating RF carrier sine wave by simply manipulating the amplitudes of separate I and Q input signals! With this method, we no longer have to try to directly vary the phase of an RF carrier sine wave. We can achieve the same effect by manipulating the amplitudes of input I and Q signals. Of course, the second half of the equation is a sine wave and the first half is a cosine wave, so we must include a device in the hardware circuit to induce a 90 degree phase shift between the carrier signals used for the I and Q mixers, but this is a much simpler design issue than the aforementioned direct phase manipulation.



Figure 9. Hardware Diagram of an IQ Modulator

Here is a block diagram of an IQ modulator. The circles with an 'X' in them represent mixers, devices that perform frequency multiplication and either upconvert or downconvert signals (upconverting here). The IQ modulator mixes the I waveform with the RF carrier sine wave, and mixes the Q signal with the same RF carrier sine wave offset by 90 degrees in phase. The Q signal is subtracted from the I signal (just as in the equation shown in line 3 above) producing the final RF modulated waveform. In fact, the shifting of the carrier by 90 degrees is the source of the names for the I and Q data - I refers to in-phase data (since the carrier is in phase) and Q refers to quadrature data (since the carrier is offset by 90 degrees). This technique is known as quadrature upconversion and the same IQ modulator can be used for any modulation scheme. This is because the IQ modulator is merely reacting to changes in I and Q waveform amplitudes, and I and Q data can be used to represent any changes in magnitude and phase of a message signal. The flexibility and simplicity (relative to other options) of the design of an IQ modulator is the reason for its widespread use and popularity

lijunping

哎,好象贴不了图片.
anyway,上面的文章说了为什么要使用IQ调制.
"It is difficult to precisely vary the phase of a high frequency carrier sine wave in a hardware circuit according to an input message signal."这句话是重点，也就是如果使用直接的调制,对高频信号而言,在硬件的实现上面是很难做到频率和相位这些非线性的调制,而使用调幅则容易多了，特别是对于多进制的调制来说。[em02]