Baseband signal modulation

Baseband signal modulation:

The signal from the source is called baseband signal, such as: computer output of various text or image file data signal. Baseband signal contains low-frequency components, even DC components, the channel can not transmit this low-frequency components or DC components, so the baseband signal needs to be modulated. modulation method: 1. Baseband modulation

Converts a digital signal to another form of digital signal, also known as an encoding. The essence is to transform the waveform of the signal, so that it can adapt to the channel characteristics, and the transformed signal is still the baseband signal. Common coding methods:

<1>: The non-zero code is 0 and 1, respectively, with two different levels of positive and negative.

<2>: Positive pulses represent 1, negative pulses represent 0.

<3> Manchester code: There is a level change in the middle of the non-zero code element, the receiver can use this as a synchronous signal, the number 0 corresponds to the signal from low to high, the number 1 corresponds to the signal from high level to low level.

<4>. Differential Manchester code: There is a level change in the middle of the non-zero code element, the receiver can use this as a synchronous signal, the fact that the number 0 is the opposite of the end level of the previous digit, there is a jump, the starting level of the number 1 is the same as the end level of the previous digit.

Note:

In baseband transmission, attention should be paid to the synchronization problem, that is, the data sequence sent by the receiving end and the sending end must be synchronized in time in order to distinguish each bit of data sent. The receiving end must accept the data according to the repetition frequency and the start and end time of each code sent by the sender, and the acceptance process continuously calibrates the time and frequency.

For example: Self-synchronization method: refers to the data signal waveform can be extracted from the synchronous information method. In the Manchester code, each bit has a jump in the middle, both as a clock signal and as a data signal. While transmitting the code information, the clock synchronization signal is also transmitted to the other side. 2. Band-Pass modulation

Using carrier modulation, the frequency range of baseband signal is shifted to the higher band, and the analog signal can be transmitted in the analog channel. The signal after the carrier modulation is called the band Communication number (which can pass through the channel only within a certain frequency range). Basic Band-Pass modulation method:

<a> amplitude modulation (AM): Causes the amplitude of the HF carrier to change with the signal. 0 or 1 corresponds to no carrier or carrier output, respectively. such as: Image signal of TV

<B> (FM): A modulation method that changes the instantaneous frequency of a carrier according to the changing law of the desired transmission signal.

<c> phase modulation (PM): Phase modulation and FM are closely related, the phase modulation is the initial phase of the carrier with the baseband digital signal and change. Strong anti-jamming ability.

Note:

code element: In digital communication often uses the same time interval symbol to represent a binary number, so that the time interval of the signal is called the binary code element, and the interval between is called the code element length.

Example: A binary number is a code element, containing 1bit of information, but a four-element unit, each code element contains 2bit of information.