In the receiver, if the intermediate frequency signal obtained after mixing is lower than the original signal, this mixing method is called down conversion.The purpose of down conversion is to reduce the signalcarrier frequency Or directly remove the carrier frequencybaseband signal 。Because of its simple circuit and low cost, the down conversion mode is widely used in civilian equipment and military equipment with low performance requirements.The biggest disadvantage of the down conversion mode is its poor ability to suppress image interference.
In the communication system, in order to facilitate signal transmission and implementationChannel multiplexingThe frequency of the transmitted signal is very high, so the frequency conversion of the signal is an important part of the communication system research.According to the situation before and after the frequency conversion, it can be divided into down conversion (frequency reduction) and up conversion (frequency increase).Down conversion is performed in the receiver.The method of down conversion is to multiply the received signal and the local oscillator signal generated by the local oscillator, and then pass thelow pass filter Obtain the signal after frequency conversion.Two multiplied signals can be divided into real mixing and complex mixing through real or complex number representation.
Real mixing down conversion
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In order to down convert the signal containing the required channel, the received RF signal can be mixed or multiplied with an actual LO signal, and low-pass filtering is performed. If the LO frequency is fLO, if frequency is fIF。Real mixing down conversion can set the center frequency at fc=fLO+fIFThe signal of is converted to the center frequency of fIFThe signal of.Due to the use of real mixing, there is also a frequency conversion process, that is, the center frequency is located at fM=fLO-fIFThe signal of can also be converted to the center frequency of f by multiplying and filtering with the LO signalIFThe signal of.
The signal observed at IF contains two different signals, which are called the required signal and its mirror image signal respectively. This phenomenon is also called mirror image signal interference.On the other hand, the image frequency interference is caused by the aliasing of the positive frequency part and the negative frequency part of the real signal moving to the IF respectively.
According to the frequency f of the local oscillatorLOIf it is lower or higher than the center frequency of the required signal, the mirror signal frequency will be lower or higher than the required signal frequency.Either way, the space between the required channel and the mirror channel is 2fIF。
For the image signal phenomenon, the traditional method is to use image suppression filtering (IR), that is, by usingBandpass filterSuppress the image signal before frequency conversion.This requires correct selection of IF frequency: on the one hand, the IF frequency should be high enough, because the interval between the required signal and the mirror signal is 2fIF, and the image suppression filter works at RF. In order to suppress the image signal, high IF is required;On the other hand, in order to achieve a band-pass filter with high Q value to suppress interference signals in the band and achieve highly selective channel filtering, low IF is required.The receiver design needs to be considered comprehensively.[1]
Complex mixing down conversion
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The spectrum of the signal is divided into positive frequency part and negative frequency part. The image interference problem is caused by the shift of the positive and negative frequency parts of the signal to the intermediate frequency during downconversion.If the positive frequency part and the negative frequency part can be separated, the image problem can be easily solved, which can be realized by complex signal processing technology.
In an ideal case, the quadrature down conversion technology has complete image signal suppression capability, eliminating the use of RF image suppression filters, and greatly relaxing the simulationRF filterThe general requirements of the RF front-end are simplified and the receiver integration is more convenient.However, this requires that the two branches of in-phase signal and quadrature signal (I, Q signal) be completely balanced.However, in practical applications, this is impossible to achieve. The image signal suppression capability is 20-40dB.
In addition to the image frequency problem, in the frequency and phase modulation system, the receiverdemodulationOrthogonal down conversion is required.
Bandpass sampling direct
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In bandpass sampling, if the sampling rate is fs, the sampled spectrum will reflect or fold into DC to fs/At this time, the absolute frequency information is lost in the 1st Nyquist region of 2, which means that the spectrum is shifted.If an appropriate sampling rate is selected, the signal can be down converted directly through sampling.For analog signals with carrier frequency fc, if the sampling rate fsSatisfied: fc=Kfs≥ k · 2B, the carrier frequency fc will be folded into the 1st Nyquist area after sampling, and the frequency will be converted to 0, that is, the signal will be directly down converted to baseband.However, for analog signals with double sidebands, spectrum aliasing will inevitably occur, so multiple sampling must be adopted.If the carrier frequency of a special single sideband signal is the same as the highest frequency (when the sideband is removed) or the lowest frequency (when the sideband is removed) of the signal, the conventional sampling method can be used for direct down conversion.
From another point of view, sampling is actually a process of multiplication and mixing, because sampling is achieved by inputting analog signal and periodic impulse signal δT(t) Implemented by multiplication.This down conversion method can make the receiver structure very simple, not only realizing digitization, but also realizing down conversion.[2]
