◆Cause: Generally, the volt-ampere characteristic of the mixing device at the static working point is i=f(υ)=α0+α1υ+α2υ2+... When υ=υs+υL=Vsmcosωct+VLmcosωLt, i will contain many combinations. Frequency components, (their amplitudes decrease rapidly as (p+q) increases). This situation is like mixing, where there are countless transform channels, fc is transformed into fpq, and each transform channel is represented by a pair of p and q values ​​and the sign taken. Only one of the transform channels (p=q=1) is useful (transforms the input signal frequency to the required intermediate frequency). The remaining large number of transform channels are useless, some of which are also very harmful (which cause interference), and the harmful ones are collectively referred to as combined frequency interference.
â—†Combined frequency interference type:
1) Interfering whistle
Cause: fpq corresponding to a pair of p and q values ​​(except p = q = 1), if its value is very close to the intermediate frequency, F is the audio frequency. In the mixer, the input signal is converted into an intermediate frequency signal by a useful channel of p=q=1, and can also be converted to a signal close to the intermediate frequency by some channels satisfying the above formula by p and q, and all will be smooth. Ground through the IF amplifier.
Phenomenon: While listening to the useful signal sound, the receiver also hears the beat signal frequency F detected by the detector to form an audio whistle.
Features: Useful signals are formed by certain parasitic channels;
a) the frequency point fc at which the whistling sound is generated is infinite, discontinuous, and is an integer or fractional multiple of fI;
b) If the frequency band is determined, the interference fc point that can fall within the segment is still a limited number;
c) the strongest interference to avoid the strongest interference whistle, can
The IF is set outside the band.
2) Parasitic channel interference
Cause: If an interference signal with a frequency of fM is applied to the input of the mixer, the satisfied channel can convert fM into a parasitic fI and pass through the intermediate frequency amplifier.
Phenomenon: The receiver can hear the sound of the interference signal.
Features: Interference signals are formed by certain parasitic channels;
a) The fM points that generate parasitic channel interference are infinite, discontinuous, symmetrically distributed on both sides, and separated;
b) If the frequency band is determined, the number of interference points that can fall within the segment is a limited number;
c) the strongest interference
In order to avoid the strongest interference, the intermediate frequency must be taken outside the band.
2, nonlinear distortion1) Envelope distortion and strong signal blocking
Envelope distortion: υ=υs+υL=Vsmcosωct+VLmcosωLt Substituting i=f(υ)=α0+α1υ+α2υ2+...
After triangulation, it can be known that the quadratic term, the fourth power term and the higher even power term of Ï… will produce the intermediate frequency current component, ignoring the å››th power of the fourth power or more.
It can be seen that there is a nonlinear relationship between IIm and Vsm. Therefore, when Vsm=Vsm0(1+MacosΩt), IIm cannot correctly reflect the variation law of Vsm. The larger the Vsm, the more serious the envelope distortion.
Strong signal blocking: When Vsm is too large, since α4 or some higher order term coefficients are negative, and the positive coefficient terms cancel each other, IIm will hardly change with Vsm. Thus, although Vsm varies according to the modulation law, IIm is approximately constant. The listener will not hear the sound of a useful signal in this situation.
2) Cross modulation distortion
When the input end of the mixer acts on both υs and the interference signal υM, υ=υL+υs+υM, then substituting i=f(υ) means that the quadratic term of υM (2α2υLυs), the fourth power term and higher The even-order term produces an intermediate frequency current component, wherein the generated intermediate frequency current component amplitude 3α4VLmVsmV2Mm is related to VMm. This indicates that the current component amplitude contains an envelope variation of the interference signal, that is, the envelope of the interference signal is cross-transferred to a nonlinear distortion outputting the useful intermediate frequency signal, which is called cross modulation distortion or crosstalk distortion. When such distortion occurs, people not only hear the sound of the useful signal, but also receive the sound of the interference signal, but when the useful signal stops transmitting, the interference signal sound disappears.
When VM1m and VM2m are constant, the smaller the values ​​of r and s, the larger the corresponding parasitic current amplitude. At the same time, since the high-frequency amplifier before mixing has good filtering effect, only two interference signals with a frequency closer to fc can be effectively added to the input end of the mixer. Therefore, the frequencies of the two interfering signals that produce the strongest intermodulation distortion should satisfy:
2fM1-fM2=fc or 2fM2-fM1=fc
That is, fL-(2fM1-fM2)=fI or fL-(2fM2-fM1)=fI
Since r+s=3, it is called third-order intermodulation distortion.
Summary: The above-mentioned several major nonlinear distortions are generated by the fourth power term of the nonlinear volt-ampere characteristic and the higher even-order term. Among them, the third-order intermodulation distortion is the most harmful to the receiver, and the third-order intermodulation distortion is serious, and other nonlinear distortions are correspondingly serious. Therefore, the third-order intermodulation distortion coefficient can be directly used as a performance index for evaluating the nonlinear distortion of the mixer.
In the actual mixing circuit, the input to the input is often the product of the superposition of the useful signal and the various interfering signals. The above-mentioned interference whistle, parasitic channel interference and various nonlinear distortions may exist at the same time, and often only one or several of them are dominant.
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