Mixing means usingNonlinear element, such as diodetelecomThe process of mixing the signals of the third frequency through the frequency selection loop.The device that completes this process is calledmixer 。The oscillation of two different frequencies is transformed into a new oscillation related to both through a nonlinear device.The new oscillation frequency is the difference between the above two different frequencies, and the amplitudeenvelopeOne of them.
The oscillation of two different frequencies is transformed into a new oscillation related to both through a nonlinear device.The new oscillation frequency is the difference between the above two different frequencies, and the amplitudeenvelopeOne of them.If the local oscillation and mixing are realized on the same nonlinear device, it is called "frequency conversion".It can be used for signal frequency reduction.[1]
Parameters of mixing
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working frequency
mixer It is a multi frequency working device, unless otherwise specifiedRF signalIn addition to operating frequency, attention shall be paid toLocal oscillatorandintermediate frequencyFrequency application range.mixer OfNoise definitionIs: NF=Pno/Pso Pno is when the input portNoise temperatureThe total noise utilization power transmitted to the output port when the standard temperature is T0=290K at all frequencies.Pno mainly includes signal source thermal noise, internal loss resistance thermal noise, mixer device currentShrapnel noiseandLocal oscillatorphase noise 。Pso is the noise resource power generated at the output end only by the useful signal input.mixer OfFrequency conversion lossDefined as mixerradio frequencyThe microwave signal power of the input port andintermediate frequencyThe ratio of output signal power.Mainly due to circuit mismatch loss, intrinsic junction loss and nonlinearity of diodeconductanceNet frequency conversion loss, etc.
1dB compression point
Under normal operating conditions,radio frequencyInput level is much lower thanLocal oscillatorLevel, at this timeintermediate frequencyThe output will change linearly with the RF input. When the RF level increases to a certain extent, the IF output will slow down with the increase of RF input,mixer Saturation occurs.RF when IF output deviates from linearity by 1dBInput powerIs the 1dB compression point of the mixer.For mixers with the same structure, the 1dB compression point depends on the local oscillator power and diode characteristics, which is generally 6dB lower than the local oscillator power.
dynamic range
dynamic range Meansmixer Microwave in normal operationInput powerRange.The lower limit varies with the application environment of the mixer, and the upper limit is subject toradio frequencyThe input power is limited by saturation, which usually corresponds to the 1dB compression point of the mixer.If there are two microwave signals fs1 and fs2 with similar frequencies andLocal oscillatorFLO is input tomixer , due to the nonlinear effect of the mixer, intermodulation will occur, whereThird-order intermodulationMay appear in the outputintermediate frequencyNearby places that fall within the IF passband and cause interference are usually described by third-order intermodulation rejection ratio, that is, the ratio of useful signal power to third-order intermodulation signal power, which is often expressed as dBc.Because the IF power varies withInput powerIn direct proportion, when the microwave input signal decreases by 1dB, the third-order intermodulation signal rejection ratio increases by 2dB.
Isolation
mixer Isolation refers to the mutual isolation between frequency ports, includingLocal oscillatorAndradio frequency, LO andintermediate frequency, and isolation between RF and IF.Isolation is defined as intrinsic vibration orRF signalPower leaked to other ports vsInput powerRatio, in dB.mixer OfLocal oscillatorThe power refers to the local oscillator power required under the optimal working condition.In principle, the higher the LO power,dynamic range enlarge,LinearityImprovement (1dB compression point rises,Third-order intermodulationCoefficient improvement).portstanding wavedirect influence mixer Used in the system, it is a parameter that varies with power and frequency.When the mixer operatesPhase discriminatorWhen there is only one input, the output should be zero.However, due to the poor matching of mixer tubes or balun imbalance, etc., aDC voltage, i.e. residual DC offset voltage of intermediate frequency.This residual DC bias voltage will affect the phase discrimination accuracy.[2]
Application of frequency mixing
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Frequency conversion
This ismixer A well-known use of.The commonly used ones are double balanced mixer and triple balanced mixer. The triple balanced mixer uses two diodesbridge。All three ports have transformers, soLocal oscillator、radio frequencyandintermediate frequencyHow many bandwidth can be reachedOctave, anddynamic range Large, low distortion, high isolation.However, its manufacturing cost is high and the process is complex, so its price is high.
Phase discrimination
Theoretically allintermediate frequencyyesDC couplingOfmixer Can be used asPhase discriminatoruse.Put the two frequencies and amplitudes of the sameRF signalAdded to mixerLocal oscillatorandradio frequencyThe IF terminal will outputDC voltage。When the two signals are sinusoidal, the phase detection output changes to sine with the phase difference; when the two input signals are square waves, the phase detection output isTriangular wave。The operating power is recommended to be near the standard LO power,Input powerIf it is too large, it will increase the DC deviation voltage; if it is too small, it willOutput levelToo low.
Variable attenuator/switch
This classmixer Also requiredintermediate frequencyDC coupling。Signal in mixerLocal oscillatorPorts andradio frequencyInter portTransmission lossIt is controlled by intermediate frequency current.When the control current is zero, the transmission loss is the isolation from LO to RF. When the control current is above 20mA, the transmission loss is the isolation between LO and RFinsertion loss 。In this way, the positive orNegative currentContinuous control to form a variable range of about 30dBattenuator, and ports within the entire range of changestanding waveLittle change.Similarly, a switch can be formed by square wave control.
This classmixer Also requiredintermediate frequencyDC coupling。Signal in mixerLocal oscillatorPorts andradio frequencyThe transmission phase between ports is controlled by the polarity of intermediate frequency current.Change the control current polarity and output at the intermediate frequency terminal * * * for groundingRF signalThe phase of will change alternately at 0 ° and 180 °.QPSKIt is composed of twoBPSK, a 90 degreebridgeAnd a 0 degreePower dividerComposition.It is actually a reciprocal process, which is reversible in the system.Here is the main introductionI/Q demodulator, I/Q demodulator consists of twomixer , a 90 degreebridgeIn phase with onePower dividerComposition.Filters that suppress the image frequency are generally fixed bandwidth.But whensignal frequency When changing, the mirror frequency also changes, which may move out of the filter's suppression band.In a multi-channel receiving system orFrequency agilityIn the system, this filter will lose its function.At this time, the mirror frequency suppression mixer is used,LO frequencyWhen changing, due to the phase matching relationship within the mixer circuit, the suppressed image frequency range will also change, so that it can still play the role of image frequency suppression.Because the circuit is not completely ideal, there are amplitude imbalance and phase imbalance, which may deteriorate the electrical performance of the image rejection mixer.
Single sideband modulator
In a multi-channel transmission system, due tobasebandVery low frequency, if ordinarymixer dospectrum shifting , thenChannel bandwidthThere will be two sidebands, which will affect the utilization of spectrum resources.This can be usedSingle sidebandModulatorTo suppress unwanted sidebands. Its basic structure is two mixers and one 90 degreePower dividerAnd an in-phase power divider.takebaseband signal Decomposition into two orthogonal paths andLocal oscillatorThe phase offset technique is used to suppress unwanted sidebands, and the local oscillator is suppressed due to the isolation of the mixer itself.[3]
Interference of mixing
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Noise interference
Many frequency components will be generated by the interaction of nonlinear devices between the harmonics of signal frequency and LO frequency, between interference signals and LO signals, between interference signals and signals, and between interference signals.In the receiver, when some frequencies are equal to or close to the intermediate frequency, it can pass through the intermediate frequency amplifier. After demodulation, it will cause crosstalk, noise and various interferences at the output stage, affecting the normal reception of useful signals.
Sub channel interference
The mixing circuit isSuperheterodyne receiver, transmitter and an important part of frequency synthesis technology. It plays an important role in the transceiver channel. Its role is to convert the modulated signal (or single frequency carrier) with carrier frequency of into the signal without distortion.Two (or more)interference signal, and add tomixer At the input, due to the nonlinear effect of the mixer, the two interference signalsLocal oscillatorThe signals are mixed with each other, and if the combined frequency components generated are close tointermediate frequencySo that it can pass throughIF amplifier, viadetectorInterference is generated after detection.The combined frequency interference generated by external interference and LO voltage is called parasitic channel interference.
Cross modulation jamming
When useful signals andinterference signaltwo typesAmplitude modulated waveAll added tomixer At the input end, the envelope of the jamming signal is transferred toIF signalOn.The generation of cross modulation is independent of the frequency of jamming station. Any interference signal with strong frequency added to the input end of mixer may form cross modulation interference.
