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Introduction
Mixer is one of the most important function for microwave system. Using the nonlinearity of the diode, mixer can convert input signal frequency and the LO frequency into their sum and different or their harmonic frequency's sum and difference. Its symbol and block diagram are shown in figure 19.1
1 Definition
A nonlinear three-port circuit (two inputs and one output) that generates a spectrum of output frequencies equal to the sum or difference of the two input frequencies and their harmonics. The two input ports are deferred to as RF and LO whereas the output port is called IF port.
2 Frequency convert process
Up-converter
As shown in figure 19.2a, when a mixer is used as an up-converter, the "sum frequency" (i,e., ff=fr+fLO) is utilized and the "differnce frequency" is rejected. Up-converters particularly are used in the case of transmitter of a radar or a comunication system
Down-converter
As shown in figure 19.2b, when a mixer is used as an down-converter, the "difference frequency" (i,e., ff=fr-fLO) is utilized and the "sum frequency" is rejected. Down-converters are used in the case of receiver of a radar or a comunication system
3 Specification of mixers
1) Rf frequency f0
2) LO frequency fLO
3) IF frequency fIF
4) Bandwidth Δf
5) Convert loss Ls
6) Noise figure
7) LO power level
4 Classes of mixers
1) Single ended mixer
2) Balanced mixer
3) Double banlanced mixer
4) Harmonic mixer |
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