7-5] MIXERS 359 



When more than one signal frequency is applied to the mixer, inter- 

 modulation between the signal components occurs. The output current 

 caused by these intermodulation components is of the following form for 

 each possible pairing of m signal components: 



cos (coy - CO/,)/. (7-13) 



where / = 2, -, 7n 



k = 1, -, m. 



Once again, the term in the brackets is constant for a particular local 

 oscillator voltage, so that 



/ = AiA.K,. (7-14) 



The voltage developed by these mixer currents is prevented from affecting 

 the receiver performance by the frequency selectivity of the IF networks 

 when coy — cojt falls outside the IF passband. Those components that would 

 ordinarily fall within the IF passband, could be eliminated by RF preselec- 

 tion and proper IF frequency. However, such preselection is not always 

 feasible. A balanced mixer is therefore used (see Paragraph 10-15). In 

 the balanced mixer two crystals are placed at two of the ports of a micro- 

 wave junction, and the signal is fed into one port and the local oscillator 

 into the other port. The junction may be a magic-tee, short-slot hybrid or 

 rat-race. 



Each individual crystal develops all of the intermodulation components, 

 but the relative phase of the signal-signal beats differs from that of the 

 signal-L.O. beats and therefore can be discriminated against in the IF 

 coupling circuit. Rejection of the undesired intermodulation components 

 on the order of 25 db is realized in practice. Principal factors affecting 

 the rejection of signal-signal beats are the impedance match between the 

 signal source and each crystal, the rectifier dynamic characteristics, and 

 the balance of the IF circuit. 



Among the signal-local oscillator products are two which affect the 

 performance of the mixer. These are included in the value K of Equation 

 7-12 when it is determined experimentally by measuring the coi — co^ 

 component from the mixer. These two products involve the generation of 

 an image frequency, i.e., a signal which is separated from the desired signal 

 frequency by a frequency equal to twice the IF frequency, and which is 

 separated from the local oscillator frequency by the IF frequency. The 

 image frequency is caused by second harmonic mixing and by an up con- 

 version resulting from the IF current which flows through the mixer. 



