LIMITS TO AMPLIFICATION 95 



the noise level in the amplifier, the system will be noisier because the 

 beats between the noise components are appreciable compared with 

 the beats between carrier and the noise. 



The same considerations apply to the first detector in a double- 

 detection receiving system. If, as is usual, the beating oscillator 

 voltage is large compared with the noise components, then the fre- 

 quency band of the noise will be shifted in position in the same manner 

 as the signal, and the signal-to-noise ratio of the system will be un- 

 changed by the frequency conversion. 



The signal-to-noise properties of any system are considered satis- 

 factory when the total output noise differs only slightly from that pro- 

 duced by thermal agitation in the input circuit alone, and this differ- 

 ence may be measured by eliminating the input thermal noise (as by 

 the short-circuit method) and noting the change produced in the out- 

 put noise. 



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