Sensitivity Considerations in Microwave 

 Paramagnetic Resonance Absorption 



Techniques 



By G. FEHER 



(Manuscript received February 9, 1956) 



This paper discusses some factors which limit the sensitivity of microwave 

 paramagnetic resonance equipments. Several specific systeins are analyzed 

 and the results verified by measuring the signal-to-noise ratio with known 

 amounts of a free radical. The two most promising systetns, especially at low 

 powers, employ either superheterodyne detection or barretter homodyne de- 

 tection. A detailed description of a swperhetrodyne spectrometer is given. 



Table of Coxtents 



Page 



I. Introduction 450 



II. General Background 450 



III. Q Changes Associated with the Absorption 450 



IV. Coupling to Resonant Cavities for Maximum Output 451 



A. Reflection Cavitj^ 452 



1. Detector Output Proportional to Input Power 453 



2. Detector Output Proportional to Input Voltage 454 



B. Transmission Cavity 455 



1. Detector Output Proportional to Input Power 455 



2. Detector Output Proportional to Input Voltage 456 



V. Minimum Detectable Signal Under Ideal Conditions 457 



VI. Signal-to-Noise in Practical Sj'stems 459 



A. General Considerations 459 



1. Why Field Modulation? 459 



2. Choice of Microwave Frequency 460 



3. Optimum Amount of Sample to be Used 461 



a. Losses Proportional to E- 461 



b. Losses Proportional to Hi^ 462 



B. Noise Due to Frequency Instabilities 462 



C. Noise Due to Cavity Vibrations 465 



D. Klystron Noise 465 



E. Signal-to-noise Ratio for Specific Systems ,. 466 



1 . Barretter Detection 467 



a. Straight Detection 469 



b. Balanced Mixer Detection 470 



2. Crystal Detection 472 



a. Simple Straight Detection 473 



b. Straight Detection with Optimum Microwave Bucking 473 



c. The Superheterodyne Scheme 475 



F. Experimental Determination of Sensitivity Limits 477 



1. Preparation of Samples 477 



2. Comparison of Experimental Result with Theory 478 



449 



