The reference arm microwave bridge

The reference arm bridge coupled to a microwave resonant structure operating in reflection has become the standard microwave geometry for ESR spectroscopy 

As a rule, microwave attenuators introduce phase shift as well as attenuation. An exception is the type of attenuation known as rotary vane , which has negligible phase shift. Some attenuator manufacturers have achieved fairly constant phase shift for microwave coaxial structures. Often the level set attenuator in the reference arm is of lower quality with large phase shift, but the operator encounters little difficulty because the reference level is seldom changed. The reader can appreciate the frustration that would occur if for every change in the main attenuator setting an accompanying phase shift were to occur. 

Small phase differences between the reference and resonator arms result in ESR resonance lines of the proper shape (i.e., no dispersive admixture, which may surprise 

While various automatic schemes for precise closed-loop control of the reference phase are possible [285], this deluxe feature is not usually present. Manual adjustment is accomplished by very carefully adjusting the phase for a maximum in detector crystal current (assuming that the resonator is well matched). 

Subtle difficulties in operation of the bridge occur if low levels of microwave power reach the detector crystal by paths other than the reference arm or the cavity arm. One possible route is directly from arm 1 to arm 3 of the circulator. For this reason, four-port circulators of high quality are used in commercial bridges. 

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