PHASE-SENSITIVE DETECTORS 



is wanted is a signal rectifier which will give a direct voltage output, whose 

 amplitude is proportional to the error wave amplitude, and whose polarity 

 depends on whether the retina beam or the wedge beam is greater. That is, 



Red filter 



Motor 



Green 

 filter 



Purple 

 wedge 



Galvo 



Red Red 



'>^x 



,,^^ ^.^^x r,u . ..Green Green Green >v<3=5nV 



Green Green Green Red Red 



Figure 6.47 



a rectifier which identifies which segments of the square wave belong to 

 which beam {Figure 6.47b). This is the purpose of the 'phase sensitive- 

 rectifier'. 



To achieve phase-sensitive rectification it is necessary to have an electrical 

 reference wave, usually of square form, which is generated synchronously 

 with the switching of optical beams {Figure 6.48). This may be generated 



Red 



Red 



Red 



Photocell 

 output 



Green Green 



Reference 

 wave 



Always 'up' 

 ■for Red, 

 down for 

 Green 



Figure 6.48 



by a commutator on the shaft carrying the rotating shutter, or by an auxiliary 

 lamp and photocell system, the light from the auxihary lamp being interrupted 

 by the same shutter as is used to control the main beams. However it is 

 generated the wave may have to undergo amplification, as an amplitude of 

 the order of 10 V is required for operation of the phase-sensitive rectifier. 



The simplest phase-sensitive rectifier is shown in Figure 6.49. The reference 

 wave is applied through a transformer to two similar diodes in series, via 

 similar current limiting resistors. In this circuit and in those to come the 

 diodes are used as switches controlled by the reference wave: when the 

 latter drives current through the diode in the forward direction the diode 

 resistance is low. When the reference wave attempts to drive current through 

 the diode in the reverse direction, the diode resistance becomes high. 



In this case it is important to see that, if no input is connected, by symmetry, 

 whatever the potential difference across the transformer happens to be A 



115 



