PHASE-SENSITIVE DETECTORS 



centre-tap by a potentiometer {Figure 6.51a), which is then adjusted until, 

 with no input apphed, the reference wave produces no spurious signal at 

 the output. With this arrangement the resistance to earth is still not very 

 low, and the effect of this is to cause the output wave form not to lie wholly 

 on one or other side of earth potential {Figure 6.51b). Upon taking the 

 average, the output is in consequence reduced. 



A better scheme is to use four diodes connected so that all are switched to 

 low resistance together {Figure 6.52): this device is known as the Cowan 



HK 



Input 



X 



T 



OI100'060'00"OOOD"000 



c 



Output 



± 



Ref. wave in 

 Figure 6.52 



bridge. A potentiometer is still necessary to ehminate the reference wave from 

 the output, but by putting it in the left-hand side of the bridge, as shown, 

 there is a low resistance path to earth straight through the right-hand diodes, 

 so the rectification is efficient. 



What might be called a 'full-wave' phase-sensitive rectifier is shown in 

 Figure 6.53 and this is the form used by Rushton and Campbell. Two trans- 



-I- 



G 

 B 



Load 



jUJLUJLliUULUJUJlJU 



nnnmnmnnn 



Ref. wave ' 

 in 



Figure 6.53 



.e 



formers provided with centre-tapped secondaries are required. The action 

 of the reference wave is to make first the upper pair then the lower pair of 

 diodes conducting. Thus, with the polarities shown, the upper diodes are 

 cut off and no current leaves or enters at G. The lower diodes conduct and 

 current flows along the path ABCDE, i.e. up the load. The next half-cycle, 

 polarities reverse and the current path is FGCDE, i.e. still up the load, and 

 the action is seen to be full-wave. The load current clearly reverses only if 

 the phase relationship of reference wave and signal wave reverses. 



117 



