. Electronic apparatus for biological research . Galvo Red Red '>^x ,,^^ ^.^^x r,u . ..Green Green Green >v<3=5nV Green Green Green Red Red Figure a rectifier which identifies which segments of the square wave belong to which beam {Figure ). 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 ). This may be generated Red Red Red Photocell output Green Green Reference wave A
. Electronic apparatus for biological research . Galvo Red Red '>^x ,,^^ ^.^^x r,u . ..Green Green Green >v<3=5nV Green Green Green Red Red Figure a rectifier which identifies which segments of the square wave belong to which beam {Figure ). 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 ). This may be generated Red Red Red Photocell output Green Green Reference wave Always 'up' â for Red, down for Green Figure 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 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
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