THE RF COUPLED STIMULATOR 



One way of achieving this, which is fairly successful, is to pass the stimu- 

 lator output via an iron-cored transformer of special low-capacitance con- 

 struction. The snag is that the kind of transformer which has a low capaci- 

 tance from secondary to primary and from secondary to earth (the core) is 

 precisely the kind of transformer that makes a bad stimulating pulse-trans- 

 former; the primary inductance is likely to be insufficient to pass the longer 

 pulses required without sag; and the leakage inductance is liable to be 

 considerable, spoiling the leading and trailing edges. This is the reason for 

 the introduction of the RF coupled stimulator. 



THE RF COUPLED STIMULATOR 



This ingenious device is due to Schmitt^. It consists of a radio-frequency 

 LC oscillator whose coil is inductively loose-coupled to a second coil, tuned 

 to the oscillator frequency. RF voltages appearing across the secondary 

 coil are rectified and smoothed to produce a direct voltage. Since the 



Stimulating 

 pulse in 



_r~L i 30pF 



36 turns 30 turns 



on 3-^ in. diam. same former 



former, tapped 



6 turns from 



grid end 



Figure 38.17 



amplitude of oscillation is nearly proportional to the HT voltage supplying 

 the oscillator, and since the direct voltage produced by the rectifying and 

 smoothing circuit is nearly proportional to the amplitude of oscillation, it 

 follows that the rectifying-circuit output is a good copy of the oscillator HT. 

 If the stimulator output is used to provide the HT for the oscillator, then the 

 rectifier and smoothed output can be arranged to follow the stimulator 

 output closely, but with this important difference; that by having the 

 secondary circuit 'floating' and only loosely coupled to the oscillator, the 

 direct voltage derived from it appears to come from a generator having a very 

 high impedance to earth; the conditions are fulfilled for avoiding spurious 

 stimulation and minimizing artefact, and the shock can be of unlimited 

 duration. Schmitt's original radio-frequency coupler is shown in Figure 

 38.17. Other couplers have been described by Haapanen'' and Perkins*'. All 

 these are in the form of 'add-on' units to stimulators having one side of the 

 output earthed. In order to keep the capacitance to earth of the secondary 

 circuit small, the coupler unit must be brought up close to the preparation. 

 It seemed to the author that the advantages of RF coupled stimulation 

 are so overwhelming that the RF unit might be regarded not so much as an 

 optional luxury attachment but as an essential component. In this case the 



613 



