SQUARE WAVE OSCILLATORS 



Counting down is achieved in the symmetrical multi-vibrator by injecting 

 positive 'pips' of voltage at the control or master frequency into one of the 

 grid circuits. If the control pips are at 100 kc/s and the multi- vibrator is to 

 count down 10 times, then it is arranged that, in the absence of any synchroni- 

 zation, the circuit would oscillate rather below 10 kc/s. The tenth syn- 

 chronizing pip then comes along and trips the multi-vibrator slightly early 

 (Figure 15.17 — cf. Figure 15 J). It is clear from Figure 15.17 that the amount 



Control pips 10 1 23A56789 10 1 



^--!- 1 1 1 1 \ 1 1 1 1 1 1 1 



Earth 



Cut-off -^>'-"> 



Instant at which 

 circuit would 'trip' 

 in absence of 

 tenth pip 



Figure 15.17 



of control pip injected must be carefully watched, otherwise the oscillator 

 will be tripped on the ninth if the pips get too big, and fail to be tripped by 

 the tenth if the pips get too small. 



Circuits of electrobiological interest which employ multi-vibrators have 

 been described by Dickinson^ (timer employing symmetrical multi-vibrators), 

 Attew^ (timer employing a variety of cathode coupled multi-vibrators) and 

 Attree^ (device for producing stabilized heater supplies, employing screen- 

 coupled multi-vibrators). 



REFERENCES 



^ Dickinson, C. J. Electronic engineering monograph Electrophysiological 



Technique p. 98 

 ^ Attew, J. E. Decade multi-vibrator design Wireless World 58 (1952) 114 

 ^ Attree, V. H. A low voltage stabilizer with saturated diode control Electron. 



Engng. 25 (1953) 71 



234 



