THE MULTI-VIBRATOR 



which the valve is unable to do this, and the cathode is unable to maintain 

 itself positive with respect to the grid on the extreme positive excursions of 

 the latter. When this happens we have: 



Regime 2— significant grid current flow — The effect of the grid current is 

 to give C a mean charge tending to bias F^ grid negatively. The entire grid 

 waveform is depressed, as shown in Figure 15.16, producing the change in 



mark-space ratio alluded to. It is not possible to produce a simple expression 

 for the value of R^ at which the change of regime takes place. In a cathode 

 coupled multi-vibrator of the author's, using an ECC 81 valve and an anode 

 load of 50 kO the critical value of R^ was about 3 kQi. Above this the mark- 

 space ratio remained at about unity and the amplitude of the output fell. 

 Below this the output rose and the mark-space ratio altered rapidly, as in 

 Figure 15.15. As a general rule with pulse circuits the quickest way to produce 

 a workable design is by experiment, it having been first established that the 

 working points of the valves are — at least on the average — safely within their 

 working regions. 



In conclusion, then, the cathode coupled multi-vibrator is characterized 

 by having fewer components than the symmetrical type, and the frequency 

 may be varied — whilst preserving the mark-space ratio — by altering only 

 one RC product. The mark-space ratio is determined by the ratio of anode 

 load and common cathode resistance, and so is the amplitude. Frequency, 

 mark-space and amplitude are all affected by the HT and valves used. 



Application of multi-vibrators 



Although the multi-vibrator is of no use as a frequency standard itself, it 

 can easily be synchronized to run at the same frequency as, or — more impor- 

 tant — at a sub-harmonic frequency of a master frequency standard such as a 

 crystal oscillator. Use is made of this fact in some kinds of timer, where the 

 crystal oscillator runs at 100 kc/s, say, and controls the frequency of multi- 

 vibrator No. 1, running at 100 kc/s. Multi- vibrator No. 1 controls multi- 

 vibrator No. 2, running at 1 kc/s, and so on. The process is known as 

 'counting down'. 



233 



