15 



SQUARE WAVE OSCILLATORS 



A square wave is one having the form of Figure 15.1. The ratio ajb is called 

 the mark-space ratio. Square waves are useful for testing the transient 



Voltage 



or 

 current 



t 



aba 



■« » * ^ ^ ^ 



Time— «- 

 Figure 15.1 



response of recording equipment, and square wave oscillators are comple- 

 mentary in function to sine wave oscillators, which check the steady-state 

 response. 



The 'squared-up' sine wave 



Square waves may be made from sine waves by amplification and clipping, 

 and many commercial, primarily sine wave, test oscillators improve their 

 sphere of usefulness by having a square wave outlet obtained on this principle. 

 The procedure is merely to feed a valve with a sine wave input which is, as 

 it were, much too big for it {Figure 15.2). When the input goes positive the 



HT+ 



250 volt 

 RM.S.say 



A/V 



-LTlJ- 



_n^i_ 





I J 



Figure 15.2 



Figure 15.3 



grid cannot follow it beyond a certain point because it is 'caught' by diode 

 action between it and the cathode. When the input goes negative the anode 

 cannot follow it beyond a certain point — the HT plus potential — because the 

 valve is cut off. The anode waveform is therefore moderately square in that 

 the tops and bottoms of the waves will be quite flat, but the sides will still 

 be rather sloping (Figure 15.3). However, if the process be repeated in a 

 further valve the final product is likely to be square enough for most purposes. 

 In pieces of apparatus where square waves only are required it is a waste 

 of effort to make sine waves and then spoil them again, and we use a circuit 



226 



