The Effect of jut-Circuit Non-Linearity on the Amplitude Stability of RC Oscillators

Ed. No te : In the RC osc i l l a to r the amp l i tude o f o s c i l l a t i o n i s c o n t r o l l e d b y a t h e r m a l l y s e n s i t i v e r e s i s t a n c e i n t h e f o r m o f a s m a l l t u n g s t e n l a m p . T h i s c o n t r o l e l e m e n t g i v e s the osc i l l a to r i t s cha rac te r i s t i c cons tancy o f ou tpu t l eve l ove r a w ide range o f ope ra t i ng cond i t i ons . I n p rac t i ce , f o r examp le , t he os c i l l a to r ' s response i s qu i te s l igh t to such d is t u r b a n c e s a s h u m , m i c r o p h o n i c s , a c h a n g e w i t h t u n i n g i n t h e i m p e d a n c e r a t i o s i n t h e f requency-de te rmin ing ne twork , o r a change in l oad ing a t t he osc i l l a to r ou tpu t . A t t h e s a m e t i m e , a n d i n c o n t r a s t t o t h e g r e a t m a j o r i t y o f c a s e s , e x p e r i e n c e w i t h t h e R C o s c i l l a t o r h a s d i s c l o s e d i n s t a n c e s i n w h i c h t h e o s c i l l a t o r h a s e x h i b i t e d a n e x a g ge ra ted response to such d i s tu rbances (see Fig. 2 below). In a few cases actual envelope ins tab i l i t y has been observed . A l though th i s e f f e c t w a s g e n e r a l l y a t t r i b u t e d t o t u b e a n o m a l i e s , s i n c e a g i n g o r r e p l a c i n g t h e t u b e s r e m o v e d t h e e f f e c t , o v e r a p e r i o d o f t i m e t h e i m p r e s s i o n b e g a n t o f o r m t h a t a more de f in i te fac to r was invo lved . T h e e n v e l o p e r e s p o n s e w a s t h e n i n v e s t i g a t e d a n a l y t i c a l l y b y B e r n a r d M . O l i v e r , h p v i c e p r e s i d e n t f o r R & D , a n d h i s a n a l y s i s o f t h e R C o s c i l l a t o r e n v e l o p e r e s p o n s e t o a m p l i t u d e d i s t u r b a n c e s i n t r o d u c e d i n t o t h e cont ro l e lement i s p resented in the fo l low ing a r t i c l e . W e b e l i e v e t h e a n a l y s i s w i l l b e o f g e n e r a l i n t e r e s t b e c a u s e i t t a k e s i n t o a c c o u n t t h e e f f e c t o f t h e s m a l l n o n l i n e a r i t y t h a t e x i s t s i n t h e a m p l i f i e r p o r t i o n o f t h e o s c i l l a t o r c i r c u i t , w h e r e p r e v i o u s a n a l y s e s h a v e a s s u m e d a n i d e a l l y l i n e a r a m p l i f i e r . The analys is shows that , i f the ampl i f ier were in fac t i dea l l y l i near , the enve lope response t o a m p l i t u d e d i s t u r b a n c e s w o u l d b e s o m e JOO c/ fa more than i t is . I t shows fur ther that the ampl i f ie r must have a s l igh t compress ion fo r the osc i l la to r to be prac t ica l . Th is agrees wi th observed resu l ts .