Grid Current Modulation

T h e t e r m grid c u r r e n t m o d u l a t o r is used to describe t h o s e v a c u u m t u b e circuits in which modulation is initially produced in t h e grid circuit of a three-electrode v a c u u m t u b e d u e to t h e non-linear grid c u r r e n t grid voltage relation. C o m p a r i s o n with a r e p r e s e n t a t i v e plate c u r r e n t m o d u l a t o r using t h e s a m e t u b e s a n d t h e same p l a t e potential shows t h a t b y m o d u l a t i n g at m a x i m u m efficiency in t h e grid circuit a n d using t h e plate circuit solely for amplification, t h e m a x i m u m power o u t p u t is increased a b o u t eight times, t h e power efficiency is increased a b o u t five t i m e s a n d t h e r a t i o of sideband o u t p u t to signal i n p u t is increased a p p r o x i m a t e l y t h r e e times. U n d e r these conditions more carrier i n p u t power is needed for t h e grid t h a n for t h e plate m o d u l a t o r . T h i s improved p e r f o r m a n c e h a s been m a d e possible by a detailed s t u d y of t h e f u n d a m e n t a l processes involved a n d by a design of t h e t u b e s a n d associated e q u i p m e n t , such as t r a n s f o r m e r s a n d f i l t e r s , t o p e r m i t these f u n d a m e n t a l processes t o o p e r a t e t o their best advantage. N o r m a l l y modulation is also produced in t h e plate circuit which is shown to be out of phase with t h a t produced in t h e grid circuit. By inserting high i m p e d a n c e s to t h e i n p u t frequencies in t h e p l a t e circuit, plate circuit modulation is p r e v e n t e d , a n d t h e reduction of grid circuit sideband is likewise a v o i d e d .