Case Study - Effect of Analog Buffer Amplifier Instability on Radiated Emissions

A n E M I ra d ia t io n case s tudy o f a co m p u te r sys tem w hich inc ludes a CPU and an ex te rna l e lec tro m ech an ica l p e r ip h e ra l is pe rfo rm ed . An un u su a l ly h igh band f requency n o ise hum p is d e te c te d , b o th f r e q u e n c y ra n g e an d e n e rg y level are h igh and beyond the d riv ing force o f the fundcam en ta l c lock c ircuit . T he EM I noise s o u r c e is d i s c o v e r e d f r o m an i n s t a b i l i t y o sc i l la t io n o f t ra n s is to r b u ffe r am p lif ie rs . The c ircu it configura tion is analyzed, and a solution is p ro v id e d b a sed on the c ircu i t co n f ig u ra t io n m a th e m a t i c m o d e l , a n d th e c o n s t r a in t s o f m a n u fac tu rab i l i ty o f the produc t . I n t r o d u c t i o n are : (1) C ircu it design and layout. (2) S u b a sse m b ly in te rco n n ec t io n . (3) E nc lo su re design. (4) I/O cable and connector choices. O f th e se fou r item s, c ircu it des ign and layou t p la y the m o s t im p o r ta n t ro le to d e te rm in e w hether the final design will be a quie t or noisy E M I radiating produc t . T h e f u n c t i o n a l b lo c k d i a g r a m o f th e s e p e r ip h e ra l s c an be ty p ic a l ly r e p r e s e n te d as F igure 1. A m ong all these func tiona l b locks, B lock #1 and B lock #2 are always considered as m a jo r E M I em is s io n so u rc e s in the c r i t i c a l frequency range above 10 MHz. A co m p u te r can e x h ib i t re la tive ly low rad ia ted e m is s io n s w hen i t is n o t c o n n e c te d to an e x t e r n a l p e r i p h e r a l . In so m e i n s t a n c e s a p e r ip h e ra l dev ice w il l n o t ra d ia te s ign if ican t ly i f it is no t exerc ised by the com puter or stands alone. It is no t uncom m on that a low em ission co m p u te r and a low em iss ion pe riphe ra l , w hen they a re c o n n e c te d to g e th e r , and e x e rc ise d (sending da ta back and forth be tw een C PU and p e r ip h e ra l ) th a t th e w h o le s y s te m e x c e e d s regulatory limits. The I/O cable may becom e a r a d i a t i n g d e v ic e b e c a u s e o f n o n e x i s te n t or in a d e q u a te s h i e ld in g o r im p r o p e r s h i e ld in g te rm ina tion o r p rov ide a m ean to exerc ise the p e r ip h e ra l w h ic h now b eco m es E M I ra d ia te d so u rc e . T y p ic a l c o m p u te r e le c t ro m e c h a n ic a l p e r ip h e ra l dev ices are f loppy and h a rd d isk drives , tape drives , p r in te rs and p lo t te rs . E M I rad ia tion d e s i g n c o n s i d e r a t i o n s on th e c o m p u t e r e l e c t r o m e c h a n ic a l p e r ip h e r a l d e v ic e b a s ic a l ly CPU to /from Digital Analog and ElectroPeripheral Control Servo Mechanical I/O Interface _ Control and Electronics Assembly