46 IEEE T R A N S A C T I O N S ON B R O A D C A S T I N G , VOL. 36, NO. 1 , MARCH 1990

DESIGN OF TUBE POWER AMPLIFIERS FOR OPTIMUM FM TRANSMITTER PERFORMANCE

Mukunda B. Shrestha Manager o f RF Eng ineer ing

Broadcast E l e c t r o n i c s , I nc . 4100 Nor th 24 th S t r e e t

P.O. Box 3606 Quincy, I L 62305

ABSTRACT

Tube a m p l i f i e r s a r e e x t e n s i v e l y used i n modern h i g h power FM b roadcas t t r a n s m i t t e r s . The power a m p l i f i e r bandwidth a f f e c t s n o t o n l y t h e modu la t i on performance b u t a l s o t h e t r a n s m i t t e r ' s immunity t o RF i n t e r m o d u l a t i o n . i n v o l v e d a r e d i scussed w i th recommendations on t h e cho ice o f bandwidth. The des ign o f tube power amp l i - f i e r s f o r optimum FM performance r e q u i r e s c a r e f u l c o n s i d e r a t i o n s i n t h e s e l e c t i o n (of i n p u t match ing and o u t p u t c o u p l i n g c i r c u i t t o p o l o g i e s due t o t h e i r e f f e c t s on t h e t r a n s m i t t e r amp l i t ude and group de lay responses. Resu l t s o f computer { c i r c u i t a n a l y s i s and measured amp l i t ude and group de lay responses a re com- pared f o r d i f f e r e n t c i r c u i t t o p o l o g i e s . performance da ta o f a t y p i c a l 20 kW FM t r a n s m i t t e r i s a l s o presented t o i l l u s t r a t e t h e e f f e c t s o f tube power a m p l i f i e r i n p u t and o u t p u t c i r c u i t s .

The t r a d e - o f f s

Modu la t i on

INTRODUCTION

Tube a m p l i f i e r s a r e w i d e l y used i n f r e q - uency modu la t i on (FM) broadcast t r a n s m i t t e r s t o i n - crease t h e l e v e l o f t h e FM s i g n a l a t t h e wideband e x c i t e r o r I n te rmed ia te Power A m p l i f i e r ( IPA) o u t p u t t o h i g h e r power o u t p u t l e v e l s . Tube a m p l i f i e r s a r e more e f f i c i e n t and c o s t e f f e c t i v e a t h i g h power l e v e l s than a combinat ion o f seve ra l low power s o l i d - s t a t e a m p l i f i e r s i n t h e 88-108 MHz FM b roadcas t band.

The power a m p l i f i e r (PA) i s t y p i c a l l y a h i g h g a i n s i n g l e - t u b e t y p e opera ted as a tuned c l a s s " C " r a d i o f requency (RF) a m p l i f i e r . The PA des ign goal i s t o d e l i v e r t h e a u t h o r i z e d power o u t p u t t o t h e antenna w i t h h i g h e f f i c i e n c y and r e l i a b i l i t y w h i l e p r o v i d i n g e x c e l l e n t modu la t i on performance.

Th is paper d iscusses v a r i o u s t o p o l o g i e s o f t h e i n p u t and o u t p u t c i r c u i t s o f a vacuum tube power a m p l i f i e r and analyzes t h e i r e f f e c t s on t h e t r a n s - m i t t e r amp l i t ude and group de lay responses. Resu l t s o f computer c i r c u i t a n a l y s i s and a c t u a l measured da ta o f a t y p i c a l t r a n s m i t t e r w i t h two d i f f e r e n t t o p o l - og ies a r e compared. Design c o n s i d e r a t i o n s f o r o p t i - mum t r a n s m i t t e r performance t o ac:hieve d e s i r e d l e v e l o f t ransparency t o a wideband FM b roadcas t s i g n a l i s a l s o d iscussed i n c l u d i n g recommendations f o r compen- s a t i n g t h e group de lay o f t h e t ransmiss ion system. The con ten ts o f t h e paper a r e d i v i d e d i n t o t h e f o l - l ow ing main headings:

O f Bandwidth L i m i t a t i o n On The T r a n s m i t t e r Per- formance.

1. Frequency Modulated S igna l And E f f e c t s

2. Power A m p l i f i e r Design Considerat ions. - Pr imary Design Fac to rs . - I n p u t C i r c u i t C o n f i g u r a t i o n s And

T h e i r E f f e c t s On The T r a n s m i t t e r Bandwidth.

- Output C i r c u i t Con f igu ra t i ons And T h e i r E f f e c t s On The T r a n s m i t t e r Bandwidth.

- Computed/Measured Ampl i tude And Group Delay Responses.

Modu la t i on Performance O f A T y p i c a l 20 kW Single-Tube FM T r a n s m i t t e r .

3.

FREQUENCY MODULATED SIGNAL AND EFFECTS OF BANDWIDTH LIMITATION ON THE TRANSMITTER PERFORMANCE

Frequency Modulated S igna l

A Frequency Modulated RF S igna l w i t h modu- l a t i o n i ndex "m", c a r r i e r f requency " f c " , and s i n g l e - tone modu la t i on f requency " f m " can be rep resen ted by t h e f o l l o w i n g mathemat ica l exp ress ion [ l - 61 :

E ( t ) = Ec.Cos[wc-t + m-Sin(wm-t ) ] ,

Where:

Ec = The unmodulated c a r r i e r amp l i t ude

DC = 2-ir-fc ( c a r r i e r f requency) w m = 2a - fm (modulat ing f requency)

m =Af/fm = f requency dev ia t i on /modu la t i ng

cons tan t

f requency

I n an FM s i g n a l , t h e d e v i a t i o n 'af" o f t h e instantaneous f requency f rom t h e average ( o r t h e c a r r i e r frequency) i s d i r e c t l y p r o p o r t i o n a l t o t h e instantaneous amp l i t ude o f t he modu la t i ng s i g n a l . The r a t e o f f requency d e v i a t i o n i s t h e modu la t i ng s i g n a l f requency,

i n f i n i t e s e r i e s o f d i s c r e t e s p e c t r a l components u s i n g t r i g o n o m e t r i c expansions and s e r i e s r e p r e s e n t a t i o n s o f Bessel f u n c t i o n s [l-51.

E ( t ) = E c . 3

The above FM Signa l can be expressed as an

Jn(m).Cos[(uc + n .om) - t ] , n = - m

Where Jn(m) a r e Bessel f u n c t i o n s o f t h e f i r s t k i n d and n t h o rde r w i t h argument "m". The numeric va lues o f t h e Bessel f u n c t i o n s Jn(m) f o r d i f f e r e n t "n" express t h e amp1 i tudes o f t h e va r ious frequency components r e l a t i v e t o t h e unmodulated c a r - r i e r ampl i tude. argument "m" and t h e o r d e r "n". These can be found f rom t h e mathemat ica l t a b l e s .

The va lues o f Jn(m) depend on t h e

For example:

If n = 0, Jn(m) = Jo(m), which i s t h e ampl i - t ude o f t h e c a r r i e r component.

If n = +1, Jn(m) = J l (m) and J - l (m) , which a r e t h e ampl i tudes o f t h e f i r s t o rde r s ide - band components.

I f n = ?2, Jn(m) = J2(m) and J-2(m), which a r e t h e ampl i tudes o f t h e second o rde r sideband components.

0018-9316/90/0300-0046$01.00 0 1990 IEEE

F i g u r e 1 shows a g r a p h i c a l r e p r e s e n t a t i o n o f how t h e Bessel f u n c t i o n va lues f o r t h e c a r r i e r and t h e f i r s t twe lve p a i r s o f sidebands v a r y w i t h t h e modu la t i on i ndex [ l - 41 . Bessel f u n c t i o n va lue d im in - i shes as t h e o r d e r o f t h e f u n c t i o n "n" ge ts l a r g e r . As t h e modu la t i on i ndex v a r i e s , t h e c a r r i e r o r t h e sideband f requency p a i r may van ish complete ly . r e s e n t a t i v e spectrum p l o t s f o r s i x d i f f e r e n t va lues o f modu la t i on i ndex a r e shown i n F igu res 2 and 3 [4]. I n FM t h e energy t h a t goes i n t o t h e sideband f requen- c i e s i s t aken f rom t h e c a r r i e r ; t h e t o t a l power i n t h e o v e r a l l composite s i g n a l remains t h e same regard - l e s s o f t h e modu la t i on index.

Rep-

4 3 1

1 0

0 8

0 6

g o 4

5 5 02

P O -

- 0 2

- 0 4

0 1 2 3 4 5 E 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

2 I I ,

m

FIGURE 1. PLOT OF BESSEL FUNCTION OF FIRST K I N D AS A FUNCTION OF ARGUMENT m.

I

.llljlll-- I

A4 - I I

I t ,lllllljlllllll 8

I I I

IC

16

FIGURE 2. FREQUENCY SPECTRUM PLOTS FOR SINUSOIDAL MODULATION, WITH VARIOUS VALUES OF "m" ( " fm" I S CONSTANT AND 'Af" I S VARIED).

Occupied S igna l Bandwidth

Occupied S igna l Bandwidth "BW" o f an FM s i g n a l can be c a l c u l a t e d f o r a s i n g l e tone modu la t i on by t h e f o l l o w i n g formula:

BW = 2-n.fm.

Where "n" i s t h e number o f s i g n i f i c a n t sideband components which depends on t h e va lue of Jn(m) and changes w i t h t h e modu la t i on i ndex "m". " f m " i s t h e modu la t i ng f requency.

16 kHz 0 5

. , I ~ I , / , I ~ I , . I I

1 8 I .

I I

fc

0.5 AilA#,,, 16

FIGURE 3. FREQUENCY SPECTRUM PLOTS FOR SINUSOIDAL MODULATIONy WITH VARIOUS VALUES OF I'm" ( "A f ' ' IS CONSTANT AND " f m " I S VARIED).

The va lue o f 'In" can be a c c u r a t e l y found f rom t h e mathemat ica l t a b l e s by i g n o r i n g sideband components w i t h ampl i tudes Jn(m) l e s s than a c e r t a i n d e s i r e d number. The maximum va lue o f "n" which need be considered f o r a g i v e n "m" may bp found f rom t h e f o l l o w i n g e m p i r i c a l exp ress ion [5]:

0.27 n = m + k- (m)

where "k" i s 2.4 f o r Jn(m) = 0.01, and 3.5 f o r Jn(m) = 0.001.

For a s i n g l e tone 15 kHz modu la t i on w i t h 75 kHz d e v i a t i o n , t h e modu la t i on index i s "5" . I f we i g n o r e components w i t h ampl i tudes l e s s than 1% (Jn(5)<0.01), t h e number "n" i s 9. The bandwidth r e q u i r e d i s 270 kHz.

The s i g n a l bandwidth f o r s t e r e o L e f t o r R i g h t o n l y s i n g l e tone 15 kHz modu la t i on i s t y p i c a l l y l e s s than t h a t f o r monaural modu la t i on . T h i s i s due t o t h e r e d u c t i o n i n modu la t i on index. The f requency d e v i a t i o n i s h e l d cons tan t a t 75 ' kHr b u t t h e com- p o s i t e baseband s p e c t r a l components comprise o f modu- l a t i o n f requenc ies a t 15 kHz ( " L + R" Main Channel), 19 kHz ( P i l o t ) , 23 kHz and 53 kHz ( " L - R " S u b c a r r i e r Channel). The bandwidth c a l c u l a t i o n f o r m u l t i p l e tone o r s t e r e o FM modu la t i on i s q u i t e complex because seve ra l combinat ion f requenc ies must be accounted f o r . Th i s i s caused by t h e n o n l i n e a r process i n - he ren t t o f requency modulat ion.

E f f e c t s O f Bandwidth L i m i t a t i o n s

T y p i c a l l y , bandwidth l i m i t a t i o n s occur when t h e FM s i g n a l passes th rough t h e RF p a t h compr i s ing t h e t r a n s m i t t e r , f i l t e r p l e x e r / c o m b i n e r , antenna system and t h e p a r t i c u l a r l y i n t h e r e c e i v e r .

tw in - tone (10 kHz/25 kHz) modulated FM s i g n a l t h rough a wideband and a narrowband ( - 3 dB bandwidth o f 400 kHz) RF p a t h [7]. The t o p thi-ee p i c t u r e s show t h a t t h e wideband RF p a t h has n e g l i g i b l e e f f e c t on t h e demodulated aud io s i g n a l . p i c t u r e s show t h e d i s t o r t i o n p roduc ts caused by band- w i d t h r e s t r i c t i o n .

F i g u r e 4 shows t h e e f f e c t o f pass ing a

But t h e bot tom t h r e e

..* -

48

PI

49

Stereophonic THD+N At 15 kHz Versus Bandwidth

% THD

i

0 4 0 6 0 8 1 1 5 2

O O l L 1- 1- I I I

Bandwidth (MHz)

3

Composi te THD+N A t 15 kHz Versus Bandwid th

Yo THD

Stereophon ic SMPTE IMD (60Hz I7kHz 4:l Ratio) Versus Bandwidth

Yo IMD

O l r i n--- I I

3

O.O5,',! 0.02

3

Composi te SMPTE IMD (60Hz/7kHz 1:l Rat io) Versus Bandwidth

% IMD

S t e r e o Separa t ion Versus Bandwid th

dB 3

?l$EE$= -60

-70 L i - I I 1 0 4 0 6 0 8 1 1 5 2

Bandwidth (MHz)

3

50

Synchronous AM Versus Bandwidth

dB Below 100% AM Modulation

-1: --- -20

:E ;;=, -60 -70

0.4 0.6 0.8 1 1.5 2 3

Bandwidth (MHz)

FIGURE !C. SYhCHPOi.,(!I'C P Y ilEPSLiS CFI?~LJ~YIDTII

The ccnc lus ion drawn froni ti;e above stuc'y i s t h i t a n:inimum -2 cH bandwidth c f 800 k i j ? 1 5 r e - quired f u r jood audic pcrforrr,ance a n d t h a t exce l i i . n t pt>rformancc Lan be ac!iisved with 1 t ? i . 5 Mhz bond- w d t h [S I .

t t f e c t s G n The TransiTi t ter R F interircdu1dt:on

Frequer.cj Spectruii: i z a very l i m i t e d n a t J r d l r e source . The FM bi-oddcds: b j i 16 i c shared tjj, seve ra l userr. 2: t h e saiw i o c a t i o n . b!hen m u l t i p l e s i g n a l s d re p r e s e n t , 21,y tion-lir,ear dev ice :uch d s tube i n t h e t r a n m i t t e r p o k e r dmpl i t i e r wi 1 1 g?r!ei-dte F F interriioc;ulatiot: prcducts dur to n,ixing o f t h e s e m u l t i p l e s i g n a l s . T h j s niixina w i l l hdke s o w conver- s ion l o s s c i l l led " turn-at .ouna-lois" . The degr;t of i r i teraodul i l t ion i n t e r f e r e n c e generated wi th in c given system cdn t e a c c u r a t e l y p red ic t56 i~i-,eti t h e '-urn- zrcund-loss c i f the t r a n x i t t e r i s a v a i l a b l e .

Turn-Arourtd-Loss ciepinds on t h r e e f a c t o r s

- I n - b a n d Conversior L o s s - Ititerferiric, Signal Attet iuat ion due t c PA

- Attenuat ion of Resul t ing iK products due

[9!.

O u t p u t S e l e c t i v i t y

t o P A C u t p u t S e l e c z i v i t y .

The t r a n s m i t t e r with a r,ilrrow?r bandwidth w i l l have higher s e l e c t i v i t y t t e r e b y maki!iQ i t m r e immune t o F t l i n t e rmodu la t ion . T h e w f o r e , t h e r e i s c e r t a i n l y 2 t r i lae-off between modu!dtion performance arid immunity from RF interniodu.ldtion.

'iriinary Desiry F a i i o r s

The prirdary f a c t o r s b(tiich shouib tJr con-

- Crs i r ed Powtr O u t p c t - Optimur: l'oGuiatiorl Performarce

s i d e r e d i n Power Amplif ier c e s i a n d re :

T h i : p a p e r wii ' f ccus i t s d i scus j io r i or, tt-e second i t,t:rl - t h e i e s i l j i i c o n s i d e r c t i o n s nec r s sa ry t o d :. I I i e v e t h i~ o p t i mum no dd 1 3 t I CI n p e I' f o rim nce .

The l.i-;.rcmitt.t>r power. m p l i f i r r handwiclih ( i f f e c t s t h e ii:odulat;r,ii perforiiiai-,cF. Availzt1:t: baiid-

sponsc:, 2nd group d r l a y resy.ottre. There i: ct t r s d e - o f f jiivolved b e t w e n t h e bdqdwidth, (JdiK: apd e f t i - cier.cy i n t h e c ie5 iq t i of a power c inp l i f i e r .

e s t h e amp1 i tudp resp.,ribt., phs5+ ',-c-

P o w r u ~ , p l i f i t : r bardwidth i: r e s t r i c t e c , by the equiva ' ,ent 1oOc r e s i s t a n c e ;icross p a r a l l e l t u n P d c i w u i t s . Tuned c i r c u i t s i ~ r e necess,ir:. t o cancel l o w r e a c t i v e 1i::pt'dance preier i ted by r c l a t ive l .8 hisii - i n p u t i , r d ou tpu t cdpacit;ricej cf t h e amp1:i'yiny device such <:s d vacu~i;i t i ibe.

T h P baiidwi(!ti-l t o r d s - i ; i g l ? tuned c i r c u i t i s :Iroporticni.,i t o t h e r a t i c (;f capaciLiv(2 r e a c t a n r e , Xc t c load r*siLtat ice , R L ( appea r ing ;:cr'oss t h e tutwd c i r c u i t ) [ l o ] :

xc = f c =

bdndiviCl't/! betwepp l 1 t i 1 f-?oweI- ( o r - 3 < : e ) pci t i ts p rcpor t iond i i t y c o n s t a r t lodd res i s t r i l l cc [appeaririy ac ross t:ined circLi1. j t o t a l c a > a r i t a r , c e o f LurieC c i r c u i t ( i n c l u d e s s t r a y cdpac i t ances a n d ou tpu t o r i npu t capaci tdtices of t h e t i1ix ) c a p a c i t i v t r ezc t ance c f C c a r r i e r {'requency

The U vol::;ic8c rwirig d i ro5s t h e t u r d c i r - c u i t a120 depends cn t h e load r c s i s t a i i c e . For t h e same power d n d e f f i c i e n c y , t h e bandwidth can be i n - r r cased i f tilt capaci t a n w i s reducec.

Input C i r L u i t Cor:ficjc.rations A i d The i r iffcicts On-Tkc I r o i l sm i t t e r 6ar)dwidth T .-

Keiwer t rc i r is ini t ter oe:iqns u t i l i z e s o l i d - s t a t t : i n w r s e d i & t e power a rp l i f i e r s LL provide rieces- s a r i R F d r i v e l eve l t o o p e r a t e t h e tube it: t h e c l a s s C rnnde. The output 1026 iii:pedancc: i s t y p i c a l l y 50 0hi;;s. I t i s , t h e r e f o r e , necessary t o design a match- i n s network t o tracs:-orm d high g r i d inpu t ir,'pedance t o 56 Ohms a t t h e Pi\ il.,put. The, i c l l owing t h r e e types o f i npu t matchi ig c i r c u i t con f igu ra t ions a r e used:

- S i n g l e Eleii!ent C d p i c i t i v e (C) Input Fiatcti - Siny ie Element Induc t ive (I.) ! n p u t Match - Broadband ( L - C ; i n p u t Pktch.

The f i r s t two a r c t h e popular oiies. These matck:ing c i r c u i t : have d i f f e r e n t e f f e c t s on t h e PA m p l i tude ?nO group delay responses .

5 1

This i s necessary t o t r a n s f o r m t h e h i g h g r i d imped- ance t o an e q u i v a l e n t s e r i e s 50 Ohms r e s i s t a n c e w i t h some c a p a c i t i v e reactance. The reactance i s then tuned o u t w i t h a s e r i e s v a r i a b l e i n d u c t o r "Ls" . I n t e r a c t i v e adjustment o f " L i n " and "Ls" i s requ i red . I n t h i s c i r c u i t c o n f i g u r a t i o n , a p a r t o f t h e i n p u t capaci tance i s used f o r impedance t r a n s f o r m a t i o n and t h e e q u i v a l e n t capaci tance across t h e tuned c i r c u i t becomes l e s s , t he reby i n c r e a s i n g t h e bandwidth. c o n f i g u r a t i o n has t h e c h a r a c t e r i s t i c s o f a low pass f i 1 t e r .

T h i s

Z = 50 OHMS

C a p a c i t i v e I n p u t Match

7YC1 ZYC2 7 x 3 7< c4

S i n g l e Element C a p a c i t i v e I n p u t Match ing Th is i s t h e C i r c u i t i s shown i n F i g u r e 11 [6,10].

s i m p l e s t i n des ign as w e l l as implementat ion. V a r i - a b l e i n d u c t o r " L i n " tunes o u t t h e tube i n p u t capac i - tance p a s t p a r a l l e l resonance t o make t h e i n p u t impedance s l i g h t l y i n d u c t i v e . Th is i s necessary t o t r a n s f o r m t h e h i g h g r i d impedance t o an e q u i v a l e n t s e r i e s 50 Ohms r e s i s t a n c e and some i n d u c t i v e r e a c t - ance. The reactance i s t hen tuned o u t w i t h a s e r i e s v a r i a b l e c a p a c i t o r , 'ICs". I n t e r a c t i v e adjustment o f " L i n " and "Cs" i s r e q u i r e d . T h i s c o n f i g u r a t i o n has t h e c h a r a c t e r i s t i c s o f a h i g h pass f i l t e r .

,

C,

(LOW Z 1 50 OHMS

3

7 x 1 >KC2 7Y c3 7 Y c 4

Z -

(LOW Z 1 50 OHMS

50 OHMS

i c i I C 2 I C 3 Lc4

INDUCTIVE INPUT MATCHING

CAPACITIVE INPUT MATCHING

j L p (LOW Z 1 50 OHMS

(HIGH Z 1

(LOW Z 1 50 OHMS ;f;cP

EQUIVALENT 'L' NETWORK FIGURE 12. INDUCTIVE INPUT MATCHING C I R C U I T

Broadband L-C I n p u t Match EQUIVALENT 'L' NETWORK

FIGURE 11. CAPACITIVE INPUT MATCHING CIRCUIT

Broadband L-C I n p u t Match ing C i r c u i t i s Th is i s an e x t e n s i o n o f shown i n F i g u r e 13 [lo].

L-Match c i r c u i t . w i t h each s e c t i o n p r o v i d i n g a smal l s t e p i n t h e t o t a l impedance t r a n s f o r m a t i o n . broadband impedance match w i t h o u t i n t e r a c t i v e a d j u s t -

c o n f i g u r a t i o n a l s o u t i l i z e s a p a r t of t h e i n p u t capac i tance f o r impedance t r a n s f o r m a t i o n and has t h e c h a r a c t e r i s t i c s o f a m u l t i p l e s e c t i o n low pass f i l t e r .

I t u t i l i z e s m u l t i p l e L-C s e c t i o n s I n d u c t i v e I n p u t Match

S i n g l e Element I n d u c t i v e I n p u t Match ing Th is technique p rov ides a C i r c u i t i s shown i n F i g u r e 12 [6,10]. n e x t most popu la r method f o r i n p u t match ing. The ment and improves t h e t r a n s m i t t e r o p e r a t i o n . Th is v a r i a b l e i n d u c t o r " L i n " i s used t o tune o u t t h e i n p u t capaci tance s l i g h t l y before t h e P a r a l l e l resonance i s reached, on t h e c a p a c i t i v e s i d e .

Th is i s t h e

BROADBAND 'L-C' INPUT MATCHING

(HIGH Z 1

EQUIVALENT NETYORK

FIGURE 13. BROADBAND L-C INPUT MATCHING CIRCUIT

52 Computed And Measured Amplitude,’Group Delay Responses O f C a p a c i t i v e And Broadband I n p u t Match ing C i r c u i t s Computer so f tware programs ECA [ll] and

=Superstar= [12] were used t o analyze t h e c i r c u i t de- s igns and o b t a i n t h e response p l o t s . Hew le t t Packard Network Analyzer Model 3577A was used t o o b t a i n t h e measured response p lo t s *

F i g u r e 14A shows t h e computed amp l i t ude r e - sponse (S21) and group de lay (DLY) response p l o t s f o r c a p a c i t i v e i n p u t match ing c i r c u i t . F i g u r e 148 shows the i n p u t VSWR (V11) and i n p u t r e t u r n l o s s (S11) p l o t s a t -0.05 dB response p o i n t s . The peaks o f am- p l i t u d e and group de lay p l o t s do n o t c o i n c i d e i n t h e case o f c a p a c i t i v e i n p u t match ing c i r c u i t . F i g u r e 15 shows t h e measured amp l i t ude response, group de lay response and i n p u t r e t u r n l o s s p l o t s o f a t y p i c a l 20 kW FM t r a n s m i t t e r a t t h e tube g r i d r i n g f o r capac- i t i v e i n p u t match ing c i r c u i t .

The r e s u l t s o f computer a n a l y s i s and a c t u a l measurements made w i t h two d i f f e r e n t i n p u t c i r c u i t c o n f i g u r a t i o n s , C-Match and Broadband L-C Match, i n a r e a l t r a n s m i t t e r o p e r a t i n g a t 20 kW RF power a r e presented below.

Two i n p u t match ing c i r c u i t s were des igned t o t r a n s f o r m t h e h i g h impedance o f PA i n p u t tuned c i r c u i t t o 50 Ohms r e s i s t i v e impedance. purpose of i l l u s t r a t i o n i t was assumed t h a t a t o t a l capac i tance o f 165 p i c o f a r a d s i n p a r a l l e l w i t h 375 Ohms l o a d r e s i s t a n c e appeared across t h e tuned c i r c u i t . Th i s rep resen ts t h e i n p u t impedance o f a t y p i c a l 20 kW FM t r a n s m i t t e r power a m p l i f i e r i n p u t c i r c u i t u t i l i z i n g an Eimac 8989 t e t r o d e .

-30 1.2 : : : : : : : : : 20

For t h e

.. t t t t t t t t t ’ ’

s11

t t t t t t t

1.1 ..

t t t t t t t t t t t

t t t t t t t t t

t t t t t t t t t

i t t t t t t t t

95.37 95.5 95.63 95.37 95.5 95.63

14A. AMPLITUDE (91) AND 14B. INPUT VSWR (V11) AND GROUP DELAY (DLY)

FIGURE 14. COMPUTED AMPLITUDE, GROUP DELAY, VSWR, AND RETURN LOSS RESPONSES OF A C A P A C I T I V E INPUT MATCHING CIRCUIT

RETURN LOSS (S11) RESPONSES

R E F L E V E L / D I V M A R K E R 95 509 0 O O . O O O H z 202. 50nSEC 50. O O O n S E C D E L A Y (9) -7. 275dBm 0. 050dB MARKER 95 509 000. OOOHz

199. 95nSEC

-7. 28 1 dBm SPAN = 3 0 0 ~ H z MAG <E>

FIGUKt 15. MEASURED AMPLITUDE, GROUP DELAY, AND INPUT RETURN LOSS RESPONSES OF A CAPACITIVE INPUT MATCHING CIRCUIT I N A 20 kW TRANSMITTER

54

F i g u r e 18 shows t h e comparison o f computed and measured bandwidth o f a t y p i c a l 20 kW FM t r a n s - m i t t e r a t tube g r i d r i n g . Broadband L-C i n p u t match ing c i r c u i t has a h i g h e r bandwidth than t h e c a p a c i t i v e i n p u t match ing c i r c u i t .

Computed

Measured

Computed And Measured Bandwidth Of A Typical 20 kW FM Transmitter

At Tube Grid Ring

I ~- - - .--

-7

I

. 15

Measured

0 100 200 300

Bandwidth At -.OS dB Response Polnts

0 Series C Input Match 0 Broadband L-C Input Match

FIGURE 18. COMPUTED AND MEASURED BANDWIDlH OF A TYPICAL 20 kW FM TRANSMITTER A T TUBE G R I D R I N G

F i g u r e 19 shows t h e comparison o f computed and measured asymmetry o f a m p l i t u d e l d e l a y response peaks o f a t y p i c a l 20 kW FM t r a n s m i t t e r a t t ube g r i d r i n g . C a p a c i t i v e i n p u t match ing c i r c u i t has a g r e a t e r group de lay asymmetry than e i t h e r t h e induc- t i v e o r broadband L-C matching ( c i r c u i t .

Computed And Measured Asymmetry 01 ArnplitudelDehy Resprnsa Peaks of

A Typical 20 kW FM Transmitter At Tube Grid Ring

FIGURE 19. COMPUTED AND MEASURED ASYMMETRY OF AMPLITUDE/DELAY RESPONSE PEAKS OF A TYPICAL 20 kW FM TRANSMITTER AT TUBE G R I D R I N G

Output C i r c u i t C o n f i g u r a t i o n s And T h e i r E f f e c t s On The T r a n s m i t t e r Bandwidth

Tuned c i r c u i t s a r e r e q u i r e d t o resona te t h e o u t p u t capac i tance o f t h e vacuum tube and t o p resen t a f a i r l y h i g h impedance between anode and cathode (sc reen g r i d i n case o f t e t r o d e s ) t o t h e fundamental c a r r i e r f requency component and p resen t a low imped- ance t o t h e harmonics. u t i l i z e a lumped i n d u c t o r , a s t r i p l i n e , o r h i g h e r " Q " ( l o w l o s s ) t ransmiss ion l i n e s e c t i o n t o resonate t h e tube capaci tance.

The o u t p u t c i r c u i t may

I n newer des ign t r a n s m i t t e r s , t h e tube power a m p l i f i e r i s t y p i c a l l y cons t ruc ted i n a c a v i t y enc losu re u t i l i z i n g ( l a r g e r p h y s i c a l s i z e ) c o a x i a l t ransmiss ion l i n e s e c t i o n o f e i t h e r q u a r t e r - wavelength, o r ha l f -wave leng th t o i nc rease t h e unloaded 'IQ" and m in im ize l osses .

The Power A m p l i f i e r e f f i c i e n c y depends on t h e RF p l a t e v o l t a g e swing developed ac ross t h e l o a d r e s i s t a n c e , t h e p l a t e c u r r e n t conduc t ion ang le and t h e c a v i t y e f f i c i e n c y . The PA c a v i t y e f f i c i e n c y i s r e l a t e d t o t h e r a t i o o f loaded t o un loaded " Q " as f o l l o w s [ lo] :

N = 1 - & x 1 0 0 QU

Where: N = e f f i c i e n c y i n p e r c e n t QL = loaded " Q " o f c a v i t y QU = unloaded 'IQ" o f c a v i t y

The loaded 'IQ" i s dependent on t h e equ iv - a l e n t p l a t e l o a d r e s i s t a n c e presented ac ross t h e tuned c i r c u i t and o u t p u t c i r c u i t capaci tance. Un- loaded " Q " depends on t h e c a v i t y volume and t h e RF r e s i s t i v i t y o f t h e conductors due t o s k i n e f f e c t s . A h i g h unloaded " Q " i s d e s i r a b l e , as i s a l ow loaded " Q " , f o r b e s t e f f i c i e n c y . As t h e loaded "Q" goes up t h e bandwidth decreases. For a g i v e n tube o u t p u t capac i tance and power l e v e l , 1 oaded " Q " decreases w i t h decreas ing p l a t e v o l t a g e o r w i t h i n c r e a s i n g p l a t e c u r r e n t . The inc rease i n bandwidth a t reduced p l a t e v o l t a g e occurs because t h e l o a d r e s i s t a n c e i s d i r e c t l y r e l a t e d t o t h e RF v o l t a g e swing on t h e tube e l ement .

For t h e same power and e f f i c i e n c y , t h e bandwidth can a l s o be i nc reased i f t h e o u t p u t capac i - tance i s reduced. Power tube s e l e c t i o n and min imiza- t i o n o f s t r a y capac i tance a r e areas o f p a r t i c u l a r concern i n PA des ign f o r maximum bandwidth. w i d t h can be f u r t h e r improved by m i n i m i z i n g added t u n i n g capac i tance and by p r o p e r l y s e l e c t i n g t h e o u t - p u t c o u p l i n g method.

o f o u t p u t c o u p l i n g c i r c u i t s used:

Band-

The f o l l o w i n g a re t h e two popu la r methods

- Ser ies C a p a c i t i v e Output Coup l i ng - Magnet ic Output Coupl ing Loop.

These c i r c u i t s have d i f f e r e n t e f f e c t s on t h e PA amp l i t ude and group de lay responses.

Se r ies C a p a c i t i v e Output Coup l i ng

F i g u r e 20 shows a schematic o f a t e t r o d e power a m p l i f i e r w i t h a c a p a c i t i v e o u t p u t t u n i n g and c a p a c i t i v e o u t p u t c o u p l i n g c i r c u i t [6,10].

5 5

t "b'

I i ~~ c

F i g u r e 2: i s a s i m p l i f i e d diacjraii showins ttx c o n s t r u c t i o n o f t h i s t y p e o f o u r p u t c i r c u i t i r : a cuar ter -wave l e n g t h c c a x i a l c3Vit.y [6,1O]. The tube cnode i s , c c b p i e a tl-LrcJuSh 6 DC h l o c k i r g c a p a c i t o r t o the transr,i:csion l i n e . The t u b e ' s c i i t p t i t capa i s S r o u g h i LO resonant? hy t h e induc-Live comporetil. cif t h e t r a n s r i s s i o n l i n t t h a t i s p h 2 s i c a l l y l ess t h a n a q u a r t e r - w v e l e n g t h l o n g . The coarse outpL;t t u n i n g i s i c n e by rnuving t h e Groui-i?, p l a n e d i ',mi iwpedanie e t i d of t h e l i w . V a r i a b i i capac i to r : ti1.e used tc. .I ine- tu r ie t h e o u t p u t rescr,anl; c i r c [ ! i t and t o coirple t h e power f r o m t h e h i g h KF v o l t a g e p o i n t l o c a t x l a t t h e iir;ode end o f t h e quar ter -wave l i n e t c t h e j o d d . An i n t e r a c t i v e ad jus tmer l t C J ~ run ing and l o a d i n g i s r e q u i r e d i n t h i s t y p e o f power a m p l i f i e r c a v i t y des;yti. T h i s t y p e o f o u t p u t c o u p l i n g i s s i m i l a r t o a s e r i c 3 c a p a c i i i v e i n p u t rnatct,it:g c i r c u i t d iscussed above.

A c i i ~ ; i i t i v e . o c ? p l ; t coupl i t :9 c i r c u i t SS d e s i y i i e d t o t r c r l s f o r m til? h i 2 l i iiripebanct: o f t h e ?A o u t p u t tuned c i r c u i t ( w i t h t o t o i capac i t t i i cc . c f 23 ;3 ico far?ds ur?d 15510 Ohn.2 l o a d r e s i s t t r i c e dcros:, i t ) t o 5C Ohms. l h i s i l l u s t . r a t e s t h e ouj.bCt c i r c u i t of 2 t y p . l ~ < ~ ! 20 kW FY t r a i i s i : : i t t e r P;, i i t i l i z i r - 9 ; i t i Einiac b%'1 ye t rode. h i p u t e r softv.i.re prograi:.ii; Lei; 2 n d =SuperStar- 1121 were i : s t d t o s n a l y y e t h e c i r c L i t des ig i i o b t a i n i i i f response p l c t s .

Fi9ut-e 7 2 1 strows t h e c w p o t t d amp1 i tude rc.spc;ril;e ( 5 2 1 ) and group d e l a y (C1.Y: respons? p1oi.s. Figurt: :?E shpws ?he :;utput \:WP ( V l l ) arr! o u t p u t r e t u r r l o s s ( S l l ) p l o t s a t -('i.Ob dB resp,'li:t i j o i r , t s . The peaks o f u r i p l i t u d e and group delGy p l o t s do n o t c o i n c i d e ~ I I t h e cast) U: c z p a c i t i v e w t p b t c o u p l i n g c i r c u i t.

56

EXHAUST A I R 4 tHV A

SHORTING GROUND PLANE COARSE/FINE TUNING

P t I F C

4

F t

SHORTING GROUND PLANE COARSE/FINE TUNING

H I G H RF P O T E N T I A L

RF OUTPUT m jONATING' I I E L E C T R I C

OUTPUT COUPLING (LOADING)

DLY

t + + t + + + + +

t t t t t i i t t

t t t t t t , + t

t + t t + + + + ,

95.33 95.5 95.67

1 . 2

95.33 95.5 95.67

Magnetic Output Coupl ing Loop

F igu re 23 shows t h e s i m p l i f i e d diagram showing the c o n s t r u c t i o n o f a magnet ic o u t p u t cou- p l i n g l o o p c i r c u i t i n a f o l d e d hal f -wave c o a x i a l c a v i t y w i t h a secondary r e - e n t r a n t l i n e s e c t i o n [5,9]. The t u b e ’ s o u t p u t capac i tance i s b rough t t o resonance by t h e i n d u c t i v e component o f t h e t r a n s - m iss ion l i n e t h a t i s p h y s i c a l l y l e s s than a hal f -wave l e n g t h long. The coarse o u t p u t t u n i n g i s done by p r e s e t t i n g t h e depth o f t h e secondary l i n e i n t o t h e c a v i t y . The PA o u t p u t f i n e t u n i n g i s accomplished by v a r y i n g t h e p h y s i c a l l e n g t h o f a f l e x i b l e ex tens ion ( b e l l o w s ) on t h e end o f t h e secondary t ransmiss ion l i n e . Th is t ype o f o u t p u t c i r c u i t does n o t r e q u i r e a p l a t e b l o c k i n g c a p a c i t o r i n t h e anode. The o u t p u t power i s coupled t o t h e l o a d by means o f a magnetic l oop p o s i t i o n e d near the RF v o l t a g e n u l l p o i n t l o c a t e d near t h e c e n t e r o f t h e p r imary t ransmiss ion l i n e where t h e RF c u r r e n t i s maximum.

The c o u p l i n g t o t h e c a v i t y v a r i e s as t h e square o f t h e e f f e c t i v e l o o p area and i n v e r s e l y as t h e square o f t h e d i s t a n c e o f t h e l o o p c e n t e r f rom t h e c a v i t y c e n t e r a x i s . There i s a min imal i n t e r - a c t i o n between t h e t u n i n g and l o a d i n g adjustments i n t h i s t y p e o f power a m p l i f i e r c a v i t y design.

Furthermore, t h i s t y p e o f c a v i t y has a g r e a t e r turn-around- loss due t o i t s h i g h e r s e l e c t i v - i t y and p rov ides b e t t e r p r o t e c t i o n a g a i n s t RF i n t e r - ference. An a d d i t i o n a l 10 dB p r o t e c t i o n i s a v a i l a b l e f o r an i n t e r m o d u l a t i o n p roduc t due t o an i n t e r f e r i n g s i g n a l f requency separated f rom t h e c a r r i e r by 4 MHz.

A magnet ic o u t p u t c o u p l i n g l oop c i r c u i t was designed t o t r a n s f o r m t h e h i g h impedance o f t he PA o u t p u t tuned c i r c u i t ( w i t h t o t a l capaci tance o f 20 p i c o f a r a d s and 1590 Ohms l o a d r e s i s t a n c e across i t ) t o 50 Ohms. Th is i l l u s t r a t e s t h e o u t p u t c i r c u i t o f a t y p i c a l 20 kW FM t r a n s m i t t e r PA u t i l i z i n g an Eimac 8989 t e t r o d e . Computer so f tware programs ECA [ll] and =Superstar= [ 1 2 ] were used t o analyze t h e c i r c u i t des ign and o b t a i n t h e response p l o t s .

F igu re 24A shows t h e computed amp l i t ude response (S21) and group de lay (DLY) response p l o t s . F igu re 248 shows t h e o u t p u t VSWR (V11) and o u t p u t r e t u r n l o s s (S11) p l o t s a t -0.05 dB response p o i n t s . The peaks o f amp l i t ude and group de lay p l o t s c o i n c i d e i n t h i s case p r o v i d i n g a symmetr ica l group de lay response.

COARSE TUNING

SECOND HARMONIC MODE SUPPRESSOR

~I /

II : ]NE TUNING

A

’rr

I I

rn +Hv +HV

RF

WTPUT COUPLING (LOADING)

RF

WTPUT COUPLING (LOADING)

I // I I IJ

HIGH DC AND RF POTENTIAL

FIGURE 23. FOLDED HALF-WAVE COAXIAL CAVITY WITH I N D U C T I V E TUNING AND MAGNETIC OUTPUT COUPLING LOOP

58

-30

+ + + + +

t t t t i t t

sz1 I , t + + + t + i t ''

.. + + t + + t t i +

.. t + t + + t t t t

.. + + + + + + + t t ''

l , t * + + t + i t l

.. + + t + + t t i +

.. t + t + + t t t t

.. + + + + + + + t t ''

9 5 . 4 1 95.5

The i h - ? C B t ranr i1 , i i ter wai; uced t n i l l u s - t r a c e i r Pxmplp r l f a redl wor-d t u b e power ; : l ,b l i f ie i . , desigced f o r o p t i r t u i p e r f u i m n c e . The i )wtr dinpi i- f i e r 1 : w 5 an Ein!ac &W3/ 4cXI?,-(J(jA high r,ain t e t r c d t tube ir: t h e tuldecl half-wave ct :vi ty outpiit c i r c u i t with inapnctic coliplir!c, l c rop. -:hi. FY-?(;Lj a l s o uses a Lircddbdili I.-C rnatchiiig c i r c u i t d t t h p t u b e g r i d i n p u t t o p-itliiliizf sicJrial df?grac'atioii.

An RF s w p l e froni c.i:,Per t he exciter o u t - p u t , o r the PA tube c r i d r i n g , cir .cl:? P.4 oiit.put was conr.ected t o t k t F l W 2 . The c wideba:ld composite o u t p u t s \';er+ useo itii- :oinposi t F t e s t i . l h e coinposite baseband 'ha: used t c J < r i v e the FMS-2. The i!?codec - , e f t and r-lcht output5 G : tile FP5-2 were u s e d f r r the s t ' r e o per fomsr , ce t e s t s .

a. 3 t- W cn t- cn W t- W 0 Z 3 [I: 0 LL [I: LLI a, Z 0 i= 4 3 n 0 2 a v 2

a

0

LL

I

&$I-

a W

E I 0 L U a t rn 0 N

._

L

- c

. .. , i.

60

VOLTAGE CURRENT POWER O U T P U T D I S S I P A T I O N

P L A T E S C R E E N G R I D 1 A U T H O R I Z E D 2 0 . 0 0 K W = 1 0 0 $ 0.OKW E R P 8 . 9 4 K V 7 1 0 V - 2 9 8 V 1 A C T U A L 2 0 . O O K W = l O O % O.0KW E R P

2 . 8 1 A 1 4 9 M A 3 1 M A 2 0 . 0 0 K W R E F L E C T E D O.OOKW= 0 %

5 . 1 2 K W 106W 1 VSWR 1 . 0 : 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I

I N T E R M E D I A T E POWER A M P L I F I E R ( I P A ) -1- - 2 -

VOLTAGE 2 7 . 9 V 2 8 . 0 V CURRENT 1 1 . 4 a 11.3A i FORWARD POWER 2 0 4 W 2 0 3 W I R E F L E C T E D POWER 2W 1 w D I S S I P A T I O N 1 1 4 W 1 1 3 W ~

T O T A L POWER FWD= 3 6 7 W R F L = I W

E X C I T E R FORWARD POWER 3 6 W E X C I T E R R E F L E T E D POWER 1 W I

EXHAUST A I R T E M P = 4 6 . C _---____---________-____________________ 1 A P C ON

R E F LEVEL /D:V M A R K E R 95 499 5 0 0 . O O O H z 520. O O r i S E C 50. O O O n S E C D E L A Y <B> 509. 5 4 n S E C -5. 4 n n d ~ m 1. ocndB MARKER 95 499 5 0 0 . 0 O O H z

M A G < B > -5. 406dBm T - - - --- - - - - r--

D B INPVPU 7 M a i q

XMrR O U T P Y T --- ___-_._

-

61

028 002 r-- ,022

FM-20B Synchronous AM

~

F i gu re 28 shows t h e measured synchronous AM (amp1 i tude modu la t i on ) performance o f t h e t r a n s m i t t e r . The performance degrada t ion i s 11 dB f rom t h e e x c i t e r t o t h e tube g r i d r i n g and 2 dB f rom t h e g r i d r i n g t o t h e t r a n s m i t t e r o u t p u t .

FM-POB Synchronous AM

dB Below 100% AM Modulation

-,:r;..-ril ~~ -20

-30 -40 -50 ~

-60

-70

-,

~~~

I ~ ~ _ _ _ ~ ~

U Exciter Output

0 Transmitter Output

U Tube Grid Ring

FIGURE 28. FM-206 SYNCHRONOUS AM

FM-EOB Composite Ampl i tude Response A t 100 kHz

F i g u r e 29 shows t h e measured composite amp l i t ude response performance degrada t ion o f 0.08 dB f rom t h e e x c i t e r t o t h e tube g r i d r i n g and a f u r t h e r 0.08 dB f rom t h e g r i d r i n g t o t h e t r a n s m i t t e r ou tpu t .

FM-POB Composite Amplitude Response At 100 kHz

dB

-0.15

-0.2

U Exciter Output 0 Transmitter Output

FIGURE 29. FM-2OB RESPONSE

17 Tube Grid Ring

COMPOSITE AMPLITUDE AT 100 kHz

FM-20B Composite THD+N A t 15 kHz

F igu re 30 shows an inc rease i n T H D t N a t 15 kHz o f 0.006% f rom t h e e x c i t e r t o t h e tube g r i d r i n g and 0.052% f rom t h e g r i d t o t h e t r a n s m i t t e r o u t p u t .

FM-POB Composite Total Harmonic Distortion And Noise At 15 kHz

% (THD+N)

008- -

0 04

U Exciter Output

n Transmitter Output

0 Tube Grid Ring

FIGURE 30. FM-206 COMPOSITE TOTAL HARMONIC DISTORTION AND N O I S E AT 15 kHz

FM-20B Composite THDtN A t 400 Hz

F i g u r e 31 shows 0.001% inc rease i n THDtN a t 400 Hz f rom t h e e x c i t e r t o t h e tube g r i d r i n g and no performance degrada t ion f rom t h e g r i d r i n g t o t h e t r a n s m i t t e r ou tpu t .

FM-POB Composite Total Distortion And Noise At

% (THD+N)

0.003

0.002

0001 I

Harmonic 400 Hz

I E rl Exciter Output n Transmitter Output

U Tube Grid Ring

FIGURE 31. FM-206 COMPOSITE TOTAL HARMONIC DISTORTION AND N O I S E AT 400 Hz

FM-206 Composite SMPTE I M D (60Hz/7kHz 1:l R a t i o )

F igu re 32 shows t h e performance degrada t ion o f 0.002% I M D f rom t h e e x c i t e r t o t h e tube g r i d r i n g b u t no change f rom t h e g r i d r i n g t o t h e t r a n s m i t t e r ou tpu t .

FM-POB Composite SMPTE lntermodulation Distortion (60Hz/7kHz 1:l Ratio)

% IMD

1 I I I

n Tube Grid Ring U Exciter Output

U Transmitter Output

FIGURE 32. FM-206 COMPOSITE SMPTE INTERMOOULATION D I S T O R T I O N (60 Hz/7 kHz 1:l RATIO)

FM-PO0 Composite FM Signal-to-Noise Ratio

dB Below + I - 75 kHz Deviation At 400 Hz 0

-20

-40

-60

-80

-100

FM-PO0 Stereo Total Harmonic Distortion And Noise At 15 kHz

% (THD+N)

0 08

0 06

[A Exciter Output 0 Transmitter Output

fl Tube Grid Ring

FM-PO0 Stereo Total Harmonic Distortion And Noise At 400 Hz

% (THD+N) ~- 002 -~

0015 - -~

001 - --I r1 017 To1i-r

0 005 -1 I

0 --

FM-PO0 Stereo SMPTE lntermodulation Distortion (60Hz/7kHz 4:l Ratio)

% IMD

0 0 4 ~ ~ _ _ _ _ _ _ _ _

0 0 3 k ~

i

63

ri Exciter Output

n Transmitter Output

n Tube Grid Ring

Figurr. L O shows ?I G cT- a rcp in st;>r?:c :epdi.ition f ro r , t b t c x c i t e r L O tbe +ubi. g r i d r inc; uno a t t i r t b e r 3 r!P rirop f r o m t h e g r i d r i n g t,o i k ~ t r a n s - 1-1; b t e r outFtit.

FM-PO0 Stereo Separation At 400 Hz

A ii l h LI k n Exciter Output U Transmitter Output

n Tube Grid Ring

FILbKC 3G. Fi"-?OB STEF.CG Sti'AKAT:TiF* F T 4!10 Hi!

C C N C L U S ION-!

The d e s i g r i Gf tuhe p o w r ;n .pl i f i t?rr f t r :)ptimum FP, t rdnsn i i t t c r p r f o r m r c e r e q u i re5 c s r e f u l cclnsiderat icns i n t h e clicice of i n p L t 3 r d ou tp i i : c i v - c u i t s dut t o t h e i r e f f e c t s on t h e Lrdnsmi t t e r jiiipli- tude and g r o u p delay respon:,es. The cociclusiiitls rrdclied a r c <is fo l lows :

1. P F bandwidtti a f t e c t ; a u d i o perfori.:ancti. I t i s , t h e r e f o r e , neces;ary t o iniririize bdrldwidttl l i m i t i n g coinpLnents i i i t k P F Gath t o reolice F1erforrn;ricr deg radd t ion .

2. Good engi i ieer ing judcjcvent i s c a l l e d f o r t.o tla:aiice t h e t r a d e - o f f s Lictwern bandkidtli and i r imni t y t o RF iriternod- u l a t i o n . A hzndwidth of 1 .0 t o 1 . 5 W z s e e i i , ~ aaequatc. f o r exc;.] lei:t inoduliition :)erformane€ w b i I s prcvidiny a r eascnab le degree o f ir;iiiun.ity t o hF intermoddlc- t i on.

64

REFERENCES

[l] Reference Data f o r Radio Ehgineers, Howard W. Sams & Co., Inc. , S i x t h E d i t i o n 1983.

[ Z ] F r e d e r i c k Terman, E l e c t r o n i c and Radio Engineer ing, McGraw-Hil l Book Company, 1955.

[3] Samuel Seely, E lect ron-Tuhe C i r c u i t s , McGraw-Hil l Book Company, 1950.

r 4 1 Herbe r t L. Krauss. Char les W . Bos t ian . and _ _ F r e d e r i c k H. Raab; S o l i d S t a t e Radio Engineer ing, John W v S o n s , 1980.

[5] K e i t h Henney ( E d i t o r i n C h i e f ) , Radio Eng ineer ing Handbook, McGraw-Hil l Book Company, 1950.

[6] E.B. C r u t c h f i e l d ( E d i t e d By) , NAB Eng ineer ing Handbook, N a t i o n a l A s s o c i a t i o n o f Broadcasters , Seventh E d i t i o n , 1985.

[ 7 ] Geo f f rey N. Mendenhall , "The Composite S igna l - Key t o Q u a l i t y Broadcast ing," Broadcast E l e c t r o n i c s , I nc . , 1981.

[8] Edward J . Anthony, "Optimum Bandwidth f o r FM Transmission," Broadcast E l e c t r o n i c s , Inc. , 1989.

Geof f rey N. Mendenhall , " A Study o f RF I n t e r m o d u l a t i o n Between FM Broadcast T r a n s m i t t e r s Shar ing F i l t e r p l e x e d o r CO-located Antenna Systems," Broadcast E l e c t r o n i c s , I nc .

i.T.M. L y l e s and Mukunda B. Shrestha, T r a n s m i t t e r Performance Requirements f o r

S u b c a r r i e r Operation," Broadcast E l e c t r o n i c s , I nc .

" E l e c t r o n i c C i r c u i t Ana lys i s 2.31 MS-DOS Version," Copy r igh t 1988 by Tatum Labs, I n c .

"=SuperStar= Vers ion 3.2," Copyr igh t 1988 by C i r c u i t Busters , I nc .

[9]

[ l o ]

[ l l]

[12]