By Electronics/BE-The... · baseband without clipping figure 6a (output from bei fs -30 stereo generator one channel only modulated @ 10khz) figure 6b baseband after 1.25db composite

1=E

I -E

"THE COMPOSITE SIGNAL - KEY TO QUALITY FM BROADCASTING"

By

Geoffrey N. Mendenhall, P.E.Manager FM Products

Broadcast Electronics, Inc.Ouincy, IL

BROADCAST ELECTRONICS INC.4100 N 24th ST P 0 BOX 3606 QUINCY. IL 62301 PHONE 217 224-9600

SUMMARY

"The Composite Signal - Key to Quality FM Broad-

casting" is a technical paper oriented toward helping

the station engineer better understand how the trans-

mission system affects the composite signal.

The author explains how new circuitry can help

reduce distortion of the composite signal.

The final portion of this paper describes tech-

niques that the station engineer can use to improve

the performance of the existing equipment.

THE COMPOSITE SIGNAL - KEY TO QUALITY FM BROADCASTING

This discussion of the FM composite baseband deals more

with the practical, operational aspects of an FM transmitting

system rather than an in-depth, theoretical analysis of the

modulation/demodulation process.

The composite signal is the sum of all the individual

components that make up a mulitplex system. FM broadcast-

ing usually includes these individual components, as shown

in Figures 1A, 1B, and 1C:

COMPOSITE BASEBANDSPECTRAL COMPONENTS

1. L + R information at the audio modulating

frequencies (30Hz to 15KHz).

2. The 19KHz pilot tone.

3. L -R information at 38KHz plus and minus the

audio modulatirg frequencies (23KHz to 53KHz).

4. The 67KHz FM modulated SCA subcarrier (53-81KHz).

Figure lA

-2-

TIME DOMAIN SCOPE WAVEFORM OF COMPOSITE BASEBAND CONTAININGSTEREO (ONE CHANNEL ONLY AT 10KHz) PLUS UNMODULATED 67KHz

(OUTPUT TAKEN FROM COMPOSITE TEST JACK ON BEI FX-30 EXCITERDRIVEN BY BEI FS -30 STEREO GENERATOR AND BEI FC-30 SCA GENERATOR)

Figure 1B

FREQUENCY DOMAIN SPECTRUM OF COMPOSITE BASEBAND CONTAININGSTEREO (ONE CHANNEL ONLY AT 10KHz) PLUS UNMODULATED 67KHz SCA

(SPECTRUM AT COMPOSITE TEST JACK ON BEI FX-30 EXCITER DRIVENBY BEI FS -30 STEREO GENERATOR AND BEI FC-30 SCA GENERATOR)

Figure 1C

-3-

Figure 1B shows what the composite baseband looks

like if viewed on an oscilloscope with peak -to -peak ampli-

tude shown as a function of time. It is difficult to identi-

fy the various components as a function of time.

Figure 1C shows the composite baseband as viewed on

a low frequency spectrum analyzer. This is a representation

of amplitude as a function of frequency. It is now easy

to identify the various frequency components within the

baseband.

Non -linearity within the modulation/demodulation process

will alter the composition of the baseband, which results

in distortion of the demodulated signal.

Let's assume that we have a perfect demodulator, and

focus our attention on the transmission portion of

the total system. I will also assume that we have perfect

output signals from our stereo and SCA generators. Now

we are left with three areas for signal degradation to

occur as shown in Figure 2.

AREAS AFFECTING THE COMPOSITE BASEBAND

I. The composite link to the FM modulator (stereo

generator, SCA generator, composite processor,

and STL equipment)

II. The FM modulator

III. The RF path to the demodulator (FM exciter, :PA,

PA, and antenna system)

Figure 2

Each of these three areas has its own special effect on

the baseband signal. I'll discuss each of these in detail.

-4-

I. The Composite Link

The composite path from the stereo and SCA

generators to the FM modulator should be linear

in both amplitude vs. frequency and in phase vs.

frequency response. Simply stated, this means that

no frequency component within the baseband should

be attenuated more than any other frequency component.

Furthermore, all frequency components should propa-

gate thru the system at the same speed (constant

group delay) and thus arrive at the modulator

at the same time. EQ. lA and EQ. 1B mathematically

relate stereo separation to amplitude response.

EQ. 2A and EQ. 2B mathematically relate stereo

separation to phase response.

-5-

EQ. 1A

EQ. 18

STEREO SEPARATION AS A FUNCTIONOF AMPLITUDE RESPONSE

_I [COS "e" "1)2 4- (5/AI el GENERALfue-)SEPARATION (A, a

(C 0 s (soy e-)2- FORM

SEPARAT/ON (A) ((7-,92 49- c (PERFECT PHASE)

568- AtIRI/TUDE RATIOWHERE: A =1,74/A/

..= fA1ASE ERRO/i /N DEGREES

STEREO SEPARATION AS AFUNCTION OF PHASE RESPONSE

(GROUP DELAY)

EQ. 2A SEPARAT/ON (A, e-)-(cos 6#A)2 # (SIN -11 2

GENERAL(COS e-A)2+(s//v-e)2 FORM

[cos -e,i)2#(stiv e)21' SF A=/E -C, 28 S I -PA RAT I OA/ ($) = (COS e-1)2. (SI AI -69z (PERFECT

AMPLITUDE)

WHERE: -e- = RIM SE ERROR IN DEG/41-Es

5v8 -L R - /IMP 7-44D F R/017-70

-6-

Figure 3A and Figure 36 show time domain pictures

of non -ideal amplitude response.

Figure 4A and Figure 4B show time domain pictures

of non -ideal phase response.

-7-

0

Correct phasing and equal group delay of

the pilot tone is also essential to achieving stereo

separation.

The final stereo performance of the complete

system will be determined by the algebraic summa-

tion of the individual composite amplitude response

and composite phase response of each device within

the composite signal path.

The exciter, STL link, and any other composite

device should specify these composite performance

parameters so that total system Performance can

be easily predicted. In order to maintain a system

separation of greater than 45dB, the composite

amplitude response must be within +/- 0.07dB or less, 30Hz

to 53KHz and the composite phase response must be less than

+/- 0.45° from linear phase 30Hz to 53KHz as calculated

in Figure 5A,

CALCULATED SEPARATION

SEPARAT/OA/ (4, 6) - [Dos e.A)2-# (s/A1(C40.549 --A)2,- (SiNte-)z

WHERE: A = 0.99209 OR /. 00 798 (t 0.06 9 04ANDe 045°S ,19.) = SEPARATION AS A FU/vc7/0A/

OF AND -0-

;2-

(A,,e)-rcos 0.4-5°4-0.99209)z* (Sw 0.45

(coS 0.45°- 0-99.20V1,4 (sin/ o 45°)z

5 (d6) = 20 LoG, 79)= 4. 5 06 dB SEPARATION

Figure 5A

STEREO WAVEFORM WITH CORRECTAMPLITUDE AND PHASE RESPONSE

(OUTPUT OF BEI FS -30 STEREO GENERATOR, WAVEFORM EXPANDED TENTIMES, PILOT EXCLUDED FOR EXAMINATION OF BASELINE FLATNESS)

Figure 5B

-10-

An amplitude and delay equalizer for the

composite baseband is now available as part of

the stereo generator. Equalization for amplitude

and phase deficiencies in the STL or exciter will

improve the overall system performance. An FM

exciter with flat amplitude and phase response

utilizing a balanced composite input will avoid

ground loop problems as well as minimize equali-

zation requirements.

The use of any non-linear devices, such as

clippers or limiters in the composite line will

alter not only the peak amplitude of the baseband,

but also the frequency spectrum of the baseband.

This generates several types of distortion at

the receiver.

Figure 6A and Figure 6B show the waveform

and spectrum of unprocessed baseband while Figure 7A

and Figure 7B show the same waveform and spectrum

after 1.25d6 of composite clipping. Figure 8

summarizes the types of distortion caused by

composite processing.

-11-

BASEBAND WITHOUT CLIPPING

Figure 6A

(OUTPUT FROM BEI FS -30 STEREO GENERATORONE CHANNEL ONLY MODULATED @ 10KHz)

Figure 6B

BASEBAND AFTER 1.25dB COMPOSITE CLIPPING

Figure 7A Figure 7B

(OUTPUT FROM BEI FS -30 STEREO GENERATOR FOLLOWED BY 1.25dBOF COMPOSITE CLIPPING -ONE CHANNEL ONLY MODULATED @ 10KHz)

-12-

SUMMARY OF TYPES OF DISTORTIONCAUSED BY COMPOSITE PROCESSING

1. Intermodulation of all baseband frequency components

causing extraneous spectral components.

2. Harmonic distortion of baseband causing degradation

of crosstalk and separation.

3. Modulation of pilot injection level causing loss

of lock at the synchronous detector.

Figure 8

The received audio is high in intermodulation

distortion and non -correlated information due to

aliasing of the extraneous spectral components

added by composite processing. For more detailed

information on the effects of composite processing,

see Reference [1] 121.

If minimum system distortion is the goal,

composite processing should not be used. Audio

processing should be performed before the audio is

multiplexed into baseband.

Distortion of the composite baseband signal

can also be caused by transient intermodulation

distortion (TIM) within the amplifier stages.

Transient intermodulation distortion of the

baseband signal is caused by the same mechanisms

that produce TIM in audio signals. The composite

amplifiers must have sufficient feedback bandwidth

-13-

to accept baseband frequencies to 100KHz anc should

slew symmetrically to minimize slew -induced distor-

tion. The TIM performance becomes largely a matter

of operational amplifier selection and circuit

configuration.

II. FM Modulator Linearity

The composite baseband signal is translated

to a frequency modulated carrier frequency by

the modulated oscillator. Frequency modulation is

produced by applying the composite baseband signal

to a voltage tunable RF oscillator. The modulated

oscillator usually operates at the carrier frequency

and is voltage tuned by varactor diodes, operating

in a parallel LC circuit.

To have perfect modulatior linearity, the RF

output frequency must change it direct proportion

to the composite modulating voltage applied to the

varactor diodes. This requirement implies that

the capacitance of the varactor diodes must change

as nearly the square of the modulating voltage as

shown in EQ. 3A, EQ. 3B, and EQ. 3C.

Eq, 3A rc oc L4, (G)e-S.,,469 Li/YE-AR VOLTAGE- 70 /cRE xle7vCTRAA 5 LATic AO

Q 3 B Fc -2 ir l/TET

Q. 3C Cv cc (IF CA -/D > 0)(Vm)z

WHERE _Z //STA N Tz4NE" 0 c. / 5 CARRIER ERE°i/e7vCy= BAS 4N.,0 /70.0L/LATilV6 VOL TA G

= _TN,De./CTANC E OF RESONANTc/Rc(// TCT TOTAL IPA C/ TA A/C E ACRoSS L

(C. )((c) # C vARACToRs)Cv = C OF VARA CTOR TLiAN/va D/0 DE5

-14-

Unfortunately, the voltage versus capacitance

characteristic of practical varactor diodes is not

the desired square law relationship. All varactor-

tuned oscillators have an inherently non-linear

modulating characteristic. This non -linearity

is very predictable and repeatab-e for a given circuit

configuration, making correction by complementary

predistortion of the modulating signal feasible [31.

Suitable predistortion can be applied by using

a piece -wise linear approximation to the desired

complementary transfer function. Figure 9 shows

a network of switching diodes and resistors for

complementary predistortion of the composite base -

band.

PREDISTORTION NETWORK

STEREO ANDSCA INPUTS COriPosirE->-f\AA,- BASES/WO

->

i\A.Ay---

-- ACTIVE5uMmtiv4AMP? /E/ER

Figure 9

z2)/

0,2

/q3 03

O

mot0R4 Di -

/5 .05

0

PRE - 7-0/i' TED Cofr,Po S/ 7E -BAS BAND TO /vo.ncizAIrcleDia ES

-15-

Figure 10 shows how the predistortion retwork

is cascaded with a non-linear voltage -tuned oscillator

to produce a linearized FM modulator

LINEARIZED FM MODULATOR BLOCK DIAGRAM

5 TERI 0

AND SCA1NPVTS PRE- D/SToRTzont VOLTAGE

/V GA/E TWDRICAcff'Z f TUNED FM

OSCILLATCli/4":

Oc/TPtir

COPipos/rrBASER/IND \- PRE -D/S70R7ED

comeosiTE BASE -BAND

Figure 10

Modulator linearization has reduced harmonic

and intermodulation distortion to less than .05%

in newly -developed equipment.

Any distortion of the baseband signal caused

by the modulated oscillator will have secondary

effects on stereo and SCA crosstalk, which are

quite noticable at the receiver in spite of the

rather small amounts of distortion to the base -

band. For example, if the harmonic distortion

to the baseband is increased from .05% to 1.0%,

as much as 26dB additional crosstalk into the

SCA can be expected.

For illustrative purposes, Figure T1A, 11B,

and 11C give representations of the fundamental and

second order terms in the composite baseband spectrum

with increasing amounts of harmonic distortion in the

modulated oscillator.

-16-

_TOEAL DE/10.01/L4TEIO CoAfPos/ re BASES4A/D 5"/YC7,4'.NY h//77/ NO/00%=- OcK8-

-60/8 (LiR)naDUL 4 roR Disr.0,2r/ON

///i

F/ //..e

F/G. //C.

-/c3 -

-20-

-30

-40

O -50-i' -60-

-70 -

-80

(0D7L.73r)

- /2d8 (L -R)

-20dB - -/09.; //V-TECT/0/1/

I I5 /5 /9 23 33 38 4,3 53 76

PRE-QC/EA/CY Wiz)R ONLY MODULATED /00°9g 5-04. Onoiati/2472-4 5C4 /60% /A/JEcnoN

DEMODULATED COMPOS/TE e4SEe3.4A40 SPFCrewy In.// TA/ 0. O5 A/A 4WD/WC/00% = oc/8_ 0/570,q7/0A/ /A/ /%101)tiZAT&Z.

-6d8 <L +R)

-/0 -

-30 -

-40 -

,Q1 -50 -

- 60 -

-70 -

-80

Cercor)-

-72d8

I

-/ 2 ./49 -

-761dB -78de

-20dB

-78c'

0 5 10 /5 /9 23 33 38 1.3 53FRE 0 11 ENC y (KHz)

R 0A/Ly IsioDut4Tep /00 % ski/z. LIN,40DziLATED SCA di) /047c1 //V-TECT/.0A/._.7-A.rrEAFE.c/evq- SEcoN0 //.1.4/4onfiC srEkio 5/0684.v.O.,5_ "We Q'8 Reza k/ 5c4.on/LK oivodifoflenir14 AND SECOND 1.-/4R/44DNIC TERMS ARE" 51/dowN.

/0(14 /N.TECTion/

4667

- 78d8

76 8G

/00% 018- DE-A,01)1/L.47-ED ColiPoS/ re- BASE6A/VD -SPEC7RLI/11 kliTH A 0% IMAY-feiv/C_-600 tR) b/57-oRr/o,V //i/ /400frz 4 raiz

-/0 -

-20

(2--R)-1 (P/40r)

0 -30

v

O -go -

-70-

-80 -

-1'6d8

-52 da

-60 de

-0 S8

-52c0 I I I -52d8

0 5 / /9 23 33 3,9 43 53 '6 47

/0% /N-rec7zon/

0.4? R cwzy AioptizAire-z) /Coold 5Alk . e/NAlooe/LATED -ce..4 /04:. /NrEcT/O, k_T/vre-.veRwv0 st-ccvw, IlivotioN/C STEREO sADERANzS ARE 12°45 BELOW SCAONLY fONDAMENIAL AND SECOND 1-/A4/44,6N/c. TERMS ARE S//d141/V.

-17-

Figure 11B shows this spectrum after 0.05% harmonic

distortion has been added to each component. Note

that the second order stereo (L -R) sidebands are 78dB

below 100% modulation or about 58dB below a 67KHz SCA

with a 10% injecticn. With normal energy distribution

in L -R and the SCA, crosstalk from stereo into the

SCA will be more than 60dB below the SCA subcarrier.

Figure 11C shows the same baseband spectrum with

1.0% harmonic distortion. The second o''der stereo

sidebands are only 32dB below the SCA. Crosstalk may

now increase as much as 26dB, depending on the respective

energy distributions in (L -R) and the S:A.

Assuring that the composite baseband signal

undergoes minimal distortion in the modulation

process will supress undesired harmonic and inter -

modulation products in the baseband, making the

FM exciter transparent to the signals coupled into

it.

TIM distortion is usually not a factor in varactor

tuned modulated oscillators. The modulation band-

width capability is generally more thar ten times

the composite bardwidth and no negative feedback

is used to maintain linearity.

III. The RF Path

The FM modulator converts the composite base -

band signal into the frequency modulated RF signal

containing a complex array of sidebands 14]. The

amplitude and phase of the FM sidebands are determined

-18-

EQ

by the modulation index, while the frequencies of

the sidebands are determined by the modulating

frequencies.

If the modulating frequency and the FM frequency

deviation are known, the modulation index can be

calculated as shown in EQ. 4A.

of-

WHORE: /-1 = /slope/LAT/0N _INDEX.6,pc,REVIEA/CY DEV/ArIoNfm x/%10,01/LAT/A/6 FREQUENCY

By making the modulation index (M) the argument

of a Bessel function, (EQ. 4B), we can determine the

amplitude and phase of the carrier (J0) as well as

the higher order sidebands (J1 thru JN).

SIMPLIFIED BESSEL FUNCTION

k7.921i,e9

E=A j To (m)S/A/ L-' (i)

OvV[sw (6_4 # 6.J,) z, ,.57A/(44 -c4.4,)]

(Al) [SW(u # tc.4,7)f 0 SW (6./c- 2CAJerdd

0.73 (,4) p7A, ruk f 3 4/4f - 5//V (Lac -3 A47-,)1

WHERE: A = 1/N/TODUZATED CARRIER AMPLirc/DETo =/%70.01/LATED CARR/EA AmpL/Tv.oh--7; , T1 ,g3 LTN = ,11-1Pz/re.hor OF NrN ORDER SIDEBANDS1,./c 277' fc ( CARR/ER FREQUENCY)WPn 2. re ic,1 n op U1_4 T/NCB FRED L/EA/C Y)

0'f f THE "40.0uL.9710A/ "Nogx

-19-

Figure 12 shows a graphical representation

of the Bessel functions for the first eight orders. [5]

AMPL TI/OF OF THE CARR /ER AND THE FIRST E GAITS 40E6AA40.5 AS A /"VA/c7/ON Q. /Y0,04/1Arion, -7A/PEX

+1.0

+0.9

+0.8

+0.7

+0.6

+0.5

+0.4

+0.3

(1.4 +0.2

+0.1

0

Q1

-Q2

0.3

-0.4

e, cs,i4ss,A;,3 4 6

CAR /EARNVC

02

0 2 3 4 5 6 T 6 9

= /VOL) z/ z4 770/V -INDEXWHERE: ,..75 REPRESENTS CARR /ER AMPL/ TUDE

J Re -PRE S EA/ TS NT/I ORDER S / DE -8,1A40/TUBEftIMP[

Figure 12

10 11 12

Figure 13 shows the frequency spectrum for a

modulation index of 2.405, which is the first carrier

(J0) null.

-20-

FM RF SPECTRUM SHOWING OCCUPIED BANDWIDTHAND BESSEL NULL SINGLE TONE @ +/- 75KHz DEVIATION

10d8* 0100 MHZ 3 Trig ea

(FOR M=2.405, Fm=31,185Hz, Fc=100.00MHz)

Figure 13

After examining the Bessel function and the

resulting spectra, it becomes clear that the occupied

bandwidth of an FM signal is far greater than the

amount of deviation from the carrier frequency. The

occupied bandwidth is actually irfinite if all the side-

bands are taken into account. It is also interesting

to note that at certain modulation indices, the carrier

amplitude goes to zero with all the transmitted power

distributed at frequencies other than the carrier frequency.

This carrier null phenomenon is useful as an extremely

-21-

accurate method for the calibration of modulation moni-

tors. (See carrier null in Figure 13.)

We have established that the FM spectrum occupies

a bandwidth far greater than the deviation width that

one might incorrectly assume is the bandwidth.

Practical considerations in the transmitter RF

circuitry make it necessary to restrict the RF band

width to less than infinity. As a result, the higher

order sidebands will be altered in amp-itude and phase.

Bandwidth limitation will add to the distortion in

any FM system.

Consider the model shown in Figure 14A, where

a perfect FM modulator is connected to a perfect

demodulator via an RF path of infinite bandwidth.

The demodulated audio contains no distortion com-

ponents.

fi/q /4A

qb-->--. F/v1

/MODULATOR

WIDEBAND RF PATH

-->--

c0/iPOS/TE_i/v/DurSPECTR0/1v.,/ 7-1/oUTD / -S7-0.97 / on/

FMDE MODUL,ITOR -->--

VI/DE-8.1NL)R.F.

SPEC TR ei/T

Figure 14A

DEMODULATEDOUTPUTSPEcTR(//%1WITHOUT0iSTOA770/V

BANDWIDTH LIMITED RF PATH

F/Q /-9a

FM R. F.' /c/v-->---

\_.CoivPoShrE

MO 40Z/L "ITO R BA/ vD ,c,,Iss/ 7 LreiR

--* acit-tao tit 41ToR

_INPUTSPECTRUMW I 7:47,01/7-DiSTORT/ON

/c/zTERcc,R . A:5P i cTR ()Ai

Figure 14B

DEMODULATEDOUTPUTSPECTRUMWI TN0/S TOR T/ o/V

-23-

In Figure 14B, a passive LC bandpass filter is

inserted between tie modulator and demodulator in order

to restrict the bandwidth. Audio distcrtion products

now appear at the output of our perfect demodulator,

due solely to the bandwidth restriction imposed by the

passive BPF.

As you can see, the distortion in any practical

FM system will depend on the amount of bandwidth avail-

able versus the ncdulation index being transmitted.

Relating the specific quantative effect of the band-

width limitations imposed by a particular transmitter

to the actual distortion of the demodulated composite

baseband is a complicated problem indeed. Some of the

factors involved are shown in Figure 15.

LIMITING FACTORS WITHIN FM TRANSMITTER

1. Total number of tunec circuits involved.

2. Amplitude and phase response of the total

combination of tuned circuits in the RF

path.

3. Amount of drive (saturation effects) to

each class "C" stage.

4. Non-linear transfer function within each

amplifier stage.

Figure 15

-24-

In Figures 16 thru 18 we view the various spectra

generated by the composite basebard at different Points

within the system. As the bandwidth is reduced, the

RF spectrum and demodulated spectrum will change. The

change in the RF spectrum is subtle, but the resulting

spectrum after demodulation is clearly modified.

SINGLE TONE (10KHz) MODULATION THRU WIDEBAND RF PATH

BASEBAND SPECTRUM TO BEI MODELFX-30 MODULATOR

RF SPECTRUM TO DEMODULATOR

Figure 16A

SINGLE TONE (10KHz) MODULATION THRU NARROWBAND RF PATH

-3104 M 3i xrt r!..


BANDWIDTH LIMITED RF SPECTRUMTO DEMODULATOR

Figure 16E

DEMODULATED BASEBAND SPECTRUMFROM BOONTON MODEL 82AD

1

T 1

IAte

i

i r r r- +1,4 4.4.4.1.4tit+4 .+4-t-i-tit

441.32J6


-_u-

TWO TONE (10KHz & 25KHz) MODULATION THRU WIDEBAND RF PATH


2008, 00017hOrjtkorq REs

RF SPECTRUM TO DEMODULATOR DEMODULATED BASEBAND SPECTRUMFROM BOONTON MODEL 82AD

Figure 17A

TWO TONE (10KHz & 25KHz) MODULATION THRU NARROWBAND RF PATH



Figure 17B


-27-

STEREO (L or R=4.5KHz) PLUS SCA (UNMOD.) MODULATION THRU WIDEBAND RF PATH


RF SPECTRUM TO DEMODULATOR DEMODULATED BASEBAND SPECTRUMFROM BOONTON MODEL 82AD

Figure 18A

STEREO (L or R=4.5KHz) PLUS SCA (UNMOD.) MODULATION THRU NARROWBAND RF PATH



Figure 18B

-28-


Figure 19 shows we can arrive at some basic con-

clusions with regard to optimizing the RF path.

IMPROVEMENT OF RF PATH

1. Maximize bandwidth by using a broadband

exciter and broadband IPA stage.

2. Use a single tube design or a broadband,

completely solid state design where feasi-

ble.

3. Optimize both grid circuit and plate

circuit of the tuned stage for best possi-

ble bandwidth.

4. Use a broadband antenna system with a low

standing wave ratio on transmission line.

Figure 19

-29-

TIPS ON HOW TO FIELD ADJUSTA TRANSMITTER FOR BEST AUDIO PERFORMANCE

We should start out by remembering that all

optimization should be done with the transmitter

connected to the normal antenna system rather than

to a dummy load.

The transmitter is first tuned for normal out-

put power and proper efficiency. A simple method

for centering the transmitter passband on the car-

rier frequency involves adjustment for minimum

synchronous AM. Synchronous AM is AM modulation

of the carrier caused by frequency modulation of

the carrier frequency. If the bandwidth is narrow

or skewed, increasing amplitude modulation of the carrier

will result.

A typical adjustment procedure is to FM modulate

100% at 400Hz and fine-tune the transmitter for

minimum 400Hz AM modulation as detected by a wide -

band envelope detector (diode and line probe).

It should be possible to minimize synchronous

AM while maintaining output and efficiency.

Another more sensitive test is to tune for

minimum intermodulation distortion in left only or

right only stereo transmissions. Stereo separation

will also vary with tuning.

For stations employing a 67KHz SCA, transmitter tuning

becomes very critical to minimizing crosstalk into the SCA.

Modulate one channel only on the stereo generator to 100%

with a 4.5KHz tone. This will place the lower second harmonic (L -R)

stereo sideband on top of 67KHz SCA. Activate the SCA at normal

injection level without modulation in the SCA. Tune the

transmitter for minimum output from the SCA demodulator.

This adjustment can also be made by listening to the

residual SCA audio while normal stereo programming is being

broadcast.

Figure 20 lists these field adjustment techniques in

ascending order of sensitivity.

FIELD ADJUSTMENT TECHNIQUES

1. Tune for minimum synchronous AM noise.

2. Tune for minimum IMD in left or right

only channel.

3. Tune for minimum crosstalk into unmodu-

lated SCA subcarrier.

Figure 20

In any of these tests, the grid tuning is frequently

more critical than the plate tuning. This is because the

impedance match into the input capacitance of the grid becomes

the bandwidth limiting factor. Even though the amplitude

response appears flattened when the grid is heavily driven,

the phase response has a serious effect on the higher order

FM sidebands.

-31-

CONCLUSION

I have discussed the three areas in the transmission

system that affect the composite baseband performance as

listed in Figure 2.

I. The composite link to the FM modulator

(stereo generator, SCA generator, compo-

site processor, and STL equipment)

II. The FM modulator

III. The RF path to the demodulator (FM ex-

citer, IPA, PA, and antenna system)

Figure 21

Each of these subsystems must be individually optim-

ized before your complete transmission system can

give you the best possible performance.

I hope that this presentation will encourage you

to investigate your particular system and will help

you optimize your composite signal, the key to quality

broadcasting at your station.

ACKNOWLEDGMENTS

The author is grateful to Mr. Robert Weirather

for his encouragement and his assistance in editing

this presentation. The author wishes to thank

Nancy Quellhorst and Lori Ash for their assistance

in preparing this manuscript.

-32-

REFERENCES

[1] Robert Orban - Orban Associates, "An Investigationof Baseband Clipping", presented at the 1980 NRBAEngineering Conference.

[2] Robert Orban - "Degradation of the FM Signal byBaseband Clipping", BME Magazine, p. 51 (1980 Aug.).

[3] Geoffrey Mendenhall - "Designing a New FM Exciter",Broadcast Communications, Vol. 3, No. 2, pp. 30-36 (1980 June).

[4] Frederick Terman - Electronic and Radio Engineering,pp. 586-600, McGraw-Hill Book Company, Copyright 1955.

[5] Reference Data for Radio Engineers, pp. 44-38 andpp. 45-20 thru 45-22, Howard W. Sams & Co., Inc.,Copyright 1968.

THE AUTHOR

Geoffrey N. Mendenhall earned a BEE degree from the

Georgia Institute of Technology, Atlanta, Georgia.

Mr. Mendenhall has designed communications equipment

for various manufacturers, including E.F. Johnson Co. and

Harris Corporation. He led the design efforts for both the

Harris MS -15 product line and the Broadcast Electronics

FX-30 FM Exciter. His practial field experience has involved

engineering and operations work for several radio and television

stations, including Storer Broadcasting.

The author is presently Manager of FM Products for Broadcast

Electronics in Quincy, Illinois. He is an active amateur radio

operator and is experimenting with home satellite reception.

Mr. Mendenhall holds two U.S. Patents for electronic designs

and is a registered professional engineer in the State of Illinois.

He has authored numerous technical papers and is a member of the

Institute of Electrical and Electronics Engineers.

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By Electronics/BE-The... · baseband without clipping figure 6a (output from bei fs -30 stereo generator one channel only modulated @ 10khz) figure 6b baseband after 1.25db composite