FM STEREO SIGNAL GENERATORThe construction projectdescribed here consists oftwo units: an FM stereomultiplex generator and athree-stage VHF FM exciter.Together, they form an FMstereo signal generatorintended for testing andaligning FM stereo receivers.

Design by J. Barendrecht

GENERATING an FM stereo stgnalwith a clean spectrum. and sutt-

able for test purposes is not simple.However. it can be done with relativelysimple means, as demonstrated here,prcvtded you have some experience inworking with high-Irequency ctrcutts.The stgnal generator described has anoutput frequency range that covers theVHF FM broadcast band (87-108MHz). and supplies an output power ofabout 150 mW into a 50-n load. Owingto the absence of an output filter, har-monies are insufficiently suppressedto enable the generator to be con-nected to an antenna. Fortunately,that ts not a problem for the main ap-plication of the slgnal generator: test-ing and aligning FM receivers. where adummy load will be used.

The FM stereo multiplexsignalFtgurc 1 shows the theoretical Ire-quency spectrum of the stereo multr-plex (MPX) stgnal applied to themodulation input of a VHF FM broad-cast transmitter. The two stereo stg-nals L (Ieft) and R (right) are added asweIl as subtracted to gtve the corre-span ding sum (L+R) and difference(L-R) components. The L+R componentoccupies the lowest part of the spec-trum. up to about 15 kHz, and affordscompatibility with mono receivers. TheL--R component is converted into adouble-sideband signal with a sup-pressed carrter at 38 kHz. This type ofmodulation is called DSCC (double-stdeband, suppressed carrterl. Itcauses an upper eideband (USB) and alower sideband (LSBl, mirrored againsta suppressed ('Invlstble'} carrter (here,38 kHz). The carrier is su ppressed to

Ei %-;i's~'C

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f ,:: ~~_R ----l-~'ot -, .-- ..,

~ 10 --------------- ---- i I L - R L - R

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15 19 230.03 3. 53

f (kHz)-----'920155 -11

Fig. 1. Theoretical spectrum 01 the stereo MPX signal (EBU recommendation).

19kHz

*' phaseoscillatordivider shift

IC2•=.,.38kHz

IC1

rcsRQ IC8

'VI)(,I)(,

LQ 'VI)(,I)(,

PLL

I')(, IC3I)(,'V

38kHz 19kHz

L-R MPX•[>IC7

920155-12

Fig.2. Block diagram 01 the stereo MPX generator. A PLL is used to synchronize the 19-kHz(pilot) and 38-kHz signals.

ELEKTORELECTRONICS MAY 1993

FM STEREOSIGNALGENERATOR

111111111

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Fig. 3. Circuit diagram of the stereo MPX generator. The two XR2208s have different functions: IC3 is wired as a phase-Iocked toop (PLL),while leG works as an analogue multiplier.

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keep the total deviation of the trans-mitter within limtts.

At the receiver side, lhe 38-kHz car-rier ts recovered with the aid of the 19-kHz pilot tone contained in the MPXsignal (relative level: 10%), which alsoserves to indicate a 'stereo' transmis-sion. The 19-kHz pilot tone ts doubledto gtve 38 KHz, and enables coherentdemodulation of the L-R infonnation.Next, a matrtx is used La 'dtsttll' the Land R signals from the componentsL+R and L-R.

Retuming to the transmitter slde,the levels of the components in theMPX signal are fixed to optimize thechannel separation gtven the availablebandwidth for the FM stgnal, and alsoto ensure that the sound quality on amono receiver is not Impaired.

ELEKTORELECTRONICS MAY 1993

XGAIN

"

MPXgenerator blockdiagram

The block diagram of the stereo MPXgenerator. or stereo coder. 15 gtven inFtg. 2. A quartz-controlled digital clockoscillator/divider supplies 19-kHz and38-kHz signals to Iow-pass filters thatturn the reetangular waves into sine-waves. In between the 19-kHz clockoutput and the input of the associatedlow-pass filter sits a phase shifting cir-cuit controlled by a PLL (phase lockedloop). The PLL compares the phase ofthe 19-kHz and the 38-kHz stne-waves,and supplies an errar stgnal to thephase shtfter ahead of the 19-kHz low-pass filter. Stncc the PLL will altemptto achieve an error signal of nought,the 19-kHz and 38-kHz stne-waves willhave a Iixed phase relattonshtp.

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9t0155·13

The left and rtght audro stgnals aretaken through pre-emphasts networksbefare they are summed and sub-tracted to gtve the L+R and L-R cornpo-nents. The L-R component ismultiplied with the 38-kHz stgnal togive the DSSC bands in the MPX spec-trum. A surnrmng circuit combines the19-kHz pilot tone. the L+R component.and the DSSC signal. The result ts thestereo multiplex signal, whlch ts fed tothe modulation Input of the FM trans-mitter.

MPXgenerator circuitdescriptionThe circuit diagram of the MPX gener-ator. Ftg. 3, clearly reflects the blockdiagram discussed above. The centraloscillator is butlt areund a 2.432-MHz

RADIO AND TELEVISION

920155 • 1488204 2N5109

BF981

~,~~BF982

+-"° *see text

BAT85 S

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Fig.4. Circuit diagram of the VHF FM exciter. The output power is adjusted with preset PS.

quartz crystal and the familiar CMOSType CD4060 oscillator/divider (lCI).The phase shifter at the 19-kHz outputis realized with the aid of a dual vart-cap (DI) and an opamp (lC2). The L-Cpi-type lew-pass filters convert the rec-tangular stgnals produced by the di-vider and the opamp tnto sine-wavesthat can be applied to the inputs of theanalogue multiplier. lCB. an XR2208from Exar, which functions as a PLLhere. The PLL error stgnal controls thecapacitance of D 1, and thus the phaserelation between the 19-kHz and the38-kHz sine-wave. The phase is accu-rately adjusted with the aid of presetPI.

The left (L) and rtght (R) audio sig-nals are first given a 50-~s pre-empha-sts by the networks at the -inputs ofopamps ICöa and ICSb. Next, theopamps contained in Ie5 and ICs turnthe audio input stgnals into L+R andR-L components. The second XR2208in the circuit. IC6. multiplies the 38-kHz sinewave with the R-L component.The multiplier also inverts the R-Lcomponent, so that its DSCC outputstgnal has the L-R component as re-quired.

The 19-kHz pilot tone, the DSCCstgnal and the L+R signal are summedby resistors at the -input of opampIC7. The levels of the pilot tone and theL+R component are adjusted wtth pre-

sets P3 and P2 respectively. The opampfuncttons as a buffer for the multiplexsignal. Aseparate AC-coupled output.marked 'A', rs avai1able for test pur-poses.

Finally, opamp IC4 serves as a 6-V(half the supply voltage) referencesource.

VHF FM exciterThe VHF FM signal generator (exeiter)is a stratghtforward destgn based onthree transtetors - see Ftg. 4. Thefirst. Tl, Juncttons as an FM modula-tor, and operates at half the generatoroutput frequency. i.e .. 44-54 MHz. TheMPX stgnaj arrtves on dual vartcap D3via a modulation level control. P4, andDC and RF decoupling parts C31. R46and Ls. Note that the varicap fonns theonly capacttance acrcss the oscillatorInductor, L9. This is done to keep FMdistortion (caused by U:fnon-linearity)to a minimum. The modulator is prettysensitive: the maximum devtattcn tsachieved at an MPX level of 80 mVpp

already. The feedback in the oscillatoris inductive via L9.

The second transistor, T2, works asan amplifier. bandfilter LID-L6 beingtuned to the first harmonie of the 05-

cillator (100 MHz). The bandfilter is acritically coupled type. It is very selec-tive. and gtves a suppression of about

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50 dB at the oscillator frequency(50 MHz). Preset Ps serves to adjustthe gain of T2, and thus to control thegenerators output power. Power levelcontrol ts must where two of these gen-erators are used, for instanee, for in-termodulation distortion (lMD)measurements, which require equalpower levels for the two test tones.

The power amplifier transistor. T3,is opera ted in class A, and supplies amaximum output power of aboutl50 mW into a 50-Q load. lnductor Lain the collector line of T3 ts tuned tothe output frequeney by trimmers C46and C47, which also take care of theoutput impedance matching.

The RF exciter and the MPX genera-tor are powered by a 12-V supply,which must be regulated, and capableof supplymg about 200 mA.

ConstructionThe FM stereo stgnal generarer is builton two printed circuit boards. whtchhave to be separated. One board con-tains the MPXgenerator. the other, theRF exciter. The artwork of the double-sided, though-plated, board is given inFtg. 6. Before you start cutting andpopulating the boards, however. werecommend that you make the induc-tors.

ELEKTOR ELECTRONICS MAY 1993

•(tO) •

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920155 -15

Fig. 5. lIIustrating the construction and connection of the horne-made inductors in the VHFFM exciter. The numbers in brackets indicate the number of turns of a particular winding,while the dots indicate the start of a winding.

Inductor constructionSince the success of the present pro-ject will depend to a large extent on theconstruction of the inductors. westrongly recommend that you followthts secnon closely. The basic con-structton of the horne-made inductorsis gtven in Ftg. 5. The figures in brack-ets indicate lhe number of turns of aparticular winding. In the descrtptlonbelow. 'CuL' stands for enamelled cop-per wire. The dots in the drawings in-dtcatc the start of the winding.

L9 rs wound on a Neosid 7TlS inductorassembly [yellow core). Primary wind-ing: 10 turns 0.3 mm dia. CuL.Secondary wtndtng: 3 turns 0.2 mmdia. CuL at the 'cold' (earthy) side ofthe primary wtndtng.

LIDis wound on a Neasid 7V1S induc-tor assembly (green core). Prirnarywinding: 2 turns 0.2 mrn dta. CuL atthe 'cold' (earthy) side of the secondarywtndtng. Secondary winding: 8 turns0.4 mm dia. CuL.

Lt t is wound on an Amidon Associates(Micrometals) T50-12 Ierrtte ring core.

ELEKTOR ELECTRONICS MAY 1993

Primary winding (1-3): 8 turns 0.7 mmdta. CuL, tap (2) at 4 turns. Secondarywinding (4-5): 2 turns 0.5 mrn dia. CuLat the 'cold' (earthyl side of the primarywinding.

L6 is wound on a Neosid 7VlS inductorassembly (green core). Eight turns0.4 mrn dta. CuL. tap at 4 turns.

L7 is wound on a small ferrite bead. forinstance, an FB-43-10 1 from AmidonAssociates (MicroMetals). It consists of4 turns of 0.2 mm dia. CuL.

La is an air-cored inductor. lt consistsof 7 c1osewound turns of 0.5 mrn dia.CuL. The tnstde diameter is 3 mrn.

Before fitting the Neosid tnductors onto the board, check the continuity ofthe windings at the base pins. Alsocheck for short -circuits between thewindings and the screening can.

MPXgenerator board constructionThis part of the ctrcurt is simple tobuild. The two audio input sockets, KIand K2. are mounted direct on to theboard. After fitting all the components,

FM STEREO SIGNAL GENERATOR

sold er 20-mm high tin plate screensover the dashed lines on the overlay.Set all three presets on the board tomid-travel.

RF exciter board constructionSome remarks are in order here. par-ticularly for those with little experiencein bullding RF circuits. Start the con-strucuon by ütung the three Neosid in-ductor assemblies. 00 not mount thescreentng cans as yet. Next. mount theresistors. the diodes, the presets andthe capacitors. The latter are fittedwith the shortest possible lead length.

Proceed with the trimmers and thetransistors. Attention: the type printon MOSFETTI is legtble from the corn-ponent side. while that of T2 is legtblcfrom the track side of the board. BothMOSFETs are fitted at the track side ofthe board.

The RF power transistor. T3. ismounted at a height of about 2 mmabove the board surface. It ts fittedwi th a small clarnp-on (TO-5) he at-sink.

Solder 20-rnm high brass or tinplate screens on to the board at the 10-cations marked by the dashed lines.Alternattvely, bend a 'frame' [Tom a 20-mm wide strip of tin plate, and solderit on to the board, making sure thatnearby components are not darnagedby overheating. The screen at the sideof trtmmera C46 and C47 1S drilled toaccept a BNC socket for single holemounting. The centre pin of the BNCsocket is soldered straight to the RFoutput solder pin. Fmally, cut andbend a cover plate from the same ma-terial as used for making the screens.Drill holes in the cover to enable theadjustment points (trimmers, presetand inductors) to be accessed.

MPX generatoradjustmentConnect a dual-channel oscilloscope totest points TPI and TP2. Apply power,and adjust preset PI until the zero-crosstngs of the 19- kHz signal and the38-kHz signal comctde. This 1S illus-trated in Ftg. 7.

Next, adjust the L+R and pilot lev-els. Connect the scope to the MPX out-put. Apply a j-kj-lz. I-Ypp sine-wave tothe L inpul, and a 300-Hz. I-Vpp stnewave to the R input. Turn the wiper ofP3 (pilot level) fully counter-clockwtse(to the +6 Y side). Next, adjust P2 untilthe levels of the two components at theMPX output are equal (Fig. 8). Removethe Land R input signals. and adjustP3 for a pilot level of 100 mYpp al theMPX output.

VHFexciter adjustmentStart by connecting a 50-Q dummy

RADlO AND TELEVlSION

C·) C·) 0" C===O ~(.) 0 0 0 ,',0 0 0 0 0 0 00 " 1'.' 0 1'.'1'.' 0 1'.' 0 0 oo ,', " ,.., ,..".., ".., " 0 0,., ,."

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0 0 :: 0 0 0 0 0 0 0 0 00 0 0 0 0 o 0 0 0 ,,00 00 0 0 ooon 0 0 0 0 0'·" 0 00 0 00 00 00

:: 0 0 00 0000 0 (·x·) 0 (·X·) (.)8 80 0

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TPl

920155 ~laSO~2e

3R7

Fig.6a. Component and solder side track layouts (mirror images) and component mounting plan of the MPX generator board.

ELEKTOR ELECTRONICS MA Y 1993

00

C47

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Fig. 6b. Component and solder side track layouts (mirror images) and component mountingplan of the VHF exciter board.

ELEKTOR ELECTRONICS MAY 1993

FM STEREO SIGNAL GENERATOR

oo

COMPONENTS LIST

Induclors:Winding details are given in the text1 100~H choke L52 7V18 assembly (Neosid*) L6;L101 7T18 assembly (Neosid') L91 T50-12 (Micrometals") L111 3-mm ferrite bead L7Enamelled copper wire:dia.'s 0.2mm, 0.3mm, O.4mm, 0.5mm,0.7mm

Miscellaneous:1 BNC socket K41 T05 heat-sink1 Printed circuil board 920155 (see

page 70)

• Neosid Ud., Icknield Way West,Letchworth, Herts 8G6 4A8. Telephone:(0462) 481000. Fax: (0462) 481008 ... Cirkil PLC (0992) 444111.

VHF FM EXCITER

Resistors:1 10kQ2 33Q1 1kQ2 100k!12 270k!11 39Q1 22Q1 470n1 3k!191 10kn preset H1 100kn preset H

Capacitors:1 220nF1 100~IF16V1 100pF ceramic3 4nF7 ceramic2 1OnFceramic2 8pF2 cerarme2 1pF ce rarnie2 1nF cerarme1 20pF foil trimmer2 60pF toll trimmer2 100nF ceramic

Semiconductors:1 BAT85 021 BB204 031 1N4148 041 BF981 T11 BF982 T21 2N5109 T3

R44R45;R55R46R47;R51R48;R49R50R52R53R54P4P5

C31C32C33C34;C41 ;C44C35;C45C36;C39C37;C38C40;C43C42C46;C47C48;C49

load to the generator output. Next,turn the wiper of P4 (modulation) fullycounter-clockwise (towards thescreen). Couple a frequency rneter in-ductively to LI, and adjust the core forhalf the desired generator output fre-quency. Lacking a frequency meter,use an FM radio to find the first har-monie of the oscillator.

RADIO AND TELEVISION

COMPONENTS LIST

MPX GENERATOR 1 1OOkn preset H P2 1 LF411CN IC21 250kn preset H P3 2 XR2208CP' IC3;IC6

Resistors: 1 OP77 IC41 2kn2 Rl Capacltors: 1 LF357N IC71 lMn5 R2 2 68pF ceramic Cl;C2 2 TL072 IC5;IC83 47kn51% R3;R4;R6 2 470pF ceramic C4;C61 220kn R5 3 lnF C5;C8;Cl0 Miscellaneous:5 22kn R7;R9;RI6; 9 100nF C7;C23-C29; 1 2.432MHz quartz crystal;

RI7;R34 C31 30pF parallel resonance X12 82kn R8;Rl0 1 4nF7 C9 2 PCB-mount cinch socket K1;K22 8kn2 Rll;R12 1 1~MKT Cll2 120kn R13;R14 1 680pF ceramic C12 • Rahm Electronics (UK) LId., Whitehall1 1Mn R15 2 330nF C13;C20 Avenue. Kingston, Milton Keynes MK104 l00kn R18;R21 ; 2 471lF 16V C14;C15 OAO. Telephone: (0908) 282666. Fax:

R40;R42 2 150nF C16;C18 (0908) 282528.2 lkn5 R19;R22 2 3nF3 C17;C193 15kn R20;R23;R32 1 18pF ceramic C21 Attention:11 10kn R24-R31 ;R36; 1 IOIlF 16V C22 although the artwork for the MPXR38;R41 1 4700llF 16V C30 generator board and the RF exciter1 7kn5l% R33 1 150pF ceramic C3 board are shown separately in this1 56kn R35

article, these boards are supplied as1 3kn9 R37 Inductors:ONE unit, order code 920155. This1 1kn8 R39 2 47mH (Toko 181 LY473J) Ll;L2board has to be cut as described in1 680n R43 2 100mH (Toko 181 LY104J) L3;L4the text 10 separate the MPX gener-2 4kn7 R56;R57ator and the RF exciter sections.1 47kn R58 Semiconductors:

1 1DOn R59 1 6B212 011 25kn preset H PI 1 C04060 ICI

···1 .I

er \\

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Fig.7. The zero-crossings 01 the 19-kHz and38-kHz sine-waves can be made to coincideby adjusting preset P1 in the PLL.

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LJ.-.L.L.L.L.L.L.L.L ~2.1 v

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Fig. 8. Multiplex signal with L+R level cor-rectty adjusted (no pilot).

Unless you have a lOO-MHz scopewith a low-capacttance probe, you willhave to build the simple RF signal de-tector drawn in Ftg. 9. Connect thescope or the detector to the drain of Tt.and adjust the core of LlO for maxi-

mum RF srgnal amplitude. Similarly,measure the RF signal level at the tapon L6, and peak the inductor. Measurethe DC voltage across R52. and adjustpreset P5 for a reading of about 0.44 V.This corresponds to a drajn current ofabout 20 mA through T2.

Move the probe to the secondary ofLI1, and adjust C42 for maximum RFsignal. Next, carefully re-adjust LIOand L6.Thts is necessary to correct theeffect of the load capacitance Intro-duced by the scope probe or the RF de-tector input.

Connect the probe or the detector tothe output of the generator. Adjusttrimmers C46 and C47 for maximumoutput power delivered to the dummyload. Note that the trimmer settingswill interact slightly.

Connect the Land R input stgnals(1 Vpp typ.), and use an FM radio to lis-ten to the stereo signal. Carefully ad-vance the modulation preset. P4, unttlthe desi.red deviation ts achieved. Thisis best done by comparing the soundlevel to that of a couple of stereobroadcast stattons in the FM band .Typtcally. the MPX modulation voltagewill be between 40 mVpp and 80 mVppat the wiper of P4.

If you have not done so already, fitthe ferrite cups and the screening canson the Neosid formers. This will causesome detuning of the inductors, sosmall re-adjustments may be required.

The final remarks in this articleconcern the power supply of the gener-ator. Because of the high sensttrvtty of

4p7

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2xAA119

100p mV

920155 • 17

Fig. 9. Build this RF sniffer to adjust the ex-eiter. The millivoltmeter ean be a OMM set toits most sensitive direet voltage range (usu-ally 200 mV).

the modulator, mains transformersshould be kept as far as possible fromthe VHF exciter. lf at a11possible, themains transformer should be left outof the signal generator case. •

ELEKTOR ELECTRONICS MA Y 1993