e 905050

BUDGET SWEEPjFUNCTIONGENERATOR

T. Wigmore

This month we add yet another item to our series of budget testinstruments. The signal generator described has a bullt-ln sweepfunction wh ich is ideal for audio measurements. Based on thewell-known XR2206 function generator le with very few external

components, the instrument olfers a hard-to-beatprice/performance ratio.

It is not wise to disregard the XR2206 fromExar when designing an all-round func-tion generator. The device is versatile like110 other. and guarantees a fairly simplecircuit for the given application. Further-rnore, its cost makes any atternpt at desig-ning an equivalent circuit based ondiscrete components a waste of time,while its ou tput signal distortion figuresare not spectaculer. but none the less lowcompared to those of a competitive chiplike the 8038.

For llse CIS a basic funetion generator,the XR2206 requires only a handful ofpassive parts. The frequency adjustmentand the sweep function are simple to im-plement by the addition of one dualopamp and three transistors. The outputamplifier of the instrument also followsthe general line of comprising of as fewcomponents as possible: only one poweropamp is required.

The generatorThe XR2206 forms the heart of the cir cuit(see Fig. J). With the external componentsconfiguration used here, the IC supplies asine-wave and a triangular wave at outpur

pin 2. The d.c. operafing point is set to halfthe supply voltege (6 V at pin3) by poten~tial divider R1S-RI6. The resistance at thepotential divider junction, 16.5 kO, andthe volta ge at the AM Input. pin 1, (0 V)determine the amplitude of the output aig-nal.

The waveform selection is effected byone contact of 54. In the position shown.resistor Rl7 is connected to pin 14 of theXR2206. The current flow through R17 en-ables the IC to convert the triangular stg-nal into a sine-wave. The value of R17determines to what extent the inflectionpoints of the triangle are rounded to givea sine-wave. FOT the sake of simplicity, afixed resistor instead of the expected(multiturn) preset is used to set this cu r-rent.

When the contact of 54 is opened, pin 2supplies a tri angular signal whose peakamplitude is twice that of the sine-wave.The rectangular wave is supplied viapin 11. This open-collector output of theXR2206 is pulled to ground at the gener-ator pulse rate by an n-p-n transistor.Voltage divider RIII-R19-R20 at pin 11 setsthe amplitude of the reetangular wave.The maximum and minimum vortage le-

MAIN FEATURES

Frequency ranges: 3 (10 Hz- 20 kHz)or 4 (10 Hz - 200 kHz)

Sweep frequency: 0.1 Hz -100 Hz

Sweep range: 0 -1 :20

Sweep output: 5 Vpp; sawtooth;Zo = 1 kQ

Waveforms: stne-wave, triangle,rectangle

Olstortion (sine--wave): 0.5% typ.(in AF range)

A.Coutput: all waveforms; 20 = 50 D,short-clrcuit reststant

Output amplitude (Re = 50 Q):0.1 mVpp -1 V", (slne-wave)0.1 mV", - 2.5 V", (triangle)

0.1 mVpp -1.5 Vpp (rectangle)

Output amplitude (Re = 600 Q):0.1 mVpp to 1.8 Vpp (slne-wave)

0.1 mVpp- 4.5 Vpp (triangle)0.1 mVpp - 3 Vpp (rectangle)

Current consumption:approx. 100 mAat 12 V

ELEKTOR ELECTRONICS MA Y 1990

BUDGET SWEEPIFUNCTION GENERATOR 11

'''I~

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Fig. 1. Circuit diagram of the sweep/function generator. The heart of the circuit is formed by IC2, an XR2206 from Exar.

vels are 9.1 V and 3.8 V respectively. Thisswing is elose to the opnmum drive ruar-gin of the power opamp tha t foJlows theXR2208.

The second cantaet of 54 selects eitherof the tWD le outputs and passes the rele-vant waveform to the outpur amplifier.

Frequency controlThe frequency of the signal supplied bythe XR2206 is deterrnined by two factors:the capacttance between pin 5 and pin 6,and the current drawn from pin 7.

The capacitance is determined by thethree capacitors selected by the frequencyrange switch, 53. A fourth range (up to200 kHz) may be added by providing anextra switch posttton and a capacitor of2.2 nF (see the secti on on construction fur-ther on),

Pin 7 01 the XR2206 supplies a tem-perature-cornpenseted reference volta geof 3 V, which is also available at pin 10,where it is decoupled by es. The volta geat pin 3 is loaded by a resistor, R14, and theoutput of opamp IClb. Hence, the outpurvoltage of the opamp determines the cur-rent through RI4 and with it the signalIrequency, f:

ELEKTOR ELECTRONICS MAY 1990

where /RI~ is in amperes. Factor C is thecapacitance (in Farads) between pins 5and 6.

Frequency and frequency sweep ad-justment are effected manually by poten-tiometer P3 at the -tnput of lClb. When 52is set to the 'normal' position, P3 and R9form a potential divider that Iirnits thevoltage at the wiper to a value betweeno V to 5 V. Resistcrs RIO-RIlset the ampli-fication of the inverting opamp to a valuethat results in output voltages of virtuallyo V and 3 V with P3 set to maximum andminimum (wiper to ground) respectively.The d.c. operating point -and with it thesta rt of the frequency range - is deter-mined by P4-R13and Ru, wh ich ensurethat apart of the 3-V reference voltage isapplied to the non-inverfing input of IClb.

Sweep functionWhen the generator frequency is set ma-nually, a fixed resistor, R9, provieles thedirect voltage to potentiometer P3. When52 is switched to the other postnon. how-ever. P3 is supplied with the output volt-

age of a ramp generator. In this mode, thepotentiometer sets the swept frequencyrange rather than the frequency itself. Inother words. it determi nes to what extent(in Hz/V) the ramp generator can changethe set generator frequency.

The ramp generator 15 formed byopamp ICla and integr ator Cl. The inte-gration time is set by the voltage a t thewiper of Pi: the lugher the voltage. thefaster the capacitor is charged. and thefaster the sawtooth volta ge rises. Poten-tiometer PI allows the sweep time to be setto a value between 10 ms and 10 s. Themaximurrt time is calibrated by preset P2,which also serves as an off-set compensa-tion for ICla.

The rtse of the sawtooth voltage at theIntegrator output is ended via T2 and Tl.The emitter of T2 is held at a referencepotential provided by aener diode D1.Thetransistor conducts, and TI and 13 are keptoff, as long as its base volta ge is below thereference. As soon as the sawtooth reachesa level of about 0.5 V beJow the referencevoltage, T2 is briefly turned off so that itscollector volta ge is pu lled to about 0 V viaRr. As a result, T3conducts and resets theIntegrator by making the inverting inputof [Clbpositive with rcspect to the non-in-

11I TEST AND MEASUREMENT

Fig. 2. Track layout (mirrer image) and component mounting plan of the single-sided printed circuit board for the generator.

verting Input. This is achieved with theaid of Tl. In the monostable formed byTz- T3, Cl ensures tha t the integration capa-citor is discharged rapidly to provide thetrailing edge of the sawtooth. The refer-ence volta ge provided by 01 th us deter-mines the amplitude of the sawtoothvoltage that sweeps the frequency of thefunction generator.

The sawtooth voltage is also availableat aseparate sweep output on the instru-men t. Resistor Re sets the output irnped-ance to about 1 kQ. The sweep outpur isshort-circuit resistant and may be used for

driving the X amplifier of an oscilloscopefor swept-frequency measurerncnts.

Output amplifierThe Type l165 opamp used in the ou tputarnpl ifler is capable of providing ampleoutpur current at a reasonable price. Thele is used in a conservatively ra ted con-figuration and is therefore not likely toactuate its internal overheating protec-tion. The power opamp is wired as annon-inverfing buffer (voltage fcllower).so that the amplitude and phase of the

output signal correspond to those of theinput signal at the wiper of ampli tudecontrol Ps. An electrolytic capacitor, CI3,is required to decouple the d.c. compo-nent at the output since a non-symmetri-cal supply is used. The parallel resistorcombination at the output is not strictlyrequired for overload protection (whichthe Ll65 provides by itself). lt does. how-ever. limit the output current to a safevalue. At the same time, it sets the gener-ator output impedance to 50 n, which is acommonly used value on test equipment.

ELEKTOR ELECTRONICS MA Y 1990

COMPONENTS LIST

Resistors:2 4k7 Rl;R191 10n Rz2 220n R3;Rl71 22k R43 lOk Rs;Rs;Rzo1 lMO R72 lkO R8;Rz21 68k Rg2 4l0k Rll;R132 820k RlO;Rl21 2k2 R"2 33k R15;R161 2lk R,.1 180n A2l2 100n Rzl:Rz41 lk8 R2S1 WOk !in. potentiometer P11 250n preset H Pa1 50k !in. potentiometer P31 lOOkprosei H P,1 10k !in. potentiometer Ps

Capacitors:1 3n31 10n4 100n1 68p1 1~0 16V axial1 2~MKT1 22n2 220n1 2n21 2n2 (opllonal)1 10"F 16V axial1 100"F 16V axial1 4700J.lF 16V axial1 1000J.lF25V axial

ClCzC3;C6;C16;C17C,CsClCaCS;C14ClOCi.C"C,ZC"C,S

.semiccneuctcrs:1 5V6400mW

zener dlode1 1N40Ql1 LEO1 B82502 BC557B1 CA3240E1 XR22061 1..1651 7812

Dz03TlT2;T3IC,IC,IC.1C4

Miscellaneous:1 miniature SPST switcti 811 miniature SPOTswitch S22 2-pole 3-way swltch 83;84

(Knrtler MTA206PA or C&K 7211)(53 may be repiaceo by a mlnlature1-pole 4-way rotary switch)

2 BNC socket2 TO-220 style heat-sink1 printed-circuit board1 front -panel toi!

900040900040-F

ConstructionThe stngle-stded printed-circuit board onwhich the generator is constructed Isshown in Fig. 2. Population of the pes isstraightforward with the possible excep-tion of the following points:

ELEKTOR ELECTRONICS MAY 1990

BUDGET SWEEPIFUNCTlON GENERATOR 11

IL

9000404

Fig. 3. Connections made in the 3-position switches trom C&K.

Thespindies ofpotentiometers PI, PzandP3are inserted from the track side of thepeB to enable the nuts on the shafts to belocked at the component side. Use shortwires to connect the potentiometer ter-ruinals to the relevant copper tracks.As shown in Ftg. 5, IC3and IC~are flttedat the track side of the board. Do observetheir correct orientation and the electri-cal insulation of the heat-sinks.5witches 51,53and 54,and the BNCsoc-kets are mounted on the front panel.Their positions correspond to those pro-vided on the overlay printed on theready-made circuit board. The connec-tions are made in short lengths of light-duty insulated wire.

It is not strictly necessary to use JC soc-kets, although the small additional invest-ment may prove worth while if a faulty ICis suspected. Since the instrument has itsown single-phase rectifier, smoothing ca-pacitor and 12-V voltage regulator. it maybe powered from an unregulated AC orDe supply with an output of 15 V to 18 V.lf a transformer is used, observe thenecessary safety precautions as regardsinsu.lation of the mains voltage end thefuse ra ting.

Setting uplt is recommended to adjust the com-pleted printed-circuit board before it isfitred into the enclosure. This means thatthe switches and the output sockets haveto be connected provisionally.

Apply power and allow a few minutesfor the circuit to warm up. Set 52 to NOR-MAL, and Pa to a frequency roughly at thecentre of a range, e.g., 100 Hz. Connect afrequency meter to the signal outpur andadjust P4 until the measured frequencyequals that set on the scale.

Tf you do not have access toa frequencymeter to perform this adjustment, use thebeat frequency method instead. Feed the100 Hz signal obtained with the aid of asmall mains transforrner, a bridge rectifierand aseries network of a 100-0 resistor

and a 100-JlFcapacitor, to a loudspeaker.Drive another loudspeaker with the gen-erator output signal. Listen to the two sig-nals and adjust P4 for zero frequencydifference. This method gives quite accur-ate results (for use with a 60-Hz mains. setthe generator to 120 Hz).

The adjustment of the sweep functionis carried out at the greatest sweep time,10 s. Turn PI fully counter-clockwise andconnect an analogue voltmeter or a LEDto the sweep ou tput of the instrument.Adjust P2 until a time period of 10 5 isobtained.

Tips and options[n the basic arrangement, the waveformand frequency range selection are effectedwith 3-position miniature switches frontC&K. The switching configurations areshown in Fig. 3. In the case of 53, the use

4 2

9000405

Fig. 4. Alternative switch connectionwhich enables the frequency range of thegenerator to be extended to about 200 kHz.

11I TEST AND MEASUREMENT

Fig. 5. Completed peS-front panel assembly seen trom the pes track side (Iett) and from the side (right).

of a switch that has only three positionslimits the frequency range of the instr'u-ment to abou t 20 kHz. A fourth range,whieh may be desirable in n number ofcases. may be added by replacing thetoggle switch with Cl smalI, four-position,rotary switch, whieh is wired as shown inFig. 4. The numbers 1 to 6 on the overlaymark the connections of the terminals of53. Ta create Cl200 kHz range. solder anadditional 2.2 nF capacitor, C16, to thecentre terminals, numbers 2 and 5, andsolder Cl wire between terrni nals 2 and 4.Next, connect the contacts (1,2 and 3) andthe pole of the rotary switch to the PCBterminals 1, 3, 6 and 5.

As already nored. the value of R17deterrnines the sha pe of the sine-wave. At

LOW-BUDGET TEST EQUIPMENT

This is the lifth instalment in aseries01 artieles describing test equipmentno serious electronics enthusiast ordesign engineer can da without. Allinstruments are housed in an attrac-tive metaleabinet type LC-850 fromTelet, which comes with with protee-tive strips at the sides. The switchareas on the front panels are grey,light blue or dark blue with white let-tering, and their size is g\lared to thefront panel of the LC-850 enclosure.Shown in the picture are the lnstru-ments deseribed so lar in this series.The power supply shown in front willbe nex! month's subject.The pile of four instruments behindthe sweep/function generator con-sists 01 (top to bottorn):

LFIHF signal tracer (January 1990)

Q meter (May 1990)

RF inductance meter (November 1989)

AC millivoltmeter (February 1990)

The polnter knobs used on the instru-ments are made by applying a smallarrow or triangle (available as transfersymbols) on 10 !he collet and protect-ing it with plastie spray.

relatively high generator frequencies, itmay be useful to replace the resistor by a500-0: preset to enable the distortion to beminimized , Front a number of practica ltests. the XR2206 supplies a fairly cleansinusoid a l stgnal up to about 100 kHz. To-wards 200 kl-iz, the sine-wa ve graduallychanges into a triangular waveform.

The L165 1Scapable of providing COI1-stdersbly more ou tput power than it isallowed to by the 50-Q output. If it isdesired to use the genera tor for swept- fre~quency measu rements Oll loudspeakers ordrive units, a low-impedance outpu t maybe provided Oll the instrument by fittingtwo binding posts on the rear panel. Thesignaloutlet is connected direct to the ne-gative terminal of CI3 to negate the effect

of the two 100-Q series resistors. Note,however, tha t this extension requires arather larger power suppfy. In that con-text, it is recommeuded to use a mainstransformer capable of supplying at least1 A of secondary cu rrent, a bridge rectifier(4xl N4001l and an additional 1000 ~Fsmoothing capacttor. The single-phaserectifier on the board, 02, is replaced by awire link. The I-A power supply enablesthe function generator to provide ampledriving power for 4-Q and 8~0: loudspea-kers. The use of a bridge rectifier insteadof the single-phase rectifier allows amains transformer with a secondary volt-age of 12 V to be used instead of a 15-V~~. .

ELEKTOR ELECTRONICS MAY [990

TEST AND MEASUREMENT

Fig. 2. Track layout (mirror image) and component mounting plan of the single-sided printed circuit board for the generator.

verting input. This is achieved with theaid of Tl. In the monostable formed byT2-T3,Cl ensures that the integration capa-citor is discharged rapidly to provide thetrailing edge of the sawtooth. The refer-ence volta ge provided by DI thus deter-mines the amplitude of the sawtoothvoltage that sweeps the frequency of thefunction genera tor.

The sawtooth volta ge is also availableat aseparate sweep output on the instru-ment. Resistor RBsets the output imped-ance to about 1 kQ. The sweep output isshort-circuit resistant and may be used for

driving the X amplifier of an oscilloscopefor swept-frequency measurements.

Output amplifierThe Type L165 opamp used in the outputamplifier is capable of providing ampleoutput current at a reasonable price. TheIC is used in a conservatively rated con-figuration and is therefore not likely toactuate its internal overheating protec-tion. The power opamp is wired as annon-inverting buffer (volta ge follower),so that the amplitude and phase of the

output signal correspond to those of theinput signal at the wiper of amplitudecontrol Ps. An electrolytic capacitor, Cl3,is required to decouple the d.c. compo-nent at the output since a non-symmetri-cal supply is used. The parallel resistorcombination at the output is not strictlyrequired for overload protection (wh ichthe L165 provides by itself). It does, how-ever, limit the output current to a safevalue. At the same time, it sets the gener-ator output impedance to 50 Q, which is acommonly used value on test equipment.

ELEKTOR ELECTRONICS MA Y 1990