Elektor Electronics 10/99

Anyone with only the slightest interestin radio-frequency engineering willimmediately be captivated by the tinyreceiver described here. It is con-structed on a printed-circuit board

measuring 8.5×5 cm (3.4×2 in) andconsists of only a handful of compo-nents, a whip antenna at the input anda small loudspeaker at the output. Nev-ertheless, it receives broadcasting sta-tions from all over the world: the Voiceof America; Radio Moscow; RadioPrague, not forgetting the BBC WorldService when you’re on holiday. Andall that with a minimum of controls.

F O U R B A N D S I N O N EThe receiver is designed for operationover a single range containing the25 m, 31 m, 41 m, and 49 m, short-wavebands. There is therefore no need for aband selector. It is tuned with the aid of

The design describedin this article is one ofthose with immediate

appeal. It is easy tobuild, very compact,

and offers exceed-ingly good perfor-mance. Even withonly a short whip

antenna, it receives a myriad of

broadcasting stationsat good strength.

42

Design by G Baars PE1GIC

poor man’s short-waveradio

HOOGFREQUENT

Some parameters Frequency range approx. 5.5-12.5 MHz

(25 m, 31 m, 41 m, and 49 m bands) Sensitivity (6 dB signal-to-noise) approx. 1 µV AGC range 86 dB Intermediate frequency 455 kHz Audio power output 1 watt into 8 Ω Quiescent current drain about 50 mA Supply voltage 12–15 V

small but effective

varactors (also called voltage-con-trolled capacitors or voltage-variablecapacitors – see inset) and a multi-turnpotentiometer. The latter ensures thatin spite of the very wide range, tuningis accurate and comfortable. The tun-ing indicator may be a light-emittingdiode (LED) or moving-coil µA-meter.

The receiver is designed for thereception of amplitude-modulated(AM) signals only, since that is the uni-versal mode of operation by broad-casting stations in the short-wavebands. The intermediate frequency (IF)bandwidth is 6 kHz, which ensuresgood audio reproduction.

T W O I N T E G R A T E DC I R C U I T SThe receiver is based on just two inte-grated circuits (ICs). One of these, IC2in the circuit diagram in Figure 1, is atiny audio-frequency (AF) amplifierthat boosts the demodulated signal toan output power of about one watt.The other, IC1, a Philips Type TDA1572,is the circuit on which the radio-fre-quency (RF) operation is based. In fact,it is a complete integrated AM receiver,containing an RF amplifier, mixer, oscil-lator, IF amplifier, automatic gain con-trol (AGC) and an AM demodulator.

Space in this article is insufficient togive a detailed description of theTDA1572, but much of this may befound on the Datasheet elsewhere inthis issue. All that will be said here is

that the mixer is a dual-balanced type and thatin the design of the RF amplifier anddemodulator great attention is paid toobtaining a large dynamic range andlow distortion.

The oscillator is a voltage-controlled(VCO) type which is provided withtemperature compensation. It worksvery well with a single inductor and isdesigned specially for use with varac-tors.

Finally, note that the TDA1572 has adedicated output for a field-strengthindicator.

The receiver is a single-conversionsuperheterodyne with an intermediatefrequency (IF) of 455 kHz. From anoperational point of view, this fre-quency should preferably have beensomewhat higher to avoid the problemof image frequencies (see inset), butthis is more than compensated by thesimplicity of construction, the readyavailability, and the low price of the455 kHz filter used.

There are two tuned circuits: onefor the input (L1-C2-D1-C2) and onefor the oscillator (L2-D2-C7-C9). Thesecircuits are tuned in synchrony withvaractors D1 and D2, bearing in mind,of course, that the frequency of thelocal oscillator circuit is at all times455 kHz higher than that of the inputcircuit.

The control voltage for the varactorsis provided by a 9-V supply, regulated

by IC3, via multi-turnpotentiometer P1.

Note that inductors L1 and L2 arereadily available small chokes.

The whip antenna is coupled to thehot end of the input circuit via capacitorC3. Although the input circuit couldhave been linked directly to the inputpin (17) of IC1, this is not done deliber-ately, since the network connected tothis pin inside the IC is low impedance,which would cause excessive dampingof the input circuit. There is thereforea buffer in the shape of source followerT1 between the input circuit and pin 17of IC1. The high input impedance ofthe buffer ensures a negligible load onthe input circuit as well as on the(high-impedance) whip or telescopicantenna. This arrangement has a ben-eficial effect on the sensitivity as well ason the selectivity of the receiver.

The ceramic IF filter, MF1, is a bal-anced type, both resonators of whichare interlinked by external capacitorC20. Inductors L3 and L4 match theimpedance of the filter to that of themixer. These inductors are also readilyavailable small chokes.

The demodulated and pre-ampli-fied audio-frequency signal is availableat pin 8 of IC1, from where it is appliedto integrated AF amplifier IC2 via vol-ume control P4. Note that IC2 needs noexternal components whatever — itsbalanced output makes even an outputcapacitor unnecessary.

43Elektor Electronics 10/99

C2

390p

C3

6p8

C1

22p

C4

100n

C5

100n

C6

100n

C9

3p9

C10

100n

C11

100n

C12

100n

C13

220n

C14

10n

C15

1µ16V

C16

10µ16V

C18

150p

C19

100n

C21

100n

C17

3n3

C8

220p

C7

220p

C22

100n

C20

39p

P4100klog

TDA7052

OUT1

IC2

OUT2

IN

IN

2

1

5

6

83

TDA1572IC117

1314 18 16

10

12

15 111

2 4 5 6 7 8

9

3

R6

22

Ω

R2

47

0Ω

D2

BB509

D1

BB509

L2

4µH7

L4

680µH

L3

82µH

R3

47

0k

R4

220k

T1

BF

L1

3µH9

R1

47

0k

245C

IC3

78L09

C24

220µ25V

C23

100µ16V

10k

P3

8 Ω

LS1

1W

P1100kMT

R5

22

k

R8

10k

R7

2k2

T2

BC550C

R9

82

0k

2k5

P2

M

100µA

D3

L

*

+12V

A

12V9V

9V

9V

990068 - 11

MF1SFD455

1V5...8V4

1V5...8V4

1V5...8V8

0V4V0

9V

9V

2V2

8V5

NULL

0V6

4V6

0V

6V

6V

1V

9

4V

2

4V

0

2V

2

2V

1

1V

0

0V

7

1V

9

1V

5

4V

2

1V

7

1V

0

1

Figure 1. The circuit isbased essentially on aType TDA1572 inte-grated AM receiver.

T U N I N G I N D I C A T O RAs mentioned earlier, there are twopossible tuning indicators. A mov-ing-coil µA-meter (100 µA full-scaledeflection — FSD) may be con-nected between the wipers of P2 andP3 via series resistor R7. Resistors R8and R9 must then be replaced by wirebridges and T2 may be omitted. TheFSD of the meter is set with P2 and itszero deflection with P3.

If space or another reason precludesthe use of a µA-meter, a high-efficiencylight-emitting diode (LED) may beused. This must be connected betweenresistor R7 and the 9-V supply line.Potentiometer P3 is then not needed,but T2, R8 and R9 are.

P O W E R S U P P L YVoltage regulator IC3 ensures an accu-rate, stable 9 V supply for IC1 and thevaractors. Audio amplifier IC2 may besupplied directly from the unregu-lated voltage(12-15 V), which may beprovided by batteries but, since thisrequires 8-10 cells, and the quiescentcurrent is about 50 mA, it is highlyadvisable to use a suitable mainsadaptor.

C O N S T R U C T I O NBuilding the receiver should not pre-sent even relatively inexperienced hob-byists any problems. If the receiver is

constructed on the printed-circuitboard (PCB) in Figure 2, it should bepossible to build it in about an hourand a half. Pay good attention to thecorrect polarity of the electrolyticcapacitors and ICs, and particularly tothe colour code of chokes L1–L4. Ifonly two of these chokes are placedwrongly, the receiver will definitely notwork properly.

Potentiometers P1 and P4 may bemounted directly on the board.

If an LED tuning indicator is used,solder its cathode (short terminal) tothe solder pin marked ‘L’ and its anodeto the + pin.

If a µA-meter is used as tuning indi-cator, connect it to the pins marked ‘M’with its – terminal to the – pin.

When the board is completed (seethe finished prototype in Figure 3),connect a whip antenna (or a 50 cmlength of wire) to pin A, a small loud-speaker to pins LS1, and a mains adap-tor to pins 0 and +12. At this stage,there should be some noise emanatingfrom the loudspeaker, and when P1 isturned, there may even be some musicor speech heard. If nothing is heard,

carefully check the completed board.Also, the voltages at certain pointsshould be compared with those indi-cated on the circuit diagram. Note thatthese voltages refer to a board withoutantenna and without reception of anystation.

Once the receiver works correctly, itshould be assembled in a suitableenclosure. The prototype is enclosed ina die-cast metal case from Hammond,but there are many other suitable cases.An ABS (plastic) enclosure may also beused, but this increases the risk of bodyeffects and spurious radiation affectingthe proper performance of the receiver.

44 Elektor Electronics 10/99

990068-1(C) ELEKTOR

C1

C2 C3

C4

C5

C6

C7

C8

C9

C10

C11C12

C13

C14

C15C16

C17

C18

C19

C20C21

C22

C23

C24

D1

D2

IC1

IC2

IC3

L1

L2

L3L4

LS1

MF1

P1

P2

P3

P4

R1R2

R3

R4

R5

R6

R7

R8R9

T1

T2

990068-1

MLA

0

+12

+ -

990068-1(C) ELEKTOR

2

Figure 2. The PCB forthe short-wavereceiver includingaudio amplifier isnotably small.

COMPONENTS LIST

Resistors:R1,R3 = 470kΩR2 = 470ΩR4 = 220kΩR5 = 22kΩR6 = 22ΩR7 = 2.2kΩR8 = 10kΩR9 = 820kΩP1 = 100kΩ, 10-turn potentiometerP2 = 2.5kΩ preset HP3 = 10kΩ preset HP4 = 100kΩ log. potentiometer

Capacitors:C1 = 22pF or 40pF trimmerC2 = 390pFC4,C5,C6,C10,C11,C12,C19,C21,

C22 = 100nFC7,C8 = 220pFC9 = 3.9pFC13 = 220nFC14 = 10nFC15 = 1µF 16V radialC16 = 10µF 16V radialC17 = 3.3nFC18 = 150pFC20 = 39pFC23 = 100µF 10V radialC24 = 220µF 25V radial

Inductors:L1 = 3.9µHL2 = 4.7µHL3 = 82µHL4 = 680µH

Semiconductors:D1,D2 = BB509IC1 = TDA1572 (DIL18 case)IC2 = TDA5072 (DIL8 case)IC3 = 78L09L = high-efficiency LEDT1 = BF245C or BF256CT2 = BC550C or BC549C

Miscellaneous:MF1 = SFD455 (Murata)M = moving-coil meter, 50-200 µALS1 = 8 Ω, 1 W miniature loud-

speakerEnclosure: see textPCB Order no. 990068 (see Readers

Services section towards the endof this issue)

S E T T I N G U P

The setting up of the receiver isstraightforward, since it contains onlya single calibration element: trimmercapacitor C1. This is set to its optimumposition as follows. Set potentiometerP1 to the centre of its travel, switch onthe receiver and open volume controlP4 slightly. Turn C1 until the noiseemanating from the loudspeaker is amaximum. Then connect a whipantenna or a 50 cm — 20 in — lengthof wire to the input, turn P1 slowlyanticlockwise (that is, from high to lowfrequencies) and tune to the first heardstation that gives a reasonable strongsignal. Readjust capacitor C1 for maxi-mum audio output.

When an LED indicator is used,adjust P2 until the indicator just lights atthe reception of weak stations andmuch more brightly with strong sig-nals.

When a µA-meter tuning indicatoris used, adjust P3 until the metershows zero in the absence of a signal.Follow this by adjusting P2 for full-scale deflection when a strong stationis being received. It is advisable torepeat these adjustments a couple oftimes.

[990068-1]

45Elektor Electronics 10/99

Image frequenciesIn superheterodyne frequency converters or receivers, an image frequencyis an undesired input frequency capable of producing the selected fre-quency by selecting one of the two sidebands produced by beating. Theword ‘image’ implies the mirror-like symmetry of signal and image frequen-cies around the beating oscillator frequency or intermediate frequency,whichever is higher.

Consider, for instance, Figure A. Here, the receiver is tuned to a stationat 6 MHz. Since the IF is 0.455 MHz, the oscillator is tuned to6.000+0.455=6.455 MHz. In Figure B, the same receiver is tuned to astation at 5.090 MHz, so that the oscillator runs at5.090+0.455=5.545 MHz.

Although the receiver in the second case is tuned to 5.090 MHz, theimage frequency of the 6 MHz station is also received, since this stationalso produces an IF of 6.000–5.545=0.455 MHz.

Image frequencies are precluded by the use of a high IF, since thismakes the difference between the original and image frequencies so largethat the image is rejected by the input filter circuit. A further remedy con-sists of increasing the selectivity of the circuits preceding the mixer. In asimple receiver as the present, these remedies are not easily imple-mented: the most effective remedy against image frequencies here is theuse of a short antenna.

OSC.

6MHz

a

OSC.

6MHz

b

990068-12

IF: 455kHz IF: 455kHz

5.545MHz6.455MHz

5.090MHz

Figure 3. Completedprototype receiverboard.

3

VaractorsA varactor or voltage-controlledcapacitor is a two-terminal solid-state device that uses the voltage-variable capacitance of a p-njunction. In the design of a normalsemiconductor diode, steps aretaken to minimize inherent capac-itance, whereas in a varactor thisis emphasized. Since this capac-itance varies with the applied volt-age, varactors may be used asvoltage-variable capacitors.