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Tune your favourite stations with this low-cost _IQ RECEIVER FM If you're new to the world of electronics, then here's a great project to get you started. This simple radio tunes in all your favourite FM stations, uses only two low-cost ICs & runs off a 9V battery. By DARREN YATES Many moons ago, if you were about to get stuck into your first electronics project, the odds were that you'd start with an AM radio. There were several reasons for this. First, AM radios in the 50s and 60s were relatively expensive and you could save lots of money by building one for yourself. Often, the parts could be scrounged from the junkbox and from derelict receivers and this added to the sense of achievement when the project was completed. Building an AM radio was also a lot of fun and it was educational. The constructor could learn all about radio frequency (RF) circuits and audio amplifiers, and could carry out any r.f. input 18 17 15 ,. 13 12 11 10 1,4V TDA7000 2,2k0 Vp 1+4,SVI • 7286939 . 1 a.f. output 16 2,2k0 SILICON CHIP repairs if the set later required servicing. During the last 20 years, AM radios have become progressively easier to build with the development of-specialised ICs. One such chip is the ZN414 AM radio IC. This device is a complete tuned radio frequency (TRF) receiver in a tiny 3-pin transistor style case and it only requires a 1.5V cell to power it. It gives quite useful results and many a "matchbox radio" has been built using this device. By contrast, FM receivers were always somewhat more complicated to build, although that situation has also changed during the last decade. In particular, the Philips TDA7000 FM radio IC is virtually a complete FM radio on a single chip. All that's required is the addition of an audio amplifier stage and a few external components to produce a working FM receiver. Apart from the TDA7000 FM radio chip, the design presented here uses just one other IC to give an economical 2-chip design. This second device is an LM386 audio amplifier IC. So most of the circuit functions are taken care of by the two ICs. But why build your own when you can buy a personal FM stereo radio for less than $20? The answer is to have fun and to learn about electronics. Provided you can solder and follow a simple wiring diagram, you Fig.1 (left): block diagram of the Philips TDA7000 FM radio circuit. This device is virtually a complete FM tuner on a single chip. All that's required to make a working receiver are a few external parts plus an audio amplifier stage. PARTS LIST 1 PC board, code SC06111921, 129 x 85mm 2 SPOT toggle switches (S1, S2} 1 100k.Q log potentiometer (VR1} 1 knob to suit potentiometer 1 57mm 8.Q loudspeaker 1 F29 ferrite slug 1 5mm coil former (Altronics Cat. L-5210; Jaycar Cat. LF1224; DSE Cat. L 1010) 4 3 x 25mm-long tapped spacers 4 screw-on rubber feet 1 9V battery 1 battery snap connector 1 60-160pF tuning capacitor Semiconductors 1 TDA7000 FM receiver (IC1} 1 LM386 audio amplifier (IC2} All the parts for the FM Receiver are mounted on a small PC board & this can be installed in a plastic project case. The antenna consists of a 75cm length of light-duty hook-up wire. should have little difficulty building up the design presented here. Admittedly, our design is mono only but then most of the low-cost commercial units can only drive headphones. This unit can drive a small loudspeaker to quite useful volume levels and it can produce good quality sound from all local FM stations from 88-108MHz. Why not tackle it as your first project? Block diagram FM signals are more difficult to decode than AM signals because of the way the information is transmitted. In AM radio, the audio signal is encoded by modulating the amplitude of a fixed-frequency carrier signal. After transmission, this audio signal can be quite easily recovered from the tuned carrier using a simple diode detector. With FM, however, the carrier amplitude is kept constant and it is the frequency of the carrier that is varie.d to encode the audio frequency signal. The method of detection (or demodulation) in this case is much more complicated but, fortunately, this function is taken care of by the TDA7000 IC. Fig.1 shows the various circuit blocks inside the TDA7000, as well as the external parts required to make a complete FM tuner. It is easy to get going since only the local oscillator (VCO) requires adjustment during alignment. All the other components, with the exception of the tuning capacitor (Cvl, are fixed . The TDA7000 IC functions pretty Capacitors 1 470µF 16VW electrolytic 1 220µF 16VW electrolytic 1 100µF 16VW electrolytic 2 10µF 16VW electrolytic 1 0.15µF MKT polyester 1 0.1 µF MKT polyester 2 0.1 µF ceramic 1 .01 µF MKT polyester 2 .0033µF MKT polyester 1 .0027µF MKT polyester 1 .0022µF MKT polyester 2 330pF ceramic 1 220pF ceramic 1 180pF ceramic 1 150pF ceramic 1 56pF ceramic 1 47pF ceramic 1 39pF ceramic 1 33pF ceramic Resistors (0.25W, 1%} 1 18k.Q 1 10k.Q 1 4.7.Q Miscellaneous 30cm of 0.63mm diameter ECW, 75cm of light-duty hook-up wire for antenna. much as a conventional superheterodyne tuner. This means that the incoming signal is mixed with a local· oscillator signal to produce an intermediate frequency (IF). This IF is then filtered to remove any mixer artefacts and, finally, demodulated to produce the desired audio signal. There's just one deviation from norN ovEMBER 1992 17 ONO ANTENNA MUTE S2 POWER S1 4.70 OFF .01 10k 220 + .0033 0.15 220pF 4 0.1 330pF 18 17 16VW+ 150pF 15 OFF ON <>-'---0+6-9V 12 10 13 * 39pF * 47pF 14 10 16VW + - 180pF B IC1 TOA 7000 5 8 .,. VC1 2-&0pF .,. 33pF L1 .,. 56pF 100 + 16VW+ 0.1 + .,. 16 FM RADIO Fig.2: the circuit uses just two ICs - the TDA7000 FM radio chip & an LM386 audio amplifier. Variable capacitor VC1 tunes IC1 across the FM broadcast band (88-108MHz), while L2 & its associated capacitors form a bandpass filter at the antenna input to eliminate interference from signals outside the tuning range. mal practice. The majority of FM receivers use an IF of 10. 7MHz whereas the TDA7000 uses a very low IF of 70kHz. The advantage of such a low IF is that it can be filtered with standard active op amp circuits instead of coils or ceramic filters. Normally though, a low IF results in really bad distortion when used with wideband deviation FM; eg, the normal broadcast band FM which has a maximum deviation of ±75kHz. However, the TDA7000 successfully manages to demodulate wideband deviation (±75kHz) FM while only having a 70kHz IF. What happens is that the recovered audio is used to modulate the local oscillator so that the received FM deviation is always less than ±15kHz. In effect, the recovered audio signal is compressed to internal mixer (on pins 13 & 14) via a bandpass filter. This bandpass filter consists of inductor L2 and the 39pF & 47pF NPO capacitors. Its job is to filter signals that lie outside the desired tuning range and thus eliminate interference. Tuning capacitor VCl and inductor Ll ensure that the voltage controlled oscillator (VCO) tunes across the FM broadcast band. Inductor Ll carries a ferrite slug and this is adjusted during the alignment procedure so that VCl covers the desired frequency range (88-108MHz). Pin 1 of !Cl is the mute control line. Pulling this line high via a 10kQ resistor disables the muting functions and allows weaker stations to be tuned if desired. Switch S2 is used to switch the muting function in or out of action. When S2 is set to ON, the circuit mutes the background hiss that otherwise occurs when tuning between stations. The recovered audio signal appears reduce its dynamic range. Although this isn't desirable in a hifi FM receiver, the results are still very good and this technique considerably simplifies the circuitry required. In fact , the distortion is typically less than 2.3% at ±75kHz deviatio:n, so your favourite FM station will still come in loud and clear. Circuit diagram Now take a look at Fig.2. This shows the complete circuit diagram for our simple FM radio receiver. At the centre of the circuit is the TDA7000 FM radio IC (ICl) and this drives the LM386 audio amplifier (IC2) via volume control VRl. Let's take a quick run through the various circuit functions. The RF signal is picked up by the antenna and fed to the TDA7000 's TABLE 1: RESISTOR COLOUR CODES u Q 0 0 18 ,:•:} 470 18k No. 1 1 SILICON CHTI' Value 18kQ 10kQ 4.7Q 4-Band Code (1%) 5-Band Code (1%) brown grey orange brown brown grey black red brown brown black black red brown yellow violet black si lver brown brown black orange brown yellow violet gold brown 8 This close-up view clearly shows the construction of coil Lt. The F29 ferrite slug at the top of the former is adjusted during the alignment procedure so that the receiver tunes the required frequency range. on pin 2 and is fed to volume control VR1 via a low-pass filter consisting of an 18kQ resistor and a .0039µF capacitor. This filter stage, in conjunction with VRt , provides the necessary 50µs de-emphasis for the recovered audio signal. From there, the signal passes to the LM386 audio amplifier stage (IC2). IC2 operates with ap. AC gain of 200 by virtue of the 10µF feedback capacitor connected between pins 8 and 1. The amplified output appears on pin 5 and is coupled to a miniature 8Q · loudspeaker via a 4 70µF capacitor. Power for the circuit is derived from a 9V battery and is switched on and off using Sl. The supply rail is de~ TABLE 2: CAPACITOR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Value 0.15µF 0.1µF .01µF .0033µF .0027µF .0022µF 330pF 220pF 180pF 150pF 56pF 47pF 39pF 33pF IEC Code 150n 100n 10n 3n3 2n7 2n2 330p 220p 180p 150p 56p 47p 39p 33p EIA Code 154 104 103 332 272 222 331 221 181 151 56 47 39 33 coupled using a 220µF capacitor and a 4. 7Q resistor. Construction *MOUNTED ON COPPER SIDE Fig.3: install the parts on the PC board as shown in this wiring diagram. The wiring shown dotted is installed on the copper side of the board. All of the components for the FM Radio, including the battery and loudspeaker, are fitted to a small PC board coded SC06111921. Before starting construction, check the board carefully to ensure that it has been correctly etched. Correct any faults that you do find immediately (in most cases, there will be none). Now check that the mounting holes for coil former Ll, potentiometer VRl, tuning capacitor VC1 and the two switches (Sl & S2) are large enough to accept these parts. Enlarge the holes if necessary using a small drill. Fig.3 shows the parts layout on the PC board. Install the wire link first, then solder the resistors into place. Table 1 shows the colour codes for the resistors but you should double-check them with your digital multimeter. When you've completed that job, solder in the ceramic and MKT polyester capacitors. Table 2 shows the capacitor codes and their equivalent values. Check the value of each capacitor carefully before installing it on the board. If you get them mixed up, the tuned circuits in the front end of the receiver could be thrown off frequency and the project wont work. Note particularly that one of the 0. lµF ceramic capacitors is installed on the copper side of the board, directly beneath ICl. Keep all capacitor leads as short as possible. At this stage, the two ICs can be installed. These should be soldered directly to the PC board. Take care with the polarity of each device and make sure that you don't overheat them when soldering their pins. Pin 1 of each device is adjacent to a small notch or dot in one end of the plastic body. Tuning capacitor VC1 is installed from the component side of the board, L2 L1 Fig.4: coil Lt consists of 2½ turns of 0.63mm enamelled copper wire on a 5mm former, while L2 is made by winding 6 turns of 0.63mm enamelled copper wire on a 6mm drill bit. NovEMBER 1992 19 The pen in this photograph points to the O.tµF ceramic capacitor that's mounted directly beneath ICt on the copper side of the PC board. Note also the wire link that's used to connect VR1 's metal case to ground. coil off the drill bit and push it all the way down onto the board before soldering its leads (don't forget to scrape away the enamel from the leads first). The volume control (VRl) is installed from the copper side of the PC board. Before doing this, solder three short tinned copper wire leads to its pads (ie, install these leads on the copper side of the board). The volume control can then be mounted in position and the three wire leads soldered to its terminals. VRl 's metal case should now be earthed by connecting it via a short wire link to the adjacent groundplane on the PC board (see photo). This is done to prevent hum pickup from breaking through into the audio path. Note that it may be necessary to scrape away some of the oxide on VRl's case to ensure a good solder joint. The leads shown dotted in Fig. 3 are run on the underside of the PC board. Use light-duty hook-up wire for the loudspeaker and switch leads and be sure to wire the battery snap connector so that the supply polarity is correct. The antenna lead consists of a 75cm length of light-duty hook-up wire which is soldered directly to the PC board. Once the wir~ng is completed, secure the loudspeaker to the top of the board using a suitable adhesive. The battery can be secured using a clamp, double-sided sticky tape, or even a large rubber band. Finally, fit four 25mm-long tapped spacers to the four corner positions of the board. These spacers are then fitted with screw-on rubber feet to prevent scratches to the resting surface. Test & alignment The two toggle switches (S1 & S2) are installed from the copper side of the PC board, as is the volume control pot (VR1). The four standoffs at the corners of the board should be fitted with rubber feet. with its central shaft protruding through to the copper side. Secure it using the two small screws supplied, then mount the two toggle switches in position. The next step is to wind the two inductors. 11 is simply 2½ turns of 0.63mm-diameter enamelled copper wire (ECW) wound on a 5mm former with an F29 ferrite slug - see Fig.4. Note that each end of the coil should finish on opposite sides of the former. Strip the enamel from the ends of 20 SILICON CHIP the leads before soldering the coil to the board and use a small dab of adhesive to hold the former in position. Don't just rely on the coil to secure the former. If the former (and thus the ferrite slug) moves about, it will alter the tuning. The ferrite slug should initially be screwed about half-way into the top of the former. Inductor 12 is made by winding six turns of 0.63mm diameter ECW onto a 6mm drill bit or something similar. Wind on the six turns, then slide the To test the unit, wind the volume up to about half way, set the MUTE switch to "off", and switch on. You should immediately hear noise from the loudspeaker (if not, try setting the MUTE switch to its other position). Now flick the MUTE switch to "on"; the noise level should drop appreciably, although you should still be able to hear some hiss if you put your ear close to the loudspeaker. Now try tuning in some stations. If you're in a normal house environment and within about 40 or 50km from some reasonably strong stations, you should be able to pick them up. Sweep through the entire frequency range with the tuning capacitor and ANTRIM TOROIDAL TRANSFORMERS • • • • Now made in Australia Standard range 15VA to 2kVA Large ex-stock quantities available Competitive pricing available for OEMs & resellers • Simply the best transformers all round HARBUCH ELECTRONICS PTY LTD 90 George St, Hornsby, NSW 2077. Phone (02) 476 5854 Fax (02) 476 3231 Fig.5: check your PC board carefully against this full-size etching pattern before installing any of the parts. keep a record of the radio stations you hear. Make sure you have the MUTE switch turned off. If there are some stations that you can't pick up at one end of the band and the tuning capacitor has run out of travel,adjust the slug in coil 11 by winding it out of the former a couple of turns. This should shift the stations further tov\;ards the centre of the tuning capacitor. If, after repeated adjustments, you still don 't have enough range, try adjusting the two small screws on the back of the tuning capacitor. To do this, tune in a station and then, using an insulated screwdriver, slowly turn one of the screws to shift the station position (note: only one of screws will have any effect). Make only small adjustments at any one time before re-checking the frequency range. If you move the station the wrong way, rotate the adjustment screw in the other direction. By the way, it's important to use an insulated screwdriver for this job. If you don't, the ?,dditional capacitance provided by your hand will detune the front end of the receiver and it will be difficult to judge the effect of any adjustments. In normal use, you should find that an alkaline battery lasts about 40 hours , depending on the setting of the volume control (VR1). If you strike problems at any stage, switch off immediately and check the board for wiring errors. SC Protect your valuable issues Silicon Chip Binders These beautifully-made binders will protect your copies of SILICON CHIP. * * Each binder holds up to 14 issues * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Heavy board covers with 2-tone green vinyl covering Price: $A11.95 plus $3 p&p each (NZ $6 p&p) . Just fill in & mail the order form on page 101; or fax (02) 979 6503; or ring(02) 979 5644 & quote your credit card number. NovEMBEH 1992 21