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The Mini-mitter is a miniature stereo FM transmitter using just one integrated circuit and powered by one 1.5V penlite cell. You can connect your CD player to the Mini-mitter and then listen to your favourite CDs anywhere in the house, via your Walkman personal portable. By JOHN CLARKE & LEO SIMPSON How would you like to listen to your favourite CDs and records via headphones without being tethered to your stereo system? Maybe you want to wash the car, mow the lawn or vacuum your house while you listen to your favourite music. Clearly, a Walkman-style cassette player will let you listen via headphones while you work but who wants to go to the trouble of dubbing a record to tape every time the mood hits you. Or maybe you have a portable CD player and you'd like to listen to CDs via your car radio. Most car radios do not have connections for an external program source so that 14 SILICON CHIP can be a real problem. If you have these problems, the Mini-mitter is the answer. You can connect it up to your stereo system and broadcast your own stereo music program on the FM band; ie, 88-108MHz. The Mini-mitter puts out a flea-power signal (around 200 microwatts or so) but that is enough to give clean stereo sound over the average portable FM radio within a range of 20 metres or so. You can use it in your car with a portable CD player, to avoid making any connections to your car radio. Just connect the CD player up to the Mini-mitter and tune in your car radio to the correct frequency to hear the program of your choice. The Mini-mitter is built into a small case and powered from just a single 1.5V AA size cell. Two RCA sockets are provided at one end of the case for the left and right audio channels. At the other end, there is a length of wire which is the transmitting antenna and there is a slide switch to turn the Mini-mitter on or off. Inside the case, there are a couple of trimpots to adjust the level of the left and right channel audio signals. These are important to obtain minimum audio distortion, especially when listening to compact discs. There are also internal adjustments to vary the transmitter frequency anywhere within the 88 to 108MHz FM band. We'll talk about these later. As you can see from the specs, battery cost will not be a problem. A normal penlite cell should last for many months even if you use it for several hours every day. A single IC provides all the .......,,s ____*___*_________ .01 .01 'T'l41-t-pF__..._L2.., i .001 18 F OUT * LEFT IN 47pF ,.. IC1 BA1404 RIGHT INPUT ~ 1.5V VR3 50k LEFT INPUT er t.J ANTENNA DSC lO * * 15pF osc=----41 1 RIGHT IN MPX OUT 14 PILOT OUT 13 10 + JJOpF 16VW _ STYRO ... 4.7pFJ ' AUDIO BIAS 2 MOO IN 12 10 DSC AUDIO RF BIAS GNO 4 3 8 + .,. * C1 15pFI 2·10pF -:- FREQUENCY TRIM ":" .,. 16VWJ .OOl+ L1 ,L2: 1.5T, 0.62mm DIA. ENAMELLED COPPER WIRE ON 5mm FORMER WITH F16 CORE .D1'i * CERAMIC FM STEREO TRANSMITTER Fig.1: the key component in the circuit is the BA1404 FM stereo transmitter IC from Rohm Corporation. This chip includes a 36kHz multiplexer plus all the necessary input amplifiers, mixers and oscillators required for FM stereo transmission. Specifications Operating voltage ... ..... .... ..... .. ....... ... .... .. ..... ...... .... .......... ... 1.5V Quiescent current ... ... ... .. .. .... .... .. .. ... .. .. .. ..... .. ..... ... .... ......... . 3mA Harmonic distortion ..... .. ........ .. .. ..... .. ...... .. . 3% for 200mVp-p input Stereo separation ........... ..... ......... .. .. ... ...... ... ... ... .... 45dB at 1 kHz Frequency range ..... ........ ... .. ... .. ....... ... ......... .. 88 .1 to 107.9MHz necessary functions for FM stereo transmission. This includes input amplifiers. 38kHz multiplexer. mixers and oscillators. Naturally there are a number of passive external components but the resulting circuit is very compact. How it works Fig.1 shows the complete circuit of the Mini-mitter. It is based on a BA1404 integrated circuit made by the Rohm Corporation of Japan. The circuit works as follows. The left and right audio signals from the program source (CD player, tape player etc) are connected to trimpots VRl and VR2 to set the input signal levels. These are coupled via a parallel network of a 47k0 resistor and .OOlµF capacitor to provide the standard 50µs pre-emphasis applied to Australian FM signals. Put simply, pre-emphasis is a defined amount of treble boost applied to the signal before it goes into the transmitter. Then, when the signal is received and demodulated, an equivalent inverse order of treble cut, called de-emphasis, is applied before the audio signal is fed to the tuner outputs. The result is an improvement in the high frequency signal-to-noise ratio, which means a reduction in audible hiss. The pre-emphasised audio signals are then coupled via a series 4.7µF capacitor and 100 resistor to the left and right inputs of the IC, pins 18 and 1, respectively. The 100 resistor is there as a " stopper" to prevent latch-up due to RF signals on the input leads. The internal bias supply for the audio input amplifiers is filtered by a lOµF capacitor connected to pin 2. To provide the 38kHz multiplex oscillator, a miniature 38kHz crystal (same physical size as a 32kHz watch crystal) is coupled via a lOpF capacitor between pins 5 and 6. The oscillator's bias supply is decoupled by a l OOOpF (.OOlµF) capacitor connected to pin 4. To provide a balance setting for the multiplexer, VR3 , a 50k0 trimpot, connects between pins 16 and 17 of the IC. The multiplexer output from pin 14 and the 19kHz pilot signal at pin 13 are mixed using series resistors and capacitors to set the correct phase and level to the modulation input at pin 12. The modulator oscillator tuned circuit components are those connected to pins 9 and 10; ie, L1 and associated capacitors. Cl , a 2-lOpF trimmer capacitor, provides fine O CT0 BER 1988 15 The completed assembly mounts inside a small plastic case to give a really compact assembly. The small trimmer capacitor (at end of pen) allows fine tuning of the transmitter output frequency. Fig.2: here's how to mount the parts on the PCB. Keep all leads as short as possible and note that some of the parts are mounted endon to save space. The coil formers for Lt and L2 are installed from the underside of the PCB. adjustment for the oscillator frequency. Inductor 12 and a 47pF capacitor at pin 7 form the tank circuit [ie, a parallel tuned circuit) for the RF amplifier. The output signal to the antenna is coupled via a 4. 7pF capacitor. One of two antennas may be used. If the transmission path is very short, you will find that a single wire 15cm long will suffice. For a more effective transmission, a simple dipole can be used, as described later. Construction The Mini-mitter circuit is built onto a small printed board (coded SC06-1-1088-1) measuring 4 7 x 16 SILICON CHIP 60mm. The board is housed within a plastic case measuring 83 x 54 x 28mm. A Scotchcal label can be attached to the lid of the case to indicate the controls. Start construction by checking that the PCB will neatly fit into the case so that it is supported on the stops of the PC guides. The stops are provided for mounting a printed board horizontally, as we have done, but they need to be lowered by a bout 4mm, to give sufficient clearance for the battery and its holder. You can achieve this by judiciously cutting away the guides with a pair of side cutters. If the PCB is too wide it can be filed down to size until it fits neatly into the case. The PCB requires two 5mm holes for the coil formers for 11 and 12. These formers are installed from the underside of the PCB and are glued in position [using nail polish or 5-minute epoxy). You can now insert and solder the IC as shown on the overlay diagram [Fig.2). The 1.5-turn coils can then be wound onto each former. These are wound by first stripping the enamel from one end of the wire and soldering it in place. Wind on 1.5 turns and insert the free end of the wire into the PCB. Cut it to length, strip off the enamel insulation and solder. Both coils are wound in the same way. Once the coils have been wound, the remaining parts can be installed as shown in Fig.2 . All the resistors are installed end-on [ie, vertically) and so is the 330pF polystyrene capacitor. Ensure that the four electrolytic capacitors are oriented for correct polarity. If you can't obtain a single AAcell holder you can make one by cutting down a dual AA-cell holder. The holder is cut in half longitudinally to provide a single holder. The spring in the negative end is soldered into the PCB while the positive end is connected to the PCB using a stiff piece of copper wire looped over and soldered to the clip socket. Once the PCB is complete, install it in the case and mark out positions for screwdriver access holes for the trimpots, VRl and VR2. You should also mark out holes for the panel RCA sockets and switch. The switch is mounted very close to the base of the case so that it does not foul the PCB. Drill holes for the sockets and trimpot level adjustment holes. The switch is mounted using small selftapping screws into the mounting flanges. The frequency adjust hole in the lid is found after attaching the label artwork to the lid and drilling at the hole position provided. Now wire up the switch, RCA sockets and antenna. The antenna can be a simple dipole [ie, an indoor FM antenna) or a single length of wire [about 15cm would be suitable) connected to the antenna terminal. For a dipole, the second How an FM Stereo Transmitter Works Fig.3 shows the block diagram of a stereo transmitter as typified by the Mini-mitter. The left and right channel inputs are applied to trimpots and then to 50µs preemphasis circuits (to provide treble boost above 3kHz, to the Australian FM standard). After pre-emphasis, the left and right signals are fed to buffer amplifiers and then to the multiplexer which is driven at 38kHz. This produces a sum (L + R) signal and a difference (L - R) signal which is amplitude modulated on a 38kHz carrier. The carrier is suppressed to provide a double sideband suppressed carrier signal. The (L + R) and (L - R) signals are mixed with the 1 9kHz pilot signal which is derived by dividing down the 38kHz oscillator signal by two. The resulting composite signal is then frequency modulated onto a carrier frequency in the FM band. Once filtered and amplified in the RF amplifier, the signal is transmitted via the antenna. Fig.4 shows the spectrum of the composite stereo signal. The L + R signal occupies the frequency range between O and 1 5kHz. The double sideband suppressed car- ANTENNA (L +R)+(L-R)coswst +Kcoswpt MULTIPLEXER PILOT MIXER .,. RF MIXER OSCILLATOR Fig.3: block diagram of an FM stereo transmitter, as used in this project. The accompanying text explains how it all works. LEVEL L+R L-R 0 15 19 23 PILOT L-R 38 53 FREQUENCY (kHz) Fig.4: frequency spectrum of the composite transmitted stereo signal. Note the pilot tone at 19kHz. rier signal (L - R) has a lower sideband which extends from 23-38kHz and an upper sideband from 38-53kHz. There is no subcarrier at 38kHz. The pilot carrier at 1 9kHz is also shown. The pilot tone is used by the receiver to reconstitute the 38kHz subcarrier so that the stereo signal can be decoded . connection goes to the earth track adjacent to this antenna terminal and is soldered to the underside of the PCB. You can buy a ready made dipole antenna from Jaycar (Cat. 1 T-3002) or Arista outlets (Cat. FMAl). VR3 should initially be set to the centre position. Now insert a 1.5V AA cell into the battery holder and check that there is 1.5V across pins 3 and 15 with a multimeter. Initial checks Operation can initially be checked with an FM tuner set to an unused frequency near the centre of the band. Insert slugs into 11 and 12 and adjust both so that each slug is about 2mm below the top of the former. Now adjust the 11 slug until the tuner receives the signal. This can be detected by the level and/or This close-up view clearly shows the winding details for L1 and L2. The PCB clips into guides running down each side of the plastic case (see text). OCT0BER1988 17 ..I L + ff!il~ SC 06-1-1088-1 + ffl!HIHs LEFT RIGHT FREO.e ADJ. LEVEL LEFT ~~ ON OFF LEVEL RIGHT ., + STEREO FM TRANSMITTER i 1~1~ + r Fig.5: actual-size artwork for the front panel. Fig.6: actual size PCB artwork. PARTS LIST 1 PCB, code SC06-1-1 088-1 , 47 x 60mm 1 plastic case with plastic lid, 83 x 54 x 28mm 2 RCA chassis mounting sockets 1 AA cell holder 1 1.5V AA battery 1 sub-miniature slider switch 2 Neosid 722/1 5mm coil formers 2 Neosid F1 6 4mm ferrite cores to suit formers 1 38kHz miniature crystal 1 BA 1 404 Rohm FM stereo transmitter IC 1 1 00mm-length of 0.63mm enamelled copper wire Capacitors The two small holes in the end of the case allow screwdriver access to the level adjustment trimpots, VR1 and VR2. A third hole in the lid of the case provides access to trimmer capacitor Cl for frequency adjustments. tuning meter on the FM tuner and/or by a stereo light indication. The L2 slug is then adjusted for maximum signal. To set the transmitter on other parts of the band, the slugs must be low down into the coils for the low frequency end of the band and conversely, higher for the high end of the band. Note that both slugs should be set in similar positions in the former for best results. The coils are slightly interactive so that adjusting one will affect the other. For fine tuning, use the trimmer Cl. The slugs can be secured in position using a very small amount of candle wax. Alternatively, some 18 SILICON CHIP dental floss inserted into the former before the slug will help stop movement. To adjust the input level trimpots, connect a program signal to the RCA sockets and listen to the output from the tuner, preferably via headphones. Adjust the trimpots so that distortion is not present on loud signals. This is easy to do because when the distortion is present it sounds awful. If you have an AC millivoltmeter and sine wave oscillator you can also set trimpot VR3 for maximum separaiion between channels. If not, just leave VR3 centred as this will be close to the optimum setting. 2 2 3 1 3 1 2 2 1 2 1 1 0µF 16VW PC electrolytic 4 . 7 µF 16VW PC electrolytic .01 µF miniature ceramic .0 1µF metallised polyester .001 µF metallised polyester 330pF polystyrene 4 7pF miniature ceramic 1 5pF miniature ceramic 1 0pF miniature ceramic 4. ?pF miniature ceramic 2-1 0pF trimmer capacitor (Philips 2222 808 11109) Resistors (0.25W, 5%) 1 x 1 00kD, 2 x 47kD, 1 x 2. 7kD, 2 x 1 on, 1 x 50kD miniature vertical trimpot, 2 x 1 0kD miniature vertical trimpots Miscellaneous Small self-tapping screws, hookup wire, tinned copper wire.