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The FM MiniMitter runs from a 3V supply and can drive a 300Ω dipole antenna for improved range. By JOHN CLARKE Build the MINIMITTER A miniature FM stereo transmitter Want to listen to your own selection of music through your personal stereo FM radio? Using the FM MiniMitter, you can broadcast from your CD player or from any other source so that it can be picked up within a 35-metre radius. There are many applications for an FM transmitter, particu­larly if it can broadcast in stereo. You can broadcast stereo signals from your CD player or any other source (stereo or mono) to an FM tuner or radio. The FM MiniMitter uses a single IC and a few other compon­ents and fits 58  Silicon Chip in a small plastic case. It broadcasts on the FM band (ie, 88-108MHz) so that it can be received by any standard FM tuner or portable radio. We published our first version of the MiniMitter in October 1988 and it has been a very popular project ever since. So why are we presenting it again? The main reasons are to extend its transmission range (a frequent request over the years), to make it easier to tune and to make it easier to operate the input level controls. Our new version of the FM MiniMitter operates from 3V rather than 1.5V and this can come from two AA cells or you can run it from a 6V DC plugpack. Running from two AA cells gives more than double the battery life of one cell since the circuit will continue to operate even when the supply voltage drops below 1V (although the power output and range will be much reduced). The RCA input sockets and DC socket are all PC-mounted so there Main Features • • • • • Frequency range: 95-105MHz (can be extended with component changes; see text) Transmission range: 40m with folded dipole antenna transmitter to typical FM receiver Current consumption: 10mA Distortion: typically 3% at 200mV audio signal level Separation between channels: typically 45dB at 1kHz is virtually no wiring to do. Setting the input levels is now much easier and the adjust­ able coils have been spaced further apart to minimise interaction and allow easier tuning. However, the biggest change is in the antenna circuit. You can now use a simple wire antenna or a 300Ω dipole antenna for extended range. Both antennas are matched correctly to the transmitter IC. The simple wire antenna is matched with a trimming capacitor while the 300Ω dipole is matched using a trimmer capacitor and a 75Ω-to-300Ω balun. These antenna improvements along with the 3V power supply provide the FM MiniMitter with a much greater trans­mission range. This is important, whether you are using it out­doors or between different levels in your home. Circuit details The FM MiniMitter is based on a BA1404 IC made by the Rohm Corporation in Japan. It incorporates all the processing circui­try required for stereo transmission which in itself is quite a complex procedure. See the separate section devoted to how an FM stereo transmitter works. Fig.1 shows the complete circuit. Left and right audio signals are connected via 10kΩ series resistors to trimpots VR1 & VR2. The 4.7µF capacitor at the wiper of each trimpot AC-couples Fig.1 (right): the circuit is based on the BA1404 FM transmitter IC. It is very similar to our original circuit published in October 1988 but now uses a 3V supply and can drive a 300Ω dipole antenna for increased range. April 2001  59 Fig.2: the component overlay for the PC board. The corners of the PC board must be shaped to fit around the pillars of the plastic case. the signal to the following 50µs pre-emphasis network comprising a .001µF capacitor and parallel 47kΩ resistor. The 50µs pre-emphasis is a defined amount of treble boost applied above 3kHz before the signal is processed in the trans­mitter. When the signal is received and demodulated, the boost­ed treble signal is subsequently attenuated so that the frequency response is flat over the audio spectrum. This process of pre-emphasis before transmission and de-emphasis at reception provides an improvement in the signal-to-noise ratio and a reduc­ tion in audible hiss. The 10Ω resistor following the pre-emphasis components at pins 1 & 18 is there to help prevent RF signals entering the IC. An internal bias voltage for the au- Fig.3 light duty hookup wire is used to wind the balun and 0.5mm enamelled copper wire for coils L1 & L2. 60  Silicon Chip dio section within IC1 is decoupled by the 10µF capacitor at pin 2 and this voltage is also present at pins 1 & 18. An internal 38kHz oscillator is formed using crystal X1 connected in series with the 10pF capacitor to pins 5 & 6. The oscillator drives the internal stereo multiplexer which switches between the left and right signals (at 38kHz). VR3 (between pins 16 & 17) provides the balance adjustment between the left and right channels. The multiplexer output at pin 14 and the 19kHz pilot tone at pin 13 are mixed at pin 12 to produce the modulation input. The resistors and capacitors at pins 13, 14 and 12 set the cor­rect pilot tone level which is required for detection in the stereo decoder in an FM receiver. Following the modulator input is the RF section which in­cludes the local oscillator and tuned output. The .001µF capaci­tor at pin 4 provides bypass of the bias voltages used for the RF circuitry. The RF mixer oscillator comprises L1, the 47pF parallel capacitor and the two 15pF capacitors at pins 9 & 10. Inductor L2 and the parallel 47pF capacitor filter the oscillator output to limit transmission beyond the frequency range required for stereo transmission. Finally, the RF output at pin 7 is coupled to the antenna and balun via a variable capacitor VC1. This is adjusted for best matching into the antenna. Balun L3 provides for a dipole antenna if required. As already noted, the circuit is powered from 3V, either from the two on-board AA cells or from an external 6V DC plug­pack. This is connected via a DC socket with internal switching. When the plugpack is connected, the internal switch disconnects the AA cells and the positive DC line is fed to the cathode (ie, positive terminal) of ZD1, a 3.3V zener diode. The negative return line to the plugpack goes via the on/off switch and a 56Ω resistor for current limiting. Construction All the components of the FM Mini­ Mitter are mounted on a PC board coded 06104011 and measuring 122 x 60mm. This is housed in a plastic utility case measuring 67 x 130 x 44mm. The component layout is shown in Fig.2. You can begin construction by checking that the PC board fits neatly into the case. The corners may need to be shaped to fit around the corner pillars in the box. Check that the holes for the DC socket and RCA sockets are the correct size. Also the mounting of coils L1 and L2 may differ slightly, depending on the type of coil former used. Some coil formers require a single The PC board inside the FM MiniMitter accommodates all the components and even the input and DC sockets. The only wiring is to the 300Ω antenna terminals and to the on/off switch. Note the small ceramic capacitors with the black dot at the top; these are NPO types, specified for minimum frequency drift, and must be used for the 15pF and 47pF values. central hole for mount­ing while others mount via two PC stakes which are part of the former. The central hole former will need a hole drilled for it and the former can later be held in Table 2: Capacitor Codes        with some super glue. PC stakes can be inserted into the adjacent holes for connecting the wires. Start the assembly by installing the wire links and resistors. You can Value IEC Code EIA Code .01µF   10n  103 .001µF    1n  102 330pF   330p   331 47pF   47p   47 15pF   15p   15 10pF   10p   10 Table 1: Resistor Colour Codes        No. 1 2 2 1 1 2 Value 100kΩ 47kΩ 10kΩ 2.7kΩ 56Ω 10Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black orange brown red violet red brown green blue black brown brown black black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown red violet black brown brown green blue black gold brown brown black black gold brown April 2001  61 How an FM Stereo Transmitter Works Fig.5: the block diagram of an FM stereo transmitter. The multiplexer switches the modulation input to the mixer oscillator between the left and right channels at a rate of 38kHz. The 19kHz pilot signal (ie, half 38kHz) is used to lock the 38kHz multiplex decoder (demultiplexer) in a stereo tuner. Fig.5 shows the block diagram of the BA1404 stereo trans­ mitter IC. The left and right channel inputs are applied to trimpots and then to a 50µs pre-emphasis circuit which provides treble boost above 3.38kHz. 50µs pre-emphasis is the Australian standard for FM broadcast. (75µs pre-emphasis is used in the USA and other countries). After pre-emphasis, the left and right channel signals are fed to buffer amplifiers and then to the multiplexer which is driven at 38kHz. This produces sum (L+R) and difference (L-R) signals which are modulated on the 38kHz carrier. The carrier is suppressed (removed) to provide a double sideband suppressed carrier signal. The (L+R) and (L-R) signals are mixed with the 19kHz pilot Fig.6: the frequency spectrum of the signal which is derived by div­ com­-posite transmitted stereo signal. Note the spike of the pilot tone at 19kHz. iding down the 38kHz oscillator used Table 1 as a guide to the resistor colour codes or use a digital multimeter to check the values. PC stakes are inserted for switch S1 and the 300Ω and 75Ω antenna outlets. Next, install the BA1404 IC, taking care to insert it with the correct orientation. This done, install the trimpots, trimmer VC1, the PC-mount RCA sockets and the DC socket. Zener diode ZD1 can be then be installed, followed by the capaci­tors. The electrolytic types must be inserted with 62  Silicon Chip the shown polarity, while ceramic types must be used where specified. Make sure you use a 38kHz crystal for X1. If you are mistakenly supplied with a 32kHz watch crystal, the transmitter will not work in stereo. Winding the coils Coils L1 and L2 are wound as shown in Fig.3. Wind one and a half turns around each former, using 0.5mm enamelled copper wire. The coil winding should be made within 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.6 shows the spectrum of the composite stereo signal. The (L+R) signal occupies the frequency range between 0 and 15kHz. The double sideband suppressed carrier signal (L-R) has a lower sideband which extends from 23-28kHz and an upper sideband from 38-53kHz. There is no subcarrier at 38kHz. The pilot tone at 19kHz is also shown. The pilot tone is used in the receiver to reconstitute the 38kHz subcarrier so that the stereo signal can be decoded. the lower half of the former. Insert each former into the PC board and solder the wires in position. The ferrite slugs can then be inserted and screwed in so the top of each slug is about flush with the top of the former. Use a plastic or brass alignment tool to screw in the slugs. Using an ordinary screwdriver is bad for two reasons: (1) it is very easy to crack the slug; and (2) the screwdriver badly affects the tuning of the coils. The balun is wound using red and Fig.7: these scope waveforms show the 38kHz multiplex output waveform at pin 14 of IC1 when an 8kHz sinewave is fed into the left channel. Fig.8: the 19kHz pilot tone at pin 13 of IC1 is a square wave. Fig.9: the final modulation waveform appearing at pin 12 of IC1 combines the left and right inputs (in this case, only the left 8kHz input), the 19kHz pilot and the 38kHz switching. Fig.10: if you have a 100MHz scope you can measure the RF output at the input to the balun. Use a 10:1 probe. black light duty hookup wire, also as shown in Fig.3. The colours make it easy to identi­fy each winding when you connect it to the PC board. Each wire is looped twice through the ferrite balun core, as shown in Fig.3. The finished balun is connected to the PC board stakes, taking care to connect the correct wire to each PC stake. The two AA cell holders are each wired to the PC board, taking care to orient each holder correctly. The holders can be held in place with small screws and nuts or simply glued in place using a hot glue gun, silicone sealant or even contact adhesive. The case requires holes for the RCA sockets and the DC socket. The screw terminals for the 300Ω ribbon cable antenna connections are mounted at the other end of the case. The screw terminal plate is secured with two screws which tap into the plastic case. Drill holes for the connection screws to pass through into the case and holes for the internal connection tabs. These are bent flat inside the case to allow the PC board to be easily installed without fouling. You will also need a hole in the lid for the power switch. Wire up the switch and the 300Ω terminals using light-duty insu­lated hookup wire. Testing Testing the transmitter can be done using two AA cells or with a 6VDC plugpack. Apply power and first check for a nominal 3V between April 2001  63 You can connect either a 300Ω dipole antenna to the screw terminals or run a length of wire from the 75Ω signal output on the PC board out through the adjacent hole. pins 3 and 15 of IC1. Now connect the 300Ω dipole antenna to the connecting screws or use a 1.5m length of insulat­ed hook­up wire connected to the 75Ω signal terminal on the PC board. Do not use both antennas together. You will need a stereo FM tuner or radio to tune the trans­mitter. The FM tuner and transmitter should initially be placed about two metres apart. Do not connect a program source to the FM MiniMitter at this stage. Begin by setting the FM tuner to around 100MHz, where there is no other station. The tuner should produce a lot of noise, indicating that there is no station present. The two RCA connectors and DC socket mate with holes drilled in the other end of the case (ie, opposite the antenna terminals). Now adjust the slug in L1 using a suitable trimming tool, until the transmitter is tuned in; this will cause the noise level from the tuner to drop right down. This is called “Quiet­ing” by the way. This done, adjust the slug in L2 so that the stereo indicator light on the FM tuner comes on (if there is one); background noise should be minimal. You can now connect up a stereo signal source such as a CD player to the inputs and check if you receive this in the tuner. If all is OK, carefully adjust trimpots VR1 and VR2 for best sound from the tuner; there should be no noticeable distortion and suffi­cient Fig.11: this drilling template can be used for marking out the holes for the RCA sockets and the DC socket. 64  Silicon Chip signal to be above any background noise. Set VR3 so that the left and right channels are correctly balanced (ie, equal in loudness). Adjusting VC1 for best range The variable capacitor, VC1, feeding the antenna will need to be adjusted for best transmission range. Connect the antenna you intend using to the transmitter and disconnect the receiver’s antenna (or move it as far away as practical). Adjust trim­mer VC1 for best signal strength in the receiver. If you cannot remove the antenna on the receiver, it will be necessary to place it about 20 metres or more from the transmitter and then adjust VC1 for best reception, as best you can. The ultimate range for the Mini­ Mitter depends on the orien­tation of the 300Ω antenna, its height and the sensitivity of the receiver. The 300Ω dipole antenna transmits its signal with maximum strength broadside to the dipole. Similarly, the FM receiver has best pickup broadside to its antenna. When using a single length of wire as a 75Ω antenna, best range will be obtained when both antenna and receiver have the same orienta­tion; ie, both vertical or both horizontal. Note that the FM MiniMitter will Parts List Fig.12: this is the actual size artwork for the PC board. It’s a good idea to check your etched board against this pattern before installing any of the parts. Fig.13: the front panel artwork. It too can be used as a drilling template. not quite cover the full FM band with the range of adjustment provided by the slug in coil L1. To cover the range between 105MHz and 108MHz, you will need to change the 47pF capacitors across L1 and L2 to 39pF. Alterna­tively, to cover the range below 95MHz down to 88MHz, change the 47pF capacitors to 56pF. Again, these capacitors must be NPO ceramic types (ie, zero temperature coefficient) to minimise frequency drift in the transmitter. If the FM MiniMitter will only be used with batteries, you can remove the DC socket and zener diode ZD1 and use a wire link in place of the 56Ω resistor. This will marginally improve cell life by preventing current flow through the zener and also remove the slight voltage drop across the 56Ω resistor. Connecting a mono source Even though the FM MiniMitter is specifically designed for stereo transmission, you may want to use it with a mono source. What do you have to do? If you want reception in both channels on a stereo tuner or radio, you must connect the mono signal to the left and right channel inputs. The simplest way to do this is to use a mono to stereo bridging lead which will have three RCA connectors (one for the input and two for the outputs). You can make this up yourself or purchase it as an accessory from kitset suppliers (eg, Jaycar Cat WA-7054) or from hifi stores. Of course, if you have a stereo tuner which can be switched to mono mode, the above course will not be necessary. In this case, you can simply connect the mono source to the left or right channel input on the MiniMitter. Note that operating in mono will also give a slightly better signal-toSC noise ratio. 1 PC board, code 06104011, 122 x 60mm 1 plastic case, 67 x 130 x 44mm 1 front panel label, 127 x 64mm 1 PC-mount DC socket 1 stereo PC-mount RCA sockets or two insulated RCA sockets 1 SPDT toggle switch (S1) 1 2-way screw loudspeaker terminal panel 2 AA cell holders 2 ferrite slug coil formers (L1,L2) 2 F29 ferrite screw slugs 1 6mm balun core 1 38kHz crystal (X1) 2 M3 x 6mm screws 7 PC stakes 1 60mm length of 0.5mm enamel copper wire 1 100mm length of red light gauge hookup wire 1 100mm length of black light gauge hookup wire 1 100mm length of medium duty hookup wire 1 90mm length of 0.8mm tinned copper wire Semiconductors 1 BA1404 stereo FM transmitter IC (IC1) 1 3.3V 1W zener diode (ZD1) Capacitors 1 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 2 4.7µF 16VW PC electrolytic 3 .01µF ceramic 3 .001µF MKT polyester 1 .001µF ceramic 1 330pF ceramic 2 47pF NP0 ceramic (see text) 2 15pF NP0 ceramic 1 10pF ceramic 1 3-10pF trimmer capacitor (VC1) Trimpots 2 2kΩ horizontal trimpots (code 202) (VR1,VR2) 1 50kΩ horizontal trimpot (code 503) (VR3) Resistors (0.25W, 1%) 1 100kΩ 1 2.7kΩ 2 47kΩ 1 56Ω 2 10kΩ 2 10Ω April 2001  65