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Learn about AM radio transmission Build a tJea-power AM radio transmitter Have you ever wondered how music is transmitted to your AM radio? Build this experimental flea-power transmitter & find out. By DARREN YATES Most of us are pretty blase about AM radio these days but that doesn't mean you shouldn't have some idea of the basic principles involved. By building this experimental AM radio transmitter, you can learn how AM signals are transmitted and have some fun into the bargain. We've christened the device the "AM Micromitter" because of its "micro-power" output. This low power output has been deliberately designed in so that you cannot interfere with your neighbour's radio reception. In fact, the AM Micromitter only has a line of sight range of about 10 metres, in keeping with the experimental nature of the device. Essentially, the AM Micromitter is a complete AM radio transmitter that can broadcast both speech and music signals to an ordinary radio receiver, AM MICROMITTER The AM Micromitter can broadcast music or Morse code signals to an ordinary AM radio receiver. It is an experimental device with a range of about 10 metres. 26 SILICON CHIP as well as Morse code signals. The device uses one low-cost IC plus a handful of other parts and can be assembled in just a couple of hours. Basic principles So how does a simple transmitter work? The block diagram of Fig.1 shows a simple Morse code transmitter, which consists of a radio frequency (RF) oscillator, an RF amplifier and an antenna. The RF oscillator produces a highfrequency signal (called the "carrier") which is fed to the amplifier stage and thence to the antenna when the key is closed. By opening and closing the key, bursts of RF energy radiate from the antenna and this can then be picked up by a radio receiver. The bigger the amplifier and the more elaborate the antenna, the stronger will be the signal received by the radio. If we use a simple piece of wire for the antenna, the radio waves radiate with equal strength in all directions. However, with more refined antennas, it's possible to concentrate the RF energy in one direction and so increase the range. A somewhat different technique is necessary to transmit audio signals (ie, speech and music). In a Morse code transmitter, information is sent by keying the carrier on and off as we have just seen. By contrast, in an AM transmitter, the carrier is transmitted continuously but its amplitude is varied (or modulated) to encode the signal. Hence the term "amplitude modulation", or AM for short. Fig.2 shows the block diagram of an AM broadcast transmitter. As can be seen, the modulator stage is inserted between the RF oscillator and RF amplifier stages, while the audio input acts as the modulating signal. PCB and SCHEMATIC CAD ANTENNA KEY RF OSCILLATOR RF AMPLIFIER ' Fig.I: this block diagram shows a simple Morse code transmitter. It consists of an RF oscillator, a key & an RF amplifier which feeds an antenna. ! .: ~r !!I'., ANTENNA ~ If AMPLIFIER ·::t ~ : • i f.',"., . ... ,:d•'li'l'-,<',«,+~'tt! . ~ i :~ ) __________ __ _ RF OSCILLATOR MODULATOR RF AMPLIFIER AUDIO SIGNAL Fig.2: the block diagram for an AM transmitter. In this case, the carrier is transmitted continuously but is amplitude modulated by the audio signal. ~i i n: = ~ ~~~ .--., : 1 By using this arrangement, the instantaneous amplitude of the carrier signal is varied in response to the amplitude of the audio input. The resulting AM signal output from the modulator is fed to the RF amplifier and from there to the antenna. It can then be picked up by any standard AM radio receiver, such as a personal radio, car radio or clock radio, provided it is within about 10 metres of the transmitter. Fig.3 shows what a typical amplitude modulated signal looks like when displayed on an oscilloscope. In this case, we have a lkHz sinewave signal modulating a 760kHz carrier signal. Note that the top and bottom halves of the signal are mirror images. fed via a 22kQ resistor and a .00lµF capacitor to pin 10 of !Cl. IC1 is an MC1496 balanced modulator/demodulator IC. This device forms the heart of the transmitter and functions as the modulator stage. So most of the hard work is performed by this single chip. The audio signal is fed to pin 1 of IC1 via a 3.5mm jack socket and 50kQ potentiometer VR1. This pot sets the depth of the modulation that occurs in the carrier wave. The greater the signal into pin 1, the greater the depth of modulation and the greater the volume from your radio. This circuit arrangement produces a clean AM signal at the output of IC1 (pin 12) and this is fed to transistor 'i!J I I 11 ll 1111111I1 1111 1111 I EASY-PC • Runs on PC/XT/AT/286/386 with Hercules, CGA, EGA or VGA. • Design Single sided, Double sided and Multilayer boards • Provides Surface Mount support • Standard output includes Dot Matrix/Laser/Inkjet printers, Pen Plotters, Photo-plotters and NC Drill • Award winning EASY-PC is in use in over 12,000 installations in 70 Countries World-Wide • Superbly Easy to use • Not Copy Protected Options: • 1000 piece Schematic symbol library Circuit details Let's look now at the circuit details of the Micromitter - see Fig.4. Transistor Ql, diodes Dl and DZ, and their associated components form an RC phase-shift oscillator which produces a sinewave with a frequency of about 760kHz. This frequency is determined by the three 1.5kQ resistors and the three 330pF capacitors in the feedback network. Diodes Dl and DZ stabilise the gain of the oscillator and thus virtually eliminate unwanted variations in the carrier signal. The 2Vpp sinewave output appears at Ql 's emitter and is • ' , . ~ •:.~ . . . •~ ~•: ~•~: • Surface Mount symbol library • Gerber Import facility For full info 'phone, fax or write: Fig.3: this AM signal was produced by modulating a 760kHz carrier with a lkHz sinewave signal (scope settings lV/div & 0.2ms/div). BTC PO BOX432 GARBUTT 4814 QLD. PH (077) 21 5299 FAX (077) 21 5930 ]ANUARY 1993 27 ,--------------..----------------------1~-------+12V 2x1N914 01 1.5k ANTENNA 10k 3.3k 1k 10k 3.3k 3 5 1k 10 + 10k 7 IC1 MC1496N 16VW+ 10 12 1k 14 1.5k 7 1.5k 1k A U O I0 '-7 INPUT 7 100 + 16VWi B EQc VOLUME/ MODULATION DEPTH VR1 50k VIEWED FROM BELOW 10k IGO 10k VR2 50k 12VOC 300mA PLUG-PACK ~ L1 : 65T, 0.63mm DIA ECW WOUND ON A 50mm LENGTH OF 9mm DIA FERRITE ROD V I------e•--,+ AM MICROMITTER Fig.4: the final circuit uses a phase-shift oscillator based on Qt, D1 & D2 to produce a 760kHz carrier signal. This signal is then fed· into ICl, where it is modulated by an audio signal that's fed in on pin 1. The output at pin 12 then drives Q2 which in turn drives aerial coil Lt & the antenna circuit. Q2. Q2 then directly drives antenna coil 11 and the antenna via a 2200pF capacitor to radiate the signal. VR2 is used to adjust the modulated output at pin 12 for best results. By connecting the output of pin 12 to an oscilloscope, it's also possible to produce what is called a double sideband suppressed carrier waveform (DSBSC) by rotating VR2 to somewhere near its centre position. Power for the AM Micromitter comes from a 12V DC plugpack supply. Its output is fed in via on/off · 06112921. Fig.5 shows the parts layout and the external wiring. Begin the assembly by installing the wire links, then install the resistors and the MKT polyester capacitors. Table 1 shows the resistor colour codes but it's also a good idea to check each resistor with your digital multimeter to avoid any confusion. The electrolytic capacitors can be installed next, followed by the two diodes, the IC and the transistors. Note that these are all polarised devices, so make sure they are correctly oriented. The 3-terminal regulator (REGl) must be installed with its metal tab facing towards the centre of the board. The next task is to wind the coil switch Sl and applied to 3-terminal regulator REGl to derive a regulated +12V supply rail (note: a lightlyloaded 12V DC plugpack actually puts out about 16V, which is ample for correct operation of the regulator). The lOOµF and 47µF capacitors provide supply line filtering, while D3 protects the circuit against reverse polarity connection of the supply. Construction Most of the components for the AM Micromitter fit on a PC board coded TABLE·1: RESISTOR COLOUR CODES 0 No. ·Value 4-Band Code (1%) 5-Band Code (1%) · 0 2 1 22kQ 12kQ 10kQ 3.3kQ 1.5R:Q 1k!2' 4.7pQ 390Q red red orange brown brown red orange brown brown black orarrge .brown orange orange red .b·rown bro1.-.in gr.eer:i• ·red b~own . • ........ ,-':>, brown black red brown yellow viplet brown brown orange white brawn brown red red b1ack red brown brown red black red brown brown black black red brown orange orange black brown brown brown green black brown brown brown black black brown brown yellow violet black black brown orange white black black brown 0 0 0 2 0 6- 0 0 4 0 28 ·5 1 1 SILICON CHIP 12VDC PLUG-PACK -) ® ~} 47u n g,, '. , ~ AUDIO INPUT i:, 7812 3 Fig.5: install the parts on the PC board exactly as shown in this parts layout diagram. Inductor Ll is made by winding 65 turns of 0.63mm enamelled copper wire on a 50mm length of ferrite rod. Check the supply polarjty before making the final connections to the board. 1 ¼J WARNING! This project is an experimental device only and has been designed to teach the basics of AM signal transmission. Do not in. crease the power output of the ·device or attempt to increase the range by feeding it into a directional antenna, as this could cause interference to other users. (Ll) . This consists of 65 turns of 0.63mm enamelled copper wire on a 50mm length of ferrite rod. Before winding on the wire, wrap a couple oflayers of paper (about 40mm long) around the rod and secure the ends with adhesive tape. The 65 turns of copper wire can then be closewound onto the rod. Keep the turns tight and again use adhesive tape to secure the ends of the winding. By the way, the actual number of turns is not critical. As long as it is somewhere around 65, the transmitter will work OK. When the coil is completed, trim the leads to about 100mm, then clean and tin them so that they are ready to solder to the board. Don't connect them to the PC board just yet though that step comes a little later on. Final assembly A plastic zippy case measuring 130 x 68 x 41mm is used to house the PC board. The board is mounted on the The PC board is secured inside the case using machine screws & nuts, while Ll is fastened to the side of the case using plastic cabl~ ties. Use light-duty hook-up wire to complete the wiring between the PC board & all external components. bottom of the case, while the coil is secured to one side using two plastic cable ties (one at either end - see photo). Prepare the case by first attaching the adhesive label to the lid and drilling holes to accept the on/ off switch and volume potentiometer. This done, drill three mounting holes for the PC board plus four holes in the side of the case to <J.Ccept the two cable ties. Finally, drill two holes in one end of the case to accept the power socket and the audio input socket. JANUARY 1993 29 PARTS LIST 1 PC board, code 0612921, 102 x 53mm 1 50mm length of 9mm diameter ferrite rod 1 plastic zippy case, 130 x 68 x 41mm 1 12VDC 300mA plugpack supply 2 3.5mm jack sockets 1 SPST toggle switch 1 3-metre length of 0.63mm enamelled copper wire 1 50kQ log potentiometer (VR1) 1 50kQ 5mm linear horizontal trimpot (VR2) Semiconductors 1 MC1496N balanced modulator/ demodulator {IC1) 1 7812 3-terminal regulator (REG1) When all the holes are drilled, secure the coil using the cable ties and solder its leads to the PC board. The remainder of the wiring can then be completed using light-duty hook-up wire - see Fig.5. Make sure that the supply socket is wired with the cor. rect polarity. The AM Micromitter assembly can now be completed by securing the PC board to the bottom of the case and fitting the four rubber feet. Testing Now for the smoke test. Connect your plugpack supply, set VRl fully anticlockwise, switch on and check the voltage at the output of the 3terminal regulator {7812). If you don't get a reading of+ 12V, switch off i:r;nme- 2 BC548 NPN transistors (01,02) 2 1N914 signal diodes (D1 ,D2) 1 1N4004 rectifier diode (D3) Capacitors 2 100µF 16VW electrolytic 1 47µF 35VW electrolytic 1 22µF 16VW electrolytic 2 10µF 16VW electrolytic 1 0.1µF MKT polyester 1 .0022µF MKT polyester 1 .001 µF MKT polyester 3 330pF MKT polyester Resistors (0.25W, 1%) 2 22kQ 6 1.5kQ 1 12kQ 41kQ 5 10kQ 1 470Q 2 3.3kQ 1 390Q diately and check for wiring errors. Assuming that the supply rail is OK, feed an audio signal into the input, set VRl to mid-position and tune your receiver to about 760kHz. If the transmitter is working correctly, you should have little difficulty tuning into the signal. VRZ can now be adjusted for best reception. Initially, ,you should position the receiver about 30cm away from the transmitter. Once you've tuned into the signal, you can test the range of the transmitter. You should be able to obtain a relatively noise-free signal at distances up to about 10 metres, while the maximum range will be about 15 metres. If you have problems with a nearby radio station that operates on a fre- Fig.8: this is the full-size etching pattern for the PC board. Check your board carefully for defects before mounting any of the parts. 30 SILICON CHIP Fig.6: this photo shows a carrier wave that is almost 100% modulated. This is the limit for a distortion-free signal. Fig. 7: a carrier wave that is overmodulated results in a distorted output from the receiver. quency close to 760kHz, you can change the values of the 1.5kQ resistors or the 330pF capacitors in the oscillator. This will shift the carrier frequency and so allow you to operate on another part of the broadcast band. Using the Micromitter When the Micromitter is working correctly, you can experiment with different settings for VRl. As mentioned briefly before, by changing the level of the audio source that's fed into the Micromitter, you're changing what's called the modulation depth of the carrier wave. If the modulation depth increases (ie, if the audio level fed into the transmitter increases), then so does the volume from the receiver. There is a limit to how far you can go, however. Fig'.6 shows a carrier wave which is close to 100% modulated, while Fig. 7 shows a carrier that is over-modulated. This is caused by too much signal and results in a distorted output from the receiver. If you have a oscilloscope, you can observe these effects for yourself by monitoring the collector of QZ. SC