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AMATEUR RADIO BY GARRY CRATT, VK2YBX The 6-metre summertime special With the 6-metre season now upon us, we are presenting a low power, crystal locked exciter. When used with a suitable power amplifier, it can take advantage of the favourable ionospheric conditions normally encountered during the summer months on this band. Each summer, you will notice that TV reception on channel 2 becomes a problem, particularly around sunset. The most common symptom is a "venetian blind" effect which is usually co-channel interference from another channel 2 transmitter, perhaps many hundreds or even thousands of kilometres away. But while these summer conditions, due to atmospheric "ducting", temperature inversion, and improved ionospheric conditions, mean that TV reception is a problem, it's a wonderful time for working 6-metres over incredibly long distances. With that in mind, we decided on the "6-metre summertime special". The criteria for such a project should be economy (after all, the exciter may only be used for several months of the year) and simplicity. This left us To ensure stability, the 6-metre transmitter is built on a PC board with a ground plane. Take care with component orientation & keep all leads as short as possible. The circuit should be housed in a metal case. 66 SILICON CHIP looking for a single chip FM transmitter, without too much additional circuitry. During this "search", it dawned on us that there might be an integrated circuit, normally intended for the 49MHz cordless telephones used in the USA, that could be the perfect answer. Sure enough, Motorola have such a device , the MC2831A. This chip is a complete 49MHz transmitter and includes a variable reactance modulator for FM, a pilot tone oscillator, a low-battery indicator, and an audio amplifier with limiting - see Fig.1. The complete circuit, using the MC2831A chip (IC1) and an OM361 hybrid amplifier (ICZ), is shown in Fig.2 Third harmonic crystal The onboard RF oscillator for IC1 is an internally biased Colpitts type, which can be used for crystals in the fundamental mode or the L/C standard configuration. The crystal selected should have a frequency of exactly one third the desired 6-metre channel, as the third harmonic is g1merated (amongst others) by the output buffer, at pin 14. The crystal should be calibrated for parallel resonance, with a 32pF load capacitance. Table 1 shows a list of 6metre repeaters and popular simplex frequencies. Inductor L1 is used to provide compensation for the reactance of the modulator and for fine frequency adjustment. The modulating signal, either from a test oscillator or dynamic microphone, is applied via pin 3 of the IC. Our circuit uses the microphone preamplifier, with the signal going via zokn potentiometer VR1, and then to pin 5 via a lµF capacitor. The output of the preamplifier is then coupled from pin 6 to pin 3 via a lOkQ resistor and 2.2µF capacitor. This gives plenty of gain. We found that about 2m V from the microphone is all that is required to drive the exciter to 3kHz deviation. This is the internal limit set by the IC, at the crystal frequency. As we are using a harmonic which is three times the fundamental frequency, the maximum deviation possible will be 9kHz - too much for narrow band FM use. VRl allows the deviation to be set to the desired level. We found that 5kHz corresponded to the midpoint of the potentiometer travel on our prototype. Capacitors C5 and C6 should be selected to have the same value, in order to . minimise residual amplitude modulation (AM). We have not used the pilot tone oscillator or the low battery indicator features in our circuit since they are not required for this application. As previously mentioned, the MC2831A buffer generates harmonics, one of which we require and the others we do not. Hence, the output (pin 14) of ICl is fed to a bandpass filter, centred around 50MHz, which attenuates signals below 50MHz by 30dB. L2, L3 and VCl are the filter components. Note that these two coils are located at 90° to each other on the PC board, to prevent any mutual coupling. By carefully choosing the value of the coupling capacitors on both VARIABLE REACTANCE OUTPUT (1) (16) DECOUPLING (2) (15) MODULATOR INPUT (3) (14) RF OUTPUT vcc VCC2 (4) (13) GND ... MIC AMP INPUT (5) (12) VCC1 MIC AMP OUTPUT (6) (11) BATTERY CHECKER vcc TONE SWITCH (7) (10) LED .,. TONE OUTPUT (8) (9) OSCILLATOR COIL MC2831A Fig.1: block diagram of the MC2831A FM transmitter IC. the input and output sides of this filter, the fundamental and second harmonic of the crystal oscillator can be significantly further reduced. Using the values shown on the circuit, the fundamental and second harmonics of the crystal are attenuated by 30dB, while all frequencies above the desired 6-metre frequency are attenuated by 40dB. The trap formed by L5 and VC2 serves to reduce the 65MHz output component (twice the second harmonic of the crystal). 82!l +9V +12V 4.7 + 16VWr .001I 3.3k RF OSCILLATOR RF DSC 12 y 20kr,M MICvCJ . -~I-• ... .001+ 2701l . .001+ IC1 MC2831A 56pF Fii~JE( 15 2.2 56pF! 12DpF 470!! VC1 100k ._..,..._......,.........14 I L4 5uH II 11 II .001 ~OUTPUT 1 8 t7-50pf tpF 18pF .,. 13 .,- L2 ...0047l l2. l3 : 4T. 25 B&S TCW ON 5mm BOLT l5 : 6T. 25 B&S TCW ON 12mm BOLT L3 ':' 100!!.,. VC2 -, 7-50pf .,. 6-METRE AMATEUR TRANSMITTER Fig.2: most of the circuit functions are performed by IC1 which is the Motorola MC2831A FM transmitter stage. NOVEMBER 1990 67 We also experimented with an output lowpass filter, to further reduce the harmonic output, but found that the improvement of 6dB was not worth the effort. Any tuned power amplifier which follows this exciter will attenuate these unwanted signals even further. The IC has an RF outpu t of -40dbm, which of course is insufficient for our needs. We decided that an output level of lm W or so (0dbm) would be a good starting point. RF amplifier Out initial choice for the job of RF amplification was another Motorola device, the MWA120. This is a DCDC SUPPLY AND RF OUTPUT ,-----,----n vcc 2 DECOUPLING z MWA120 COUT >,.:..--t-o .,. .,.3 GNO Fig.3: the RF amplifier stage in the transmitter circuit (Ql) is similar to-the MWA120 hybrid amplifier from Motorola. It's a common emitter circuit based on a BFR91 or MRF571 RF transistor. TABLE 1: 6-METRE BAND REPEATERS FREQUEN CY 50.010MHz 50.075 50.090 51.020 51 .030 52.013 52.020 52.100 52.200 52.225 52.250 52.310 52.320 52.325 52.345 CALL SIGN JA21GY VS6S IX KH6EQI ZL1UHF ZL2MHB P29BPL FK8AB ZK2SIX VK8VF ZL2VH2 ZL2VHM ZL3MHF VK6RTT VK2RHV VK4ABP LOCATION Nagoya Hong Kong Honolulu Auckland Hawkes Bay Loloata Island Noumea Niue Darwin Taranaki Manawatu Christchurch Wickham Newcastle Long reach FREQUENCY 52.350 52.370 52.418 52.420 52.425 52.435 52.440 52.450 52.460 52.465 52.470 52.485 52.490 52.510 CALL SIGN VK6RTU VK7RST VKOMA VK2RSY VK2RGB VK3RMV VK4RTL VK5VF VK6RPH VK6RTW VK7RNT VK8RAS ZL2SIX ZL2MHF LOCATION Kalgoorlie Hobart Mawson Base Sydney Gunnedah Hamilton Townsville Mt. Lofty Perth Albany Launceston Alice Springs Blenheim Mt. Climie Where to Get the Parts The MC2831A is available from VSI Electronics (Australia) Pty Ltd in Sydney & other capital cities. The BFR90 or BFR91 transistors can be obtained from VSI or Dick Smith Electron ics. A suitable crystal (code GC05E) can be obtained from HY-0 Crystals. Phone (03) 783 96 11. The OM361 can be obtained from stockists of Philips components and Radiospares Components (all states) . A suitable microphone, complete 68 SILICON CHIP with plastic case and PTT switch (Model "07702" or "07704") can be obtained from Benelec Pty Ltd; phone (02) 693 5111. Additional information on the MC2831A can be found in the Motorola data book "Linear and Interface Integrated Circuits", DL 128 Rev 2 1988. Also in application notes AN-HK-02 "Low Power FM Transmitter System" and AN-HK-09 "A Fundamental Transmit Frequency Design in a 46149MHz 10-Channel Cordless Phone". 400MHz hybrid amplifier containing a single transistor with good power gain, a simple 3-pin package and, best of all, 50Q input and output impedances. However, after several enquiries we realised that the device, although still current on the manufacturer's production list, was not available in Australia. The prototype we had built used a device obtained directly from the USA. Nevertheless, we were undeterred. The data sheet for the MWA120 showed the internal circuit as having a single transistor and three resistors. Surely by opening up the device, we should be able to measure these resistors, then choose a likely transistor and duplicate the circuit. This is in fact what we did. By grinding off the top of the IC, we were able to identify and measure the resistor values, as shown in Fig.3. We selected two possible candidates from the transistor data book, either the BFR90/91 device or the MRF5 71. Both devices exhibit lOdB or so of power gain in this application, which is sufficient to drive the output amplifier, an OM361 hybrid amplifier (IC2). This brings the output level up to our target. The PCB layout for the exciter will accept both types of driver transistor. The output stage of the exciter comprises an OM361 hybrid amplifier, normally seen in television masthead preamplifiers, which has a gain of 28dB when operated from 12 volts. This stage provides sufficient gain to achieve an output power of lm W. Due to the high gain of this IC, particular care must be taken to ensure that the amplifier remains stable under all conditions. This means keeping the component leads of C13, C14, C15, L4 and C16 as short as possible. The main DC supply pin is fitted with an F29 ferrite bead to ensure that RF does not get into the supply rail. Construction Construction of the exciter is quite simple. The unit is built on a doublesided PC board measuring 92 x 64mm. The upper (component) side of the PCB is a ground plane, ensuring a good low impedance earth where needed. Component leads that are connected to ground should be soldered on both sides of the PCB. Three in- PARTS LIST 1 double sided PC board, 92 x 64mm , code SC06111 901 2 5µH inductors (L 1, L4) 1 17 .50833MHz crystal 1 F29 ferrite bead 1 20kQ trimpot (VR1 ) Semiconductors 1 MC283 1A FM transmitter (IC1) 1 OM361 hybrid amplifier (IC2) 1 BFR91 , MRF571 NPN RF transistor (Q1 : see text) TO MIC + 12V Fig.4: when installing the p arts, be sure to solder the leads on both sides of the board if the groundplane comes right up to the edge of the hole. The coils are made by winding tinned copper wire onto bolts (see Fig.2 for details). 0 Resistors (0.25W, 5%) 1 1MQ 1 270Q 1 100kQ 1 100Q 1 10kQ 1 82Q 1 3.3kQ 1 3.9Q 1 470Q 0 en ..... ..... ..... 0 Miscellaneous 2 metres of 25 B&S tinned copper wire, 50Q coax cable . (,!;) 0 ..... 0 en o• :o 0 0 0 -- 0 00 00 0 0a a - 0 0 00 0 0 0 0 00 0 0 ..... ..... ..... 0 0 0 00 00 00 0 <.O 0 ..... g ct,0 8 0 0 O'> 0!c)c,! oo• :o oo: 0 () 0 00 00 00 000 Capacitors 1 22µF 16VW electrolytic 1 4.7µF 16VW electrolytic 1 2.2µF 16VW electolytic 1 1µF 16VW electrolytic 1 .0047µF disc ceramic 5 .001µF disc ceram ic 1 120pF disc ceramic 2 56pF disc ceramic (should be closely matched : see text) 1 47pF disc ceram ic 2 18pF disc ceramic 2 7-50pF trimmers 0000 0 Figs.5 & 6: here are the two full size artworks for the PC board. The bottom pattern is the groundplane and is necessary to ensure a good low impedance earth. ductors - 12, 13 and 1 5 - need to be wound. We made ours fro m 25 gauge B&S tinned copper w ire (shown as TCW on the circuit diagram). All three inductors are air cored. You can wind them on a 5mm or 12mm bolt and the bolt's threads w ill give precise spacing for each of the turns . The RF output on the prototype was terminated in a BNC connector, for easy connection to an add-on power amplifier. The exciter should be housed in a metal box to ensure minimum oscillator leakage. The microphone can either be permanently connected, or a suitable plug/ socket combination can be used to incorporate a push-to-talk switch. The circuit draws almost lOOmA when fed from a 12V supply, so battery operation is not recommended. Next issue, we will describe a power amplifier for the unit. ~ NOVEMBE R 1990 69