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80 metre DSB amateur g Ri If you are studying for your novice licence, you will want to get on the air as cheaply & easily as possible. This little 80-metre transceiver is the way to do it. It uses no integrated circuits & all the parts are cheap & readily available. By LEON WILLIAMS, VK2DOB There’s a certain fascination and challenge in extracting the most performance from the least number of components and this project is the outcome of a desire to do just that. The 3.5MHz to 3.7MHz, or 80-metre Amateur band, is ideal for the experimenter. The relatively low frequencies allow the use of semiconductors generally meant for audio applications. As well, construction techniques are not as critical as for VHF or UHF circuits. At night, signals can be quite strong and therefore receiv­ers do not need November 1994  53 54  Silicon Chip 100pF 220pF 470  Q1 470  E .001 0.1 Q7 BC549 B MIC GAIN VR2 500  2.2M 10k .0056 .012 10k 10 16VW 150  220  B Q8 BC549 E .01 Q2 BC337 C B 1 E 16VW C 100  10 16VW 1M 0.1 .01 .01 D2 1N4148 CARRIER 6T NULL VR1 200  56  T1 D1 1N4148 470  +RX .022 220W 16VW 1 1M 100 16VW 1k B 4.7k 4T VOLUME 10 16VW E C 6T Q6 BC549 4T T2 100  1 Q9 BC549 C 16VW B VR3 E 50k LOG 5.6k 0.1 56  0.1 80 METRE DSB TRANSCEIVER 10 16VW 100pF Q1 BC549 C 68k B OPTIONAL OSCILLATOR 330pF 100pF 330pF 33k F1 3.58MHz ZD1 15V 1W MICROPHONE TUNE VR4 20k LIN 0.1 100 F1 3.58MHz 10k 0.1 56  470  E Q10 BC549 C B 5.6k 100  0.1 220  1k 0.1 0.1 0.1 22  10  D5 68pF 10 16VW D6 1N4148 +TX E L1 6T D4 PLASTIC SIDE 4.7k B 470 25VW E 2. 2W 330  0.5W E C C L4 2.2uH 100pF Q5 BD139 0.1 0.1 C B E E TX RX 560pF L2 2.2uH 220pF 820pF 13.8V +V 820pF L3 2.2uH 100pF B C HEADPHONES S1 VIEWED FROM BELOW B C +RX 560pF 100 E 16VW C Q12 BC549 220  E 1k B Q11 BC337 820pF 2. 2  E B Q4 BD139 C B 2x1N4148 0.1 Q3 D3 BD139 C 1N4004 B 6T T3 820  0.5W 10  +TX +V ANTENNA to be extremely sensitive and it is possible to work long distances (DX) with low power. From my location near Canberra I have easily worked New Zealand with just a few watts of output power when conditions were favourable. This transceiver is about as simple as can be. The trans­mitter and receiver share a balanced mixer; in transmit mode it operates as a balanced modulator and in receive mode as a product detector. The carrier oscillator is also common to both transmit and receive modes and for simplicity, can be crystal controlled. Also delightfully simple is the method of using a ceramic resona­tor to form a simple variable oscillator. The main feature of this transceiver is the use of cheap and common components. Other features include a power output of about 1.5 watts PEP and easy single PC board construction. There are no expensive and hard to get integrated circuits and no difficult alignment procedures to undertake. The transmis­sions are Double Side­ band or DSB. This means that the carrier is nulled out and only the two sidebands (upper and lower) are transmitted. This is much more efficient than conventional AM (amplitude modulation) because there is no RF output when there is no modulation. This means the output stage is not wasting power and heating up while you are not talking. While single sideband (SSB) is the most used mode on the Amateur bands, an SSB transceiver is a lot more complex than a DSB type. In any case, a DSB signal is compatible with an SSB receiver and has the advantage that the receiving station can choose either USB or LSB mode. The receiver is a direct conver­sion type where the incoming signal is mixed directly with the carrier frequency to produce an Fig.1 (left): this transceiver is about as simple as can be. The trans­mitter and receiver share a balanced mixer (T1); in transmit mode it operates as a balanced modulator and in receive mode as a product detector. The carrier oscillator is also common to both transmit and receive modes and, as an option, it can be crystal controlled. PARTS LIST 1 PC board, code 06110941, 143 x 71mm 1 Jiffy box, 196 x 112 x 60mm 1 black binding post 1 red binding post 1 3.58MHz ceramic resonator (F1) 1 SPDT toggle switch (S1) 1 4-pin microphone panel socket 1 square mount S0239 panel socket 1 6.5mm stereo jack socket 1 200Ω horizontal trimpot (VR1) 1 500Ω horizontal trimpot (VR2) 1 50kΩ log potentiometer (VR3) 1 20kΩ linear potentiometer (VR4) 2 knobs 15 PC stakes 4 F14 balun formers (L1,T1,T2,T3) 3 2.2µH RF inductors (L2,L3,L4) Semiconductors 7 BC549 NPN transistors (Q1,Q6-Q10,Q12) 2 BC337 NPN transistors (Q2,Q11) 3 BD139 NPN transistors (Q3-Q5) 5 1N4148 diodes (D1,D2,D4,D5,D6) 1 1N4004 diode (D3) 1 15V 1W zener diode (ZD1) Capacitors 1 470µF 25VW electrolytic audio signal. Once again, an SSB signal is compatible, the only disadvantage being that there is equal response to both the lower and upper sidebands. Now let’s have a look at the circuit diagram of Fig.1. Carrier oscillator The carrier oscillator is formed around Q1. It is config­ured as a Colpitts oscillator with feedback provided by the capacitors connected to the base and emitter. The oscillator frequency is set by F1, a 3.58MHz ceramic resonator. This is used in preference to a crystal because it can be pulled in frequency quite easily by altering the circuit capacitance around it. This is achieved by using a variable capacitance diode, which is in fact a 15V 1W zener diode, ZD1. These are cheaper and easier to get than a dedicated Varicap. A variable resistor and a 10kΩ series resistor provide a means of varying the frequency. 3 100µF 16VW electrolytic 5 10µF 16VW electrolytic 3 1µF 16VW electrolytic 13 0.1µF monolithic 1 .022µF MKT polyester or greencap 1 .012µF greencap 3 .01µF ceramic 1 .0056µF greencap 1 .001µF ceramic 3 820pF ceramic 2 560pF ceramic 1 330pF ceramic 1 220pF ceramic 4 100pF ceramic 1 68pF ceramic Resistors (0.25W, 1%) 1 2.2MΩ 3 470Ω 2 1MΩ 1 330Ω 0.5W 5% 1 68kΩ 4 220Ω 1 33kΩ 1 150Ω 3 10kΩ 3 100Ω 2 5.6kΩ 3 56Ω 2 4.7kΩ 1 22Ω 3 1kΩ 2 10Ω 1 820Ω 0.5W 5% 2 2.2Ω Miscellaneous Screws, nuts, spacers, medium-duty hook-up wire, shielded cable, scrap aluminium. A 0.1µF capacitor is included as protection against noise on the supply rail modulating the oscillator. The prototype tuned from 3.568MHz to 3.583MHz and while this is not a big range, it allows greater flexibility than when a crystal is used. Note that the oscillator does not have any voltage regulation and it is important to use a regulated power supply to stop the oscillator changing frequency while transmitting. The small value capacitors around the oscillator are speci­fied as ceramics in the parts list. This was satisfactory in the prototype, however if excessive frequency drift is experienced, polystyrene capacitors may need to be substituted. Q2 operates as a buffer stage and provides a low impedance drive for the balanced mixer. As an alternative to a ceramic resonator, a 3.579MHz crys­tal can be used for the oscillator, to give fixed November 1994  55  10uF  470   100pF Q11 560pF 560pF   820pF  0.1   100uF 10uF 330  0.1 ANTENNA SOCKET 220pF L2 10  820   1k Q10 10uF   470uF 820pF 100pF   L3  Q9 2. 2  2. 2   L1 1k 220  4.7k 100  D6 1uF 1uF D4 13.8V B C E 0.1 L4 220  470  10uF 220    0.01  0.1 D5  .001 VR2  68pF 100  0.1 Q9 10 22 820PF 1uF 2 B C E 220   .022 0.1 1 0.1 100uF  Q5  Q3 5.6k   0.1 Q4 T3 4.7k .012   D3 0.1 1k  .01 100  .0056 Q8 Q6 T1 1M 10k 10k  Q7    2.2M 0.1 100pF  D1 5.6k 10uF  D2 .01 150  470  10k  Q2 100pF ZD1 0.1 Q1 VR1  0.1  1M F1   56   330pF T2 56W 56W 68k 33k 0.1 100uF   CONNECTIONS MADE TO GROUND PLANE 1 HEADPHONES 2 S1 MICROPHONE VR3 VR4 frequency operation. This alternative is shown on the circuit diagram of Fig.1. Microphone input Transistor Q7 is the microphone amplifier and its gain is variable by adjusting the emitter degeneration with potentiometer VR2. The circuit should provide enough gain for most microphones, however low output microphones may need an extra external ampli­fier. A .001µF capacitor is wired across the input of the ampli­fier to filter out any RF that may make its way in via the micro­phone lead. Q8 performs the dual role of buffer stage and low pass filter. The buffer stage provides a high impedance load to Q7 and provides a low output impedance drive for the balanced modulator. The low pass filter has a cutoff frequency of about 2kHz. A 56  Silicon Chip DSB transmitter occupies twice the bandwidth of an SSB signal and we must limit the audio response to avoid interference to adjacent stations. Balanced modulator The balanced modulator components are transformer T1, two 1N4148 diodes (D1 & D2) and a 200Ω trimpot (VR1). T1 is a trifilar wound transformer, where three lengths of wire are twisted to­gether and wound on a former as one. This provides close coupling between the windings and also aids in the balance or nulling of the carrier. Let’s look at how it works in transmit mode, firstly with no audio input from the microphone amplifier stages. The high level RF from Q2 causes current to flow in the secondary winding of T1. A .01µF capacitor effectively grounds the centre of the winding to RF. Due Fig.2: this component overlay diagram shows all the components which must have their leads soldered to the top & bottom of the PC board; all the relevant component leads are marked with a black star dot. to the phasing of the windings (shown by dots on the circuit), the two diodes conduct during the negative half of the RF cycle. Thus, equal currents will flow through the diodes and the resulting voltage at the wiper of VR1 will be zero. In the next (positive) half cycle, the diodes will be turned off and again no voltage will appear at the wiper of VR1. When an audio signal appears at the centre of the winding, depending on the instantaneous voltages, one of the diodes will conduct more than the other. The result is that the modulator is unbalanced and a vol­tage will appear at the wiper of VR1. This voltage follows the envelope of the original audio signal and is a suppressed carrier double sideband (DSB) signal. A 56Ω resistor provides a broadband resistive termination. Ideally D1 and D2 should be a matched pair, however we can get good results by adjusting VR1 to obtain the deepest carrier null (we’ll talk more about this aspect later). The output of the balanced modulator is coupled by trans­former T2 to the RF driver stage Q3. It is biased in class A, with a collector current of 50mA. The collector load for Q3 is transformer T3 with the secondary winding driving the output stage Q4 and Q5 which are two BD139 transistors in parallel except for their separate emitter resistors. These resistors stabilise the AC and DC gain, ensure that the current is shared more or less equally between the two transistors and prevent thermal runaway. The transistors are biased in class AB which means that the transistors are just conducting when there is no input signal. D3 and an 820Ω resistor provide a stabilised base voltage and the final stage draws 30mA under no-signal conditions (ie, with no speech into the microphone). L1 is the collector load and the output signal is fed from it through a low-pass filter before connection to the antenna. The 330Ω resistor in parallel with the collector coil is included to suppress a spurious signal that was noticed during develop­ ment. It is important that the low-pass filter is used because quite large harmonics can be produced in the RF amplifier. The filter is basically a double Pi filter with notch frequencies at 7MHz, set by L2 and a 220pF capacitor, and 10MHz, set by L3 and a 100pF capacitor. When the output signal was viewed on a spectrum analyser, all harmonics were at least 45dB below the signal fundamental. Receive circuit The signals from the antenna flow through the just men­tioned low-pass filter and this helps attenuate strong out-of-band signals. The signal then passes through a bandpass filter form­ ed by L4 and an 820pF capacitor. A 100pF and a 68pF capacitor match this bandpass filter to the impedances of the low-pass filter and receive preamplifier. Diodes D4 and D5 protect tran­ sistor Q6 from damage during transmit mode. Transistor Q6 is the receive preamplifier. The collector load of Q6 is one winding of transformer T2 and the output is coupled to the product detector via the 4-turn winding (of T2) All of the circuitry is on a double-sided PC board with a ground plane on the top. Note the two BD139 transistors which are bolted together with heatsink flags. These function as the RF output transistors. and potentiometer VR1. The product detector uses the same components as used for the balanced modulator in transmit, except that the signal path is now reversed. When there are no signals coming from the antenna, the balance is maintained and no audio signals are produced at the centre tap of T1. When a signal is tuned in, the balance is upset and a voltage representing the audio signal is produced and passed to the first audio stage Q9. A 56Ω resistor and two .01µF capacitors filter out any RF that may be on the audio signal. The signal level at this point is quite small and so Q9 is configured for high gain. The collector load is essentially a 5.6kΩ resistor in parallel with a .022µF capacitor. This combina­tion acts as a low-pass filter, where the gain of the stage is greatest at low frequencies and drops off rapidly at higher frequencies. This is necessary to filter out adjacent signal interference and it is in these audio stages that the adjacent channel selectivity of the receiver is determined. The output of Q9 is passed to the volume control VR3. This is the only gain control for the receiver and needs to be adjust­ ed for differing signal strengths as there is no automatic gain control. The second audio stage is Q10 and again low-pass filter­ing is accomplished by the collector combination of a 5.6kΩ resistor in parallel with a 0.1µF capacitor. The audio output stage is Q11 and provides enough power to drive a pair of low impedance headphones. Power supply decoupling is included to ensure amplifier stability. Transmit/receive switching Normally, a relay is used in a transceiver to switch the antenna and power supply between the transmit and receive cir­cuits. Relays are both bulky and expensive, so this design avoids them by using some novel techniques. The antenna and low-pass filter are permanently connected to both the transmit output stage (Q4) and the receive bandpass filter. During transmit, the receiver (ie, the input of Q6) is protected by a pair of back-to-back diodes (D4 & D5) which limit the voltage to about 1.2V peak-to-peak. The 100pF capacitor feeding the receive bandpass filter is small enough in value to avoid affecting the operation of the low-pass filter. During receive, Q4 and Q5 are turned off and the collectors exhibit a high enough impedance to avoid November 1994  57 Fig.3: here are the full size etching patterns for the double-sided PC board. attenuating the signal on its way to the receive section. Power supply switching has been simplified by using the transmit/ receive switch, S1. Power is permanently connected to the audio section, the carrier oscillator section and the RF output stage collectors. When the switch is in transmit, power is applied to the microphone amplifier, the RF driver and the RF output base bias circuit. Power is also applied to diode D6 which turns on Q12 and mutes the receive audio sections. In receive, power is switched to the receive RF preamp and the audio mute transistor Q12 is turned off. There is a small turn-off delay as the 10µF capacitor discharges via the 4.7kΩ resistor and the base of Q12. This is done to 58  Silicon Chip avoid any signal from the microphone feeding through to the audio amplifier stages while the microphone amplifier is turning off. Construction All parts except for the controls and sockets are mounted on a PC board coded 06110941. The PC board is double-sided, with the top side being a continuous groundplane with clearances for the component leads. Components which require a ground plane connection are soldered to the top and these points are marked with a black star symbol on the component overlay. The electrolytic capacitors get their earth connection through the earth leads of adjacent components which are themselves soldered on the bottom and top of the board. This can be seen on the wiring diagram of Fig.2. As with any RF project, keep the component leads as short as possible. The overlay diagram shows the variable frequency oscillator components installed. After you have checked the board for any defects, commence by soldering in the resistors, then install PC stakes for the external connections. Continue with the capacitors, diodes, ceramic resonator and the prewound 2.2µH RF chokes. Install the transistors, taking particular care with the orientation of the BD139s. The output pair need to be installed about 5mm above the board. Place a 3mm screw through the mounting holes of the two transistors while they dsb Fig.4: this full-size artwork can be used as a drilling template for the front panel. Rx phones Tx Ri 80mg allel to each other. While holding one end of the set of wires secure in a vice, twist the other end until there is about five twists per centimetre. A hand drill or a battery operated drill with variable speed con­trol would be handy for this job. Wind the wires on the former as discussed before. Cut off the excess and untwist the ends for identification with a multi­meter. The start of one and the finish of another winding need to mic All coils are wound on 2-hole F14 ferrite balun formers, using enamelled copper wire: • L1: 6 turns 22 B&S enamelled copper wire • T1: 6 trifilar turns 26 B&S enamelled copper wire • T2: primary 4 turns; secondary 4 turns; Q6 collector winding 6 turns 26 B&S enamelled copper wire • T3: primary 6 turns; secondary 4 turns 26 B&S enamelled copper wire • L2, L3 & L4 are prewound 2.2µH RF chokes. L1 is straightforward, as is T3 except that it has two windings. T2 has three windings. The winding ends can be identi­fied by scraping the enamel off the ends of the wires and check­ing for continuity with a multimeter. Ideally each winding would use a different colour wire or you could use a spot of paint; some form of identification needs to be used so that the winding polari­ties are as specified. When pulling the wire through the balun formers, try not to damage the enamel. This can happen as the wire passes over the sharp edges of the holes and could ultimately cause shorted turns. Note that T1 is wound using the trifilar method: take three 400mm lengths of wire and place them par- audio Coil winding details tune are being soldered in. The holes need to be in line, so that a small heatsink can be attached. This can be simply constructed from two pieces of scrap aluminium 16mm wide by 28mm long. These are formed into two “L” shapes with a bend at 8mm. A hole is drilled in the centre of the short leg of each piece. One is placed in between Q4 and Q5 and the other is placed against the metal surface of Q5. A screw is then passed through the assembly and tightened with a nut. Next comes the coil winding. Normally this involves cans and formers with slugs and can be an quite involved. This project makes it simple by requiring just a few turns of wire wound on 2-hole ferrite balun formers. These are sold in two sizes, the one required measures about 12 x 12 x 7mm. A turn is defined as passing a wire up through one hole, out the other end and feeding it back again down the other hole, so that both ends (start and finish) of the wire are at the same end of the former. be joined to form the centre tap of the secondary winding. The remaining winding becomes the primary. Final assembly The PC board is housed in a plastic jiffy box measuring 196 x 112 x 60mm. On the front panel are knobs for tuning (VR4) and audio volume (VR3), the transmit/receive switch, the microphone socket and the headphone socket. The PC board is mounted on November 1994  59 ORDER FORM BACK ISSUES MONTH YEAR MONTH YEAR PR ICE EACH (includes p&p) Australi a $A7.00; NZ $A8.00 (airmail ); Elsewhere $A10 (airmail ). Buy 10 or more and get a 10% discount. Note: Nov 87-Aug 88; Oct 88-Mar 89; June 89; Aug 89; Dec 89; May 90; Aug 91; Feb 92; July 92; Sept 92; NovDec 92; & March 98 are sol d out. All other issues are currently i n stock. TOTAL $A B INDERS Pl ease send me _______ SILICON CHIP bi nder(s) at $A12.95 + $5.00 p&p each (Australi a only). N ot avail abl e elsewhere. Buy five and get them postage free. $A SUBSCRIPTIONS ❏ New subscription – month to start­­___________________________ ❏ Renewal – Sub. 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Please have your credit card details ready 60  Silicon Chip ______________________________ Card expiry date________/________ Card No. Phone (02) 9979 5644 Signature OR Fax (02) 9979 6503 Fax the coupon with your credit card details 24 hours 7 days a week Mail coupon to: OR Reply Paid 25 Silicon Chip Publications PO Box 139, Collaroy 2097 No postage stamp required in Australia The rear panel of the transmitter carries the SO239 antenna socket & binding posts for the power supply connections (13.8V DC). the bottom of the box with four spacers secured with 3mm screws and nuts. The square mount SO239 antenna socket is mounted in the bottom right corner of the base of the box with a solder lug under one of the retaining nuts for the earth connec­tion. The power supply binding posts are mounted on the base above the antenna socket. All wiring from the PC board is done with hook-up wire except the microphone lead which should be via shielded audio cable. Twist the wires to the antenna socket, the tune control and the volume control. Keep the wiring to the front panel as short as possible but long enough so that the PC board can be accessed when the front panel is lent forward. Note that the headphone socket is a stereo type wired for mono operation. Testing Once construction is complete, check all the wiring one more time. Place the transmit/receive switch in receive and connect a power supply to the binding posts. The transceiver is designed to be run off 13.8V DC regulated and draws about 70mA, however there should be no troubles with a voltage between 12V and 15V. A supply of 15V should be considered a maximum and 12V will give a reduced power output, compared to the nominal setting of 13.8V Plug headphones into the phones socket and advance the volume control. A hiss should be heard indicat- ing that the audio stages are working correctly. Check that the oscillator is work­ing by measuring the frequency with a frequency counter at the emitter of Q2. Failing this, listen on a receiver placed nearby which is tuned to the oscillator frequency. Inject a signal into the antenna socket at 3.58MHz and check that you can hear a tone of about 1kHz – rotate the tune control until the tone is heard. You should only need very light coupling to the antenna socket for a good, clean tone. If you fail to hear a hiss, the fault will be later in the audio sections and if you hear a hiss but no tone then look for trouble in the RF sections or around the early audio stages. Unless you have a second transmitter or a friend nearby, you will probably have to wait till late after­noon to receive off-air voice signals. Before testing the transmitter, plug a 50Ω dummy load or wattmeter into the antenna socket and place a multimeter set to the 1A range in the supply positive lead. Place the modulator balance trimpot (VR1) at halfway. Switch to transmit and without a microphone connected, check the current; it should read about 180mA. A reading far from this indicates a fault and should be looked into. The next step is to balance the modulator. This can be done by using a low power wattmeter, a dummy load and an oscilloscope or a second receiver. A dummy load can be simply two 100Ω 1W resistors in parallel wired across the antenna socket. In all the methods, the aim is to rotate balance control VR1 until minimum output power is obtained. This should be at half way, but it may need adjusting a little either way to obtain balance. If you are using a receiver be careful to avoid picking up the direct signal from the oscillator which can cause misleading S-meter readings. With the carrier nulled, plug in a microphone and either listen to yourself on a second receiver or have someone else listen while whistling into the microphone. Advance the mic gain control VR2 until the signal starts to distort and just back it off a little. Driving the transmitter too hard will cause a distorted signal and should be avoided at all times. The transmitter draws about 400mA on voice peaks. Operating Before you can transmit with this project you must hold a current amateur radio licence. To obtain the best results with any radio it is important to use an effective antenna. With a QRP or low power transmitter it is even more important, because we want as much signal radiated as possible. The most popular anten­na for the 80-metre band is the half wave dipole, which is about 40 metres long and generally fed at the centre with 50Ω coax cable. While the antenna is very important, the band conditions can also play a large part in getting good contacts. Sometimes the band can be noisy or propagation poor, so do not expect to work long distances every time. When making a CQ call, it is helpful to say that you are operating QRP. This stirs the curiosity of those listening and also explains the possibility of your low signal strength. Most stations on the air will be using commercial transceivers with much greater output powers than your 1.5W, so a little patience and skill is needed to get contacts. You will, however, be pleasantly surprised with the signal reports you get. If you intend to contact other DSB stations, it will be necessary to adjust the tune control very accurately. In fact the two carrier frequencies should be exactly the same frequency and in phase to recover the audio properly. This will generally not be possible but, with a little knob twiddling a success­ ful contact will be possible. This problem does not occur with SC SSB signals. November 1994  61