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Pt.2: By LEON WILLIAMS, VK2DOB Last month, we gave the circuit details of this new Direct Conversion Receiver. This month, we conclude with the full construction and alignment details. There’s also a few tips on using the completed unit. A single PC board coded 06107021 (172 x 134mm) accommodates virtually all the parts, so building this receiver is really easy. Fig.5 shows the layout details. Before installing any of the parts, check that the holes for the larger components such as the coil formers www.siliconchip.com.au are the cor­rect size. If not, enlarge them with a suitable drill bit. The assembly can now be started by installing the seven wire links, making sure that they are straight and that they lay flat on the PC board. Follow these with the smaller components, such as the resistors, diodes, RF choke, trimpots and PC stakes. It’s a good idea to check the resistor values with a digital multimeter before installing them on the board. Because this is an RF project, it is important that you keep all component leads as short as possible to avoid any un­wanted feedback and instability. In short, make sure that all components are mounted close to the PC board. This is also the reason why IC sockets are not used for the ICs, apart from the PIC chip. Next install the headphone socket, IC socket and the ca­pacitors. Start with the smaller capacitors and progress to the larger electrolytics, ensuring they August 2002  71 Fig.5: install the parts on the PC board as shown here, taking care to keep all leads as short as possible. The PIC microcontroller (IC1) is installed in a socket and should be left out of circuit until after the power supply checks have been completed. are installed with correct polarity. Follow this with the transistors (Q1 & Q3-Q7), FET (Q2), voltage regulators (REG1-3), varicap diode package VC1, crystal (X1) and the ICs, leaving the PIC chip until later. Note that the transistors, FET, BB212 (varicap diode pack­age) and the small voltage regulators are all similar in ap­pearance, so double check that you have installed them in the correct 72  Silicon Chip locations. The 8V regulator (REG2) runs cool and doesn’t need a heatsink. Winding the coils Fig.6 shows the winding details for the coils, including the wire size, the start and finish pins and the number of turns required. If you are new to radio building and not familiar with coil winding, a few comments will probably be helpful. All the coils need to be wound before they are installed on the PC board. Let’s start with the BPF coils – T1 and T2. They comprise a 6-pin base, a metal can and a 5mm former into which a ferrite slug is screwed up and down to alter the inductance. The first step is to place a drop of superglue on the bottom of a former (make sure that none gets into the threaded section) and then press it at right angles into the centre hole of a 6-pin base. Then, once the glue has set, you begin with the winding that has the larger number of turns. www.siliconchip.com.au Table 2: Capacitor Codes              sol­dering the wire to the finish pin. You can now complete the coil by installing the wind­ing with the least number of turns over the top of the first winding, starting from the bottom. Solder this winding to its respective start and finish pins as before, then screw in a ferrite slug. The second BPF coil (T2) is wound the same way, noting the different pins for the start and finish of the windings. This coil is also fitted with a ferrite slug. The mixer transformer (T3) is a bit different in that it is wound on a 2-hole balun former – see Fig.6. A turn here involves passing the wire up through one hole and then back down through the second hole. The secondary is wound first and consists of eight turns either side of a centre tap. To wind it, first take a Fig.6: the above table shows the winding details for the various coils. T1, T2 & L1 are wound on 5mm formers fitted with a 6-pin base, while T3 is wound on a ferrite 2-hole balun transformer. Note that T1 & T2 (but not L1) are fitted with ferrite slugs. To do this, first run the wire down the inside of the start pin and solder it to the end of the pin. That done, start at the bottom of the former and wind on the required number of turns, keeping them next to each other in a single tight layer. The trick now is to hold the turns tight while you run the free wire down the outside of the winding and inside the finish pin. Finally, solder to the end of that pin. Although the heat Value IEC Code EIA Code 0.1µF 100n  104 .022µF  22n 223 .01µF  10n  103 .0047µF  4n7  472 .0033µF  3n3  332 .0015µF  1n5  152 470pF 470p  471 330pF 330p  331 220pF 220p  221 33pF  33p   33 10pF  10p   10 5.6pF  5p6   5.6 from the soldering iron should melt the wire enamel to allow soldering, you will probably find it easier if you scrape some of the enamel off the ends of the wires first. Cut off the excess wire from both pins when you have finished the winding. It may sound difficult at first but it will become easier with practice. And here’s a tip – wrapping a piece of adhesive tape around the winding makes it easy to keep it in place while Table 1: Resistor Colour Codes  No.   1   6   4   4   4   1   5   8   3   2   2   1   3   6   1   2 www.siliconchip.com.au Value 1MΩ 100kΩ 47kΩ 22kΩ 20kΩ 11kΩ 10kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1kΩ 560Ω 150Ω 100Ω 10Ω 4.7Ω 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown red red orange brown red black orange brown brown brown orange brown brown black orange brown yellow violet red brown orange orange red brown red red red brown brown black red brown green blue brown brown brown green brown brown brown black brown brown brown black black brown yellow violet gold brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown red red black red brown red black black red brown brown brown black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black brown brown green blue black black brown brown green black black brown brown black black black brown brown black black gold brown yellow violet black silver brown August 2002  73 The metal shield was made out of scrap tinplate. The two holes provide access to VR4 & VR5. length of wire about 600mm long and pass one end through one of the former holes, leaving about 50mm at the other side. While holding this short end in place, wind on eight turns around the centre of the former. Now bend the remaining wire into a sharp ‘U’ shape about 20mm from the former and twist the wires together to form the centre tap. That done, continue winding another eight turns in the same direction as before. When completed, the ends of both wires and the centre tap should be at the same end of the former. Next, move the secondary wires aside to avoid getting them mixed up. You can then complete the coil by taking another length of wire and winding on four turns over the top of the secondary to form the primary. The local oscillator coil (L1) is wound in a similar fash­ ion to the BPF coils, except that it only has one winding. You will need to make sure this coil is wound tight and the wire can’t move. Any movement will alter the local oscillator frequen­ cy and move the station off tune. The easiest way to do this is to coat it with glue or silicone adhesive after it’s wound. Before soldering the coils into place, check that they sit neatly and that the formers are perpendicular to the PC board. Also, when installing the cans for T1 and T2, make sure that they are positioned centrally about the former so that the slugs can be screwed up and down through the hole in the top of the can. The slugs used for T1 and T2 have a strip of rubbery mate­rial glued to one side. This is included to stop the slugs 74  Silicon Chip from moving during normal use and altering the tuning of the coils. You will probably find that this makes them quite difficult to move and excessive force on the brittle core can cause them to break. If you find that this is the case, scrape away some of the rubbery material with a sharp knife so that they are still firm in the former but can be moved freely with an alignment tool. Final assembly Now for the final assembly. First, cut the pot shafts to the required length using a small hacksaw and break away the anti-rotation spigot. That done, install the three pots on the PC board, making sure that the correct pot goes in each location. Next, scrape away some of the passivated coating on the top of the Main and Fine tune pots (VR2 & VR3) and connect a length of tinned copper wire between their bodies and also to ground on the PC board (see Fig.5). This stops 50Hz hum from being picked up by the pots and modulating SOFTWARE The PIC software files can be downloaded from the SILICON CHIP Web site. The files DCRX.ASM and DCRX.HEX are combined in a single zip file called DCRX-ASM-HEX.ZIP. To program your own PIC chip you will need the file DCRX.HEX, while studying the DCRX.ASM source code will reveal the secrets of how a humble PIC can measure high frequencies and sound Morse code! the local oscillator. A metallic shield must be placed over the local oscillator components so that it is not affected by quick changes in temper­ature or by external magnetic fields. This shield can be made from scrap PC board or, as in the prototype, constructed from tinplate (eg, from a food can). Fig.7 shows how the metal shield is made. Begin by cutting out the cross shape, then drill the two holes and remove all burrs along the edges with a file. That done, bend the tinplate along the dotted lines and run a bead of solder on the inside of each corner where the sides meet. Finally, place the upturned metal box over the local oscillator components – the holes line up with trimpots VR4 & VR5 – and solder it to the four PC stakes. The PC board is now finished and you can start work on the case. Using the photographs as a guide, start by drilling the holes for the power supply binding posts and for the antenna socket on the back panel. The front-panel holes can then be drilled using the accompanying artwork as a template. You will need to drill three holes for the pots, two for the pushbutton switches and one for the headphone socket. Alternatively, you can drill the holes in the front panel after first affixing the adhesive label. In each case, it’s best to drill a small pilot hole first and then carefully enlarge the hole to the correct size using a tapered reamer. Now, with the front and rear panels removed, place the PC board on the bottom of the case so that its front edge will butt up against the front panel. www.siliconchip.com.au This view shows the completed PC board assembly, prior to fitting the metal shield over the local oscillator section at lower left. The metal shield is secured by soldering it to four PC stakes. Note that the PC board will not sit flat at this stage, because some of the mounting pillars on the base interfere with the soldered connections. You can fix that by removing the offending pillars, either by drilling them out with a large drill or by cutting them off with a large pair of side­cutters. Once the case is ready, install the antenna socket and the binding posts on the rear panel. Note that an earth lug must be attached to the antenna socket (it’s secured by one of the mount­ing screws), to provide an earth connection point. The two push­button switches can also be installed on the front panel at this stage, with the red FREQ switch at the top. Once that’s done, attach the front www.siliconchip.com.au panel to the PC board and secure it by installing the pot nuts and washers (the washers go behind the nuts). You can then slide the assembly into the slot at the front of the case and fasten the PC board in place using four small self-tapping screws. Finally, fit the rear panel in place and wire the antenna socket, the power supply binding posts and the pushbutton switch­es to the PC board stakes using light-duty hookup wire. Test & alignment Before applying power, have a good look over the PC board one last time. A few moments spent here looking for components with the wrong value or in the wrong position could save you hours of frustration later on. Once you are satisfied that everything is correct, follow this test procedure to check out the receiver: (1). Set all the trimpots and the frontpanel controls to mid-position and plug a pair of headphones (or a loudspeaker) into the headphone socket. (2). Connect the receiver to a regulated DC power supply of around 12V and connect a multimeter – set to read DC current – in series with the positive lead. (3). Apply power and check that the current drawn is about 50mA. If you don’t get this, switch off quickly and check for errors. (4). Assuming all is OK, turn the Gain control (VR6) clockwise and check that you can hear some hiss in the headphones. This indicates that at least the audio stages are working. (5). Disconnect the multimeter from the supply lead, reapply power and check the voltages at the outputs of August 2002  75 The rear panel carries the two binding post terminals for the power supply plus the SO239 antenna socket. You can replace the binding post terminals with a 2.5mm DC power socket if you wish but make sure you get the polarity right. regulators REG1 and REG3. In each case, you should get a reading of +5V. You can also check for +8V at the output of REG2. Note: you will either have to temporarily remove the metal shield or remove the entire assembly from the case (so that you have access to the underside of the PC board) in order to do this. All regulator outputs should be accurate to within 250mV. Once again, if any measurements are incorrect, switch off immediately and check for errors. (6). If all voltage checks are OK, turn off the power and install the PIC chip. That done, reapply power and check that you hear three beeps in the headphones (each time power is applied, the PIC chip does a reset and generates three beeps to indicate that it is operating correctly). (7). Press the FREQ switch and the frequency of the local oscil­lator should be heard. Don’t worry about what it is at this stage – just use it to adjust the level trimpot (VR1) for an acceptable level in the headphones. 76  Silicon Chip (8). If you have a signal generator, inject a low-level signal at about 7.1MHz into the antenna socket. Set the Gain control (VR6) for a relaxed volume and adjust the cores in T1 and T2 with a suitable alignment tool for maximum volume. The BPF is fairly broadband, so there is no need to stagger tune the two coils to obtain a flat pass band. If you don’t have a signal generator simply connect an antenna, tune to a station around the middle of the band and adjust the cores in T1 and T2 for maximum volume. Freq. counter programming Now that the receiver is operating, let’s check the fre­quency counter operation and programming options. As previously stated, when you press the FREQ switch, the current frequency of the local oscillator is announced in Morse code. In addition, each time either switch is pressed and acknowledged by the PIC soft­ware, a short burst of tone is heard in the headphones. Pressing the MEM switch, however, gives one of two possible outcomes. If the next switch pressed is MEM again, the current frequency of the local oscillator is stored in the PIC’s EEPROM memory and two beeps will be heard (the EEPROM retains its cont­ents even if the power is removed). Alternatively, if the next switch pressed is the FREQ switch, the frequency stored in the EEPROM (not the current frequency) will be sounded in Morse code. This is a simple single-memory store and allows you to store a particular frequency and then retrieve it at a later stage – unless you overwrite it of course! Pressing the FREQ and MEM switches at the same time places the frequency counter in program mode and a long beep will be heard. At this point, pressing the FREQ switch toggles between long and short Morse modes. Long mode is where all the frequency digits are sounded; eg, 7123450. Short mode only sounds the kHz digits – in this example, the digits 123. This option will be the normal www.siliconchip.com.au setting and is used to speed up the Morse sounding. In any case, you will normally know what the MHz digit is and we are not usually interested in the fre­quency digits below 1kHz unless we are doing some testing or alignment. Pressing the MEM switch moves you onto the Morse speed setting, where two long beeps will be heard. Pressing the FREQ switch toggles between the three Morse speed settings. Pressing the MEM switch will return you back to the start of the program­ming mode. Each time a length or speed option is selected with the FREQ switch during programming, the current frequency is sounded using the selected options. Pressing both switches at any time exits programming mode and returns the frequency counter to normal operation. The pro­gram settings are stored in EEPROM and so do not get erased when power is turned off. The very first time you power on the receiver, the values in the stored frequency area of the PIC’s EEPROM will be unknown. As a result, strange readings may occur when the MEM and then the FREQ switches are pressed to read the stored frequency if one has not been stored previously. To avoid this situation, press the MEM button twice to store a valid frequency after the first power on. Once an initial frequency is stored in EEPROM, the MEM switch can then be used normally. Calibrating the counter To check and adjust the accuracy of the frequency counter, you will need to connect an external frequency meter to pin 6 of IC2b. That done, press the FREQ switch and compare Fig.7: here’s how to make the metal shield that goes over the local oscillator circuitry. Cut out the cross shape and drill the two holes before bending the tinplate down along the dotted lines. the frequency heard in Morse code with that displayed on the frequency meter. If they are the same or within a few tens of hertz, then no adjustment is really necessary. If you do want to improve the accuracy, this can be done by adjusting VC1 with a small screwdriver and then pressing the FREQ switch to check the change. Continue until the frequency heard in Morse code is the same as that displayed on the frequency meter. If you can’t get the frequency correct, you may have a crystal that’s too far off frequency, so try another. You may also need to alter the 33pF capacitor if you have changed the crystal and are still having no luck. Don’t be too concerned about obtaining absolute accuracy, as the base PARALLAX BS2-IC BASIC STAMP $112.00 INC GST www.siliconchip.com.au August 2002  77 If you don’t have access to a frequency meter, a reasonably accurate way of adjusting the crystal oscillator is to zero-beat to a known frequency carrier. At this point, the local oscillator and the carrier frequency will be the same. Press the FREQ switch and adjust VC1 as before. Setting the LO Fig.9: this front-panel artwork can be cut out and used direct if required. It can be protected behind a thin sheet of clear Perspex. resolution of the PIC software counter is only ±10Hz. Also, in normal use, you don’t need to know the tuned frequency to better than 1kHz accuracy. What’s more, the PIC oscillator will probably drift to a small degree over time and with changes in temperature. 78  Silicon Chip The local oscillator (LO) is a free-running HF oscillator and as a result it is quite normal for some frequency drift to occur immediately after power is applied. It stabilises within five minutes or so and drift after this warm-up period is quite small. For this reason, make sure the receiver is powered on for at least five minutes before adjusting the oscillator range. At this point, you need to decide what the range of the local oscillator – and hence the tuning range of the receiver – is going to be. In the prototype, the lower frequency was set to 7.000MHz and the upper frequency set to 7.200MHz. The LO adjust­ment procedure is as follows: (1). Set the Fine tune control (VR3) to mid-position and rotate the Main tuning control (VR2) fully anticlockwise, then move it a few degrees clockwise from the stop. (2). Press the FREQ switch to check the frequency. Adjust the “Low Set” trimpot (VR5) with a small screwdriver and check the frequency again. Repeat this procedure until the frequency is at the desired lower limit. (3). Rotate the Main tuning control (VR2) fully clockwise and then move it a few degrees anticlockwise from the stop. Now set the upper frequency limit in the same fashion as before, this time by adjusting the High Set trimpot (VR4). Note that there is some interaction between the High and Low trimpot settings, so you may need to repeat the last two steps a couple of times to obtain the desired range. It’s also possible that you may not be able to set the range correctly, because of component tolerances or because the coil (L1) is way off its intended inductance. If you can’t get the range low enough, try adding a turn or two to L1. Conversely, if the range is too low, take a turn or two off. (4). Check the frequency range of the Fine tuning control by rotating it to one stop, pressing the FREQ switch and then rotating to the other stop and again pressing the FREQ switch. You should achieve a range of around 1-2kHz either way. Note: this simple fine tuning system results in more range at the high frequency end than at the low frequency end. Once setup has been completed, attach the top case half with the two screws supplied and the receiver is ready for use. Operation Finally, here are a few tips to help you get the best from your receiver. First, when selecting a power supply, don’t be tempted to use a standard 12V plugpack. These are generally unregulated, producing up to 17V or so with no load. More importantly, they produce large levels of hum and this will be injected into the sensitive audio stages of the receiver. For this reason, it’s best to use a small 12V regulated supply or a battery that’s capable of supplying a few hundred milliamps. A regulated 12V or 13.8V DC plugpack or “power pack” is ideal. Don’t use a switchmode supply though as this will almost certainly create noise problems. Tuning SSB and CW stations is often difficult for the uninitiated. That’s because the tuning is fairly critical and also because the tone of the audio changes as you tune across the signal. The trick is to first set the Fine tune control to midway and tune in the signal as best you can using the Main tuning control. After that, it’s simply a matter of slowly rotating the Fine tune control until the voice sounds natural. For Morse code signals, just adjust the Fine tune control until the audio tone is easy to listen to. Because this receiver does not have an automatic gain control (AGC), you will need to adjust the Gain control to suit the level from different stations. Always start out with the Gain control set around three quarters and then advance it if the level of the signal is too low. If the gain is set too high and you are wearing headphones, a sudden burst from a strong signal will be most unpleasant. The receiver was designed for head­ phone use and so the output power is not particularly high. However, an efficient loudspeaker mounted in a suitable enclosure can be used if pre­ferred. Finally, to get the best from the receiver, it should be connected to an antenna resonant on the 40m band and www.siliconchip.com.au Fig.8: here are the drilling details for the front panel. The larger holes are best made by first drilling small pilot holes and then enlarging them to size using a tapered reamer. with an impedance of 50Ω. A good performing and easy-to-build type is a wire dipole fed with coaxial cable. www.siliconchip.com.au If you don’t have one alrea­dy, consult an antenna book for guidance or search the Internet for designs. 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Subscribe now by using the handy order form in this issue or call (02) 9979 5644, 8.30-5.30 Mon-Fri with your credit card details. August 2002  79