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SSB Shortwave Receiver Part 2 by Charles Kosina, VK3BAR Introduced last month, this new Shortwave Receiver covers the entire shortwave band from 3MHz to 30MHz. It is digitally tuned and has a host of useful features like squelch, USB/LSB support, good sensitivity (a -107dBm signal gives 13dB SNR), fast or slow AGC, an RSSI display and it runs from 12V DC. This month, we describe how to build, test and align it. T his is not an overly difficult device to build, as it uses no tiny components or fine-pitch ICs. However, it has two boards that are fairly packed with SMDs plus quite a few though-hole components, so you should ideally have decent soldering skills if you’re going to attempt it. You also need some test equipment to calibrate the Radio. That includes an accurate frequency counter up to 100MHz and a signal generator that will work up to at least 30MHz that can produce a signal down to 10µV or less (or an attenuator to allow that). An oscilloscope with 100MHz or more bandwidth is also nice to have, but not absolutely necessary. Some sheet metalwork is needed. 74 Silicon Chip I recommend having a stepped drill bit or two (eg, 3-12mm & 12-24mm) on hand. A drill press would be ideal, but you can do it with a hand drill if necessary. There are many components overall, but the values are marked on the circuit boards to ease construction. It pays to be careful as you go through the assembly process and make sure each part goes where it’s supposed to. Mixing up two visually identical capacitors could be enough to prevent the radio from working. Construction Virtually all the components mount on two PCBs, the Control Board (Fig.14, 152.5 × 81.5mm) and the RF Australia's electronics magazine board (Fig.15, 152.5 × 51mm). There are some through-hole components used, but the vast majority are SMDs, mostly passives (resistors and capacitors) in M2012/0805 packages, which measure 2.0 × 1.2mm. These passives are on the small side if you are used to through-hole components, but we still consider them to be in the ‘easier to handle’ category compared to really small parts. So as long as you have the right tools, a decent amount of light and reasonable vision (or magnification), you should not find the assembly too difficult. Similarly, the ICs are not in really tiny packages; they are mostly SOIC types with 1.27mm lead pitch, ie, half that of a through-hole chip. Again, siliconchip.com.au Fig.14: the Control Board has parts on both sides; fit all the SMDs first, then the throughhole parts on the underside. The ICs, diodes, electrolytic capacitors and the Arduino Nano module must all be installed with the polarities shown here for the Radio to work. these are not what we would consider difficult-to-solder parts. Control board I recommend building the Control Board first and testing it before you move on to the RF Board. There are components on both sides of the board, but most of the parts, including all the SMDs, are on the front. Start by soldering the two ICs first, making sure their orientations are correct. In each case, find the pin 1 marker (a dot or divot on the top, or a chamfered edge on the side) and make sure it’s aligned as shown in Fig.14 and on the PCB silkscreen. It’s possible to solder the pins of these SOIC package devices individually with a fine-tip soldering siliconchip.com.au iron. Add plenty of flux paste to make soldering easier and reduce the possibility of bridging pins with solder. If that happens, use copper braid with a bit of extra flux paste to remove the excess solder. In fact, we usually don’t bother trying to avoid bridges as it’s so easy to clean them up later; we’re more focused on making sure the solder flows onto each pin and pad, to avoid high-resistance connections that can be difficult to find later. Follow with the passives using a similar technique. The resistors will be marked with codes indicating their values (eg, 10kW = 103 or 1002) while the capacitors will not have any markings. In both cases, it’s best to unpack a single value, then fit them all as shown Australia's electronics magazine on the overlay diagram so you can’t get them mixed up. None of the passives are polarised so they can be soldered either way around. Note that a few of these parts, like the 68W resistor and 100μF capacitor, are slightly larger than the others and so have larger pads to suit. Also, two of the 8.2kW resistors at centre left are not fitted (marked R10 & R20 on the PCB) as these are the I2C pull-ups and the Si5351 module has onboard pull-up resistors. Follow by soldering the four identical Mosfets, which are all in three-pin SOT-23 packages. The pins are small but widely spaced, so this should not be too difficult. Don’t fit any of the through-hole July 2025  75 components on the front side of the PCB (where the SMDs have been soldered) yet. The Arduino Nano and the Si5351 modules are on the back of the board and can be plugged into socket strips. This is important as if either failed, replacing them would be difficult. 15-pin socket strips are used for the Nano and one seven-pin strip for the Si5351. The only other parts on the back of the board are the headers, two electrolytic capacitors (which are polarised, so make sure they’re fitted the right way around) and the trimpot for LCD contrast. All of those can be mounted now. As well as the speaker connector (CON4), there is another two-pin header, CON3. This is connected in parallel with the headphone jack socket and may be wired to an RCA socket on the back panel for an external powered loudspeaker. Now go back to the front of the PCB and fit the remaining through-hole components. It’s important that the switches, potentiometers and encoder are square on to the board before soldering. The way to ensure this is to attach the black front panel with 16mm tapped spacers to position these correctly; make sure that all controls turn easily before soldering. For a better appearance, rather than zinc-plated screws, I used black 6mm machine screws (which you can buy at Bunnings) to attach the front panel. The jack socket is a unique part; ensure it is pressed firmly on to the board. Next, solder the 16-pin header to the LCD module. Don’t attach the LCD yet; clean the board with de-fluxing solvent and inspect all connections with a magnifier. Pay special attention to the solder joints on the socket strips for the Nano, as they are not accessible once the LCD is mounted. Before any modules are plugged in, use an ohmmeter to ensure that there are no shorts from the 12V or 5V supply lines to ground. Finally, attach the LCD module using 5mm spacers and 12mm machine screws and nuts. The Si5351a module is held in place by 6mm M2 or M2.5 screws with 11mm threaded spacers. RF board assembly If you want to take a break from assembly now, you could skip down to the “Programming the Nano” sub-heading, complete initial testing and calibration, then come back here when you get to the part where you need the RF board to be assembled. The RF board overlay is shown in Fig.15. Parts are only fitted to one side of this board. As before, start with the ICs (two NE612s, one LMC6482 and the PCF8754) and ensure they are all orientated correctly as you solder them. All are in SOIC packages. Then move on to transistors Q1-Q7; Q7 is a Mosfet, while the others are NPN RF transistors, but they all come in the same packages, so don’t get them mixed up. Follow with Q8-Q10, which have four pins since they are dual-gate Mosfets. In all three cases, the wider source lead goes towards lower right with the PCB orientated as shown in Fig.15. Fit diode D4 with its cathode stripe as shown, then REG2 after first applying a thin layer of flux paste to its pads. That will assist in soldering its tab properly. After that, solder the SMD resistors & capacitors, noting that the capacitors are again unpolarised, and all the SMDs are on the board. Mount the axial inductors next; they have three different values, so make sure the right ones go in each location. They are not polarised, so you can fit them in either orientation. Fit diodes D2 & D3 next; they are polarised, so ensure the cathode stripes both face to the right. After that, solder RLY1 in place with its pin 1 marking towards the top. Next, fit VC1-VC3, which are polarised in a sense, because we want the adjustment screws to be connected to the ground pins in each case. So orientate them as we have shown. For the varicap diode, VD1, you may get it in a two-lead TO-92 package like we did, or in an axial package, similar to a regular diode, which can be mounted vertically. Regardless, ensure its cathode lead goes to the pad marked K; with the axial package, this will be the end with the stripe. Bend REG1’s leads down and attach it to the board using an M3 machine screw and nut, ensuring its three leads go into their pads. Solder and then trim the leads. Don’t do this before tightening the screw or you could fracture the leads. I used a 16mm-long screw to attach the tab as it makes a convenient ground point for testing later. Next, install CON1 and CON2. That just leaves the crystal filter module, XF1. The crystal filter comes with SMA sockets attached, and at least the input one has to be removed. As it’s supplied, only the top connections are soldered; I used a hot air wand to carefully heat them and slide them off, but a soldering iron with a large tip could also be used instead. Take great care that other nearby components don’t get removed as well. Attach the filter to the PCB using four 10mm-long M2 or M2.5 screws, nuts and 5mm spacers. Solder wires to connect the input and output of the filter to the circuit board, one for signal and another for ground at each end. In theory, XF1 is not polarised, but it’s a good idea to mount it like we did, with the angled capacitor on Fig.15: all the parts are on the top side of this RF Board. Polarised components to pay attention to include the ICs, dualgate Mosfets, diodes, relay and variable capacitors. It’s best to remove the SMA connectors from the crystal filter module (XF1) before mounting it on this board. 76 Silicon Chip Australia's electronics magazine siliconchip.com.au the right-hand side, since that’s how we tested it. Toroid winding The 3-10MHz toroid (T1) needs 42 turns of 0.35mm diameter enamelled copper wire (ECW) for its secondary and four turns of the same wire for the primary. The secondary will take a little while to wind; do it first and neatly, with the turns almost touching each other. There is just enough room for that number of turns with a small gap in between the ends. Attaching the transformers to the PCB is one of the most fiddly parts of the assembly process. T1’s primary is soldered between the pads labelled A & B on the PCBs, and is wound near the ‘cold’ end of the secondary, while the secondary is soldered between pads C & D. I found it easier to attach the transformers to the PCB first, solder the secondary windings, then add the primary windings. It is a bit tricky but I think it is the best approach! Make sure you scrape off the enamel from the wire ends before soldering them to pads A-D; otherwise, you won’t get a good electrical connection and the radio won’t work. It helps to tin the ECW ends after scraping them; if the solder won’t stick evenly, that means you need to scrape off more enamel first. There’s also room for a tinned wire loop to help hold the toroidal core to the PCB at upper right. We recommend you add this to prevent solder joints from fracturing due to movement over time. This does not form a shorted turn as the pads it’s soldered to aren’t connected to anything. The second toroid (T2) has 15 turns of 0.6mm diameter ECW for the secondary, which you should distribute evenly. Make sure the direction of winding is such that the ends go into holes G & H in the PCB correctly. Then add the two-turn primary using 0.35mm diameter ECW, scrape and tin the ends and solder it to pads E & F. Again, solder the piece of tinned wire to hold it to the PCB. Once all components have been installed, give the board a thorough clean to get rid of surplus flux. Inspect all soldered joints and check for any shorts with an ohmmeter. Make up the connecting 16-wire cable with IDC sockets. Use a small vice or crimping tool to siliconchip.com.au Wiring up the boards is pretty straightforward with a ribbon cable connecting the control and RF boards. There’s room inside the enclosure to fit a three-cell battery holder which can be used to power the Receiver. evenly press the parts together; make sure the cable is exactly square on to the connector. Attach the loudspeaker to the two wire connector and plug this on the control board. Programming the Nano The Nano should be programmed before it is plugged in. You can use the free programming software called AVRDUDESS for Windows that you can download from siliconchip.au/ link/aaxh or use the command-line version, avrdude, if you’re running Linux or macOS. Australia's electronics magazine Connect the Nano via a USB cable and check what COM port it appears as. Select a baud rate of either 57,600 or 115,200 depending on the version of the bootloader in your Nano. Select the programmer Arduino for bootloader using STK500v1 protocol from the drop-down list; it may be the default. Press the Detect button and it should recognise the chip. It is important that the programming is done in exactly the following way, making sure that the settings in AVRDUDESS are correct. Otherwise, you could end up with a bricked Nano. July 2025  77 There are two files to be loaded: the program file, “SSB RX xx.HEX”, where xx is the version, and the “SSB RX xx.EEP” file, which is a binary file loaded into the chip’s EEPROM. There are five boxes below Options. Tick “Disable Flash Erase”. Under the Flash box, “Format” should be “Auto (writing only)”. Locate the HEX file to be loaded into Flash by searching for it in the square to the right of the Flash window. Locate the EEP file and place in the EEPROM window. Select Raw Binary from the drop-down list. Tick the Write circle under Flash and press the Go box. This will result in progress messages in the bottom window. Tick the Write circle under the EEPROM window and press the Go box. It will also have messages in the progress window. This completes the programming, and the Nano may be disconnected from your computer and plugged into the Control Board. Make sure its orientation is correct. Initial testing Connect power to the board via CON1. Make sure polarity is correct; if not, nothing will happen as there is a protection diode. The LCD backlight should be on, but there may not be any text visible. Adjust trimpot VR6 until you see text on the screen. With the Band potentiometer (VR3) fully anti-clockwise, the top line should have 3.600.000MHz and the bottom line USB or LSB, depending on the position of switch S2. There should be a cursor visible under one of the digits. When the shaft encoder is rotated, this number should change. Depending on the particular encoder used, it may operate backwards. In that case, bridge the two pads marked DIR above the LCD to reverse the direction. Switch the power off and on; the screen will show “SSB Receiver” on the top line and version number on bottom line for two seconds. Toggle the USB/LSB switch and see that it changes on the screen. Press the switch on the shaft encoder and check that each press moves the cursor to another position under the frequency. It should allow adjustments in 10Hz, 100Hz, 1kHz, 10kHz, 100kHz and 1MHz steps. The Band potentiometer is a convenient way to cycle through the most common amateur radio bands. It sets a frequency partway through each band starting with 3.6MHz, then 7.1MHz, 10.0MHz, 14.1MHz, 21.1MHz and 28.1MHz. 10MHz is not actually in a ham band, but it has WWV transmitting time and accurate frequency information. Using an oscilloscope and a frequency counter, check the outputs of the Si5351 module. CLK2 is the BFO and that should read 8999.6kHz or 8996.6kHz depending on the position of the USB/LSB switch. CLK0 should have a frequency that is the sum of the currently tuned frequency plus the BFO frequency. The accuracy of these depends entirely on the 25MHz crystal attached to the Si5351 module, so you may get slightly different values. At this point, it is advisable to perform calibration. Calibration To calibrate the set, you need to measure the actual frequency of the 25MHz crystal on the Si5351 module. This procedure will calibrate the short-term accuracy to within less than 5Hz. Switch off the receiver, then rotate the Band potentiometer fully clockwise. Switch it on and the top line on the LCD will show “Calibration”; the bottom line will show the nominal crystal frequency of 25,000,000Hz. In this mode, the frequency on OUT0 is set to exactly 10MHz. Fig.16: the recommended hole locations and sizes for the rear panel. A stepped drill bit makes drilling these straightforward. As this is at actual size, you could copy or download and print it and use it as a template. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au Measure the frequency on OUT0 with an accurate frequency counter and rotate the tuning knob, which increments or decrements by 10Hz, until the output is as close as possible to 10MHz. It’s possible to get to within 1Hz. It may take many turns, as the crystal could be out by more than 10kHz. Turn the Band potentiometer anti-­ clockwise to leave calibration mode. The new value for the 25MHz crystal overwrites the original value in EEPROM loaded by the EEP file, and will be read every time the power is switched on. This calibration needs to be done at regular intervals as the crystal may age and can also drift with temperature. Case preparation The modifications to the case involve drilling four 3mm holes in the base to attach the RF board, plus numerous holes in the back panel for the power socket, antenna connectors and the loudspeaker. I used the Jaycar AS3025 rectangular speaker, but just about any small 8W speaker will be suitable. You will need to adjust the mounting hole positions if using a different speaking, though. Fig.16 shows the suggested hole pattern. I used a stepped drill bit, as they make clean, circular holes. If you have a drill press, that would be ideal, but you can hand-drill these holes neatly if you’re careful. A word of warning! The drill can grab the plate and spin it around, possibly injuring you, so for safety, always make sure the plate is clamped firmly while drilling the large holes. Alignment This should be done with the two circuit boards not yet assembled into the case to allow easier access to test points. The only adjustments on the RF board are the three variable capacitors, and they should be peaked at 9MHz. The way you do this depends on what equipment you have. Connect the control board to the RF board with the flat cable and carefully check that the two pin 1s are connected together, ideally with the red striped side of the cable to those pins (check for continuity between the pin 1 pads on the two PCBs). If you connect the cable backwards at one end, you could do damage! Switch on the power and set the frequency to, say, 7.1MHz or some other convenient frequency. Use an oscilloscope probe to check that you have the VFO signal at about 16MHz on TP3 and the 9MHz BFO on TP5. Connect a signal generator to CON1 with the output set to about 100µV. This is way above the lowest level, but is useful for the initial setup procedure. Tune the signal generator for a whistle from the loudspeaker, which should be very loud. Reduce the signal generator output until you get some background hiss from the speaker. Adjust the antenna tuning potentiometer (VR1) for maximum signal, measure the DC voltage on TP6 and tweak the three trimmers (VC1-VC3) for maximum output. Fig.17: the front panel PCB overlay for the SSB Receiver. It is shown here at 70% of actual size. siliconchip.com.au Australia's electronics magazine That’s all there is to it; the receiver should work across the whole range. You will find as you tune across 10MHz, the relay will click to switch between using T1 & T2. Final assembly The two boards and front panel can now be assembled into the case. The RF board uses M3 × 10mm threaded spacers to attach to the bottom of the case. The antenna input connects via a short ready-made cable between the SMA connector to a BNC connector on the back panel. Power is from a 2.1mm or 2.5mm ID (inner diameter) DC jack on the back panel to the two pin connector, CON1, on the Control Board. The front panel attaches to the case by four screws on the corners. Instead of using the zinc-plated screws that came with the case, I found some black screws that look better. All the rotary controls have 6mm diameter shafts. It is preferable that these have fluted shafts, as knobs for these are more common. Still, there are a few sellers on AliExpress that have knobs with grub screws (they are listed in the parts list last month). Those were ideal for D-shaped shafts but can also be used on fluted shafts. Conclusion Your Radio should now be operational and you can start scanning the bands for signals! You can even use it on the go, powered from a 12V battery. Note that while you could run this radio from a 12V vehicle battery, you must not do so (or connect it) while the engine is running as it doesn’t have protection from voltage spikes. Unless you add a suitable filter, it’s far safer to run it from its own interSC nal battery. When first running the Receiver you must calibrate it by following the text under the cross-heading “Calibration”. This ensures accurate timing. July 2025  79