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BUILD-IT-YOURSELF FM AM WITH and a Touchscreen Interface using an Explore100 By Duraid Madina and Nicholas Vinen L ast month’s article had all the details on this radio’s circuit design and an explanation of how it works. All the circuitry shown last month is hosted on a single, compact board as shown here. Most of the components are surfacemounting types; many of them are only available as SMDs so we decided that Last month, we introduced our new digital radio design which is a DIY world-first: a DAB+ radio which will also tune into FM and AM broadcasts. It has a slew of excellent features such as a 5-inch colour touch-screen interface, built-in stereo headphone and speaker amplifiers, digital audio outputs and infrared remote control. Let’s start building it! we might as well go the “whole hog” and use them extensively. As we explained last month, where possible, we’ve chosen larger and easier-to-solder components where possible, so anyone with a bit of practice soldering SMDs should be able to manage it. And we also explained that we are going to offer PCBs with the trickiest (RF) parts pre-soldered. We are in the process of sourcing the components to build those and we’ll have more details on how you can purchase those next month (or you can check our online shop to see when they become available). We strongly suggest that if you want The DAB+/FM/AM Tuner uses the Micromite Plus Explore 100 module as its controller, station selector, volume setting and so on. This touch-screen module is perfect for the task and also saves a bewildering array of switches and knobs! See the Micromite Plus Explore 100 articles in September and October 2016 – siliconchip.com.au/Series/304 80 Silicon Chip Australia’s electronics magazine siliconchip.com.au A WORLD-FIRST DIY PROJECT FROM SILICON CHIP! to build this radio but are not comfortable working with fine-pitch SMDs, you start with the partially pre-assembled board. It will make your life so much easier. Practically speaking, if you’ve never soldered any SMDs before, you should probably start with a simpler project first. Once you’re more comfortable working with them, you can move onto this one. You may even want to experiment with soldering some cheap SMDs onto scrap board to gain some experience before tackling this one! Sourcing the parts As well as sourcing the PCB and parts for the radio board described here, if you don’t already have one, you will also need to purchase or build a Micromite Explore 100 module. You will also need to source a 5-inch colour touchscreen to suit. They are available from a variety of sources including ebay and AliExpress. Make sure you get the common version with a 40-pin header on the righthand side of the screen. The LCD PCB is usually blue. One option for building the Explore 100 is to purchase a short-form kit from our online shop, Cat SC3834 (siliconchip.com.au/Shop/20/3834). It includes everything you need to build the Explore 100 except for the LCD screen. There are a few SMDs on that board, including the 100-pin PIC32 processor. But they are not especially difficult to solder, at least, compared to the 48-pin QFN radio chip. Then solder the 10µF SMD capacitor in place, near IC1. Next, install the through-hole components are shown on the PCB silkscreen printing. These consist of nine resistors, 13 ceramic capacitors, two electrolytic capacitors, three LEDs, one crystal, one transistor, one regulator, one tactile switch and numerous connectors. The LED cathodes (shorter leads) go into the holes nearest the adjacent PCB edge. When fitting the connectors, make sure that CON6 and CON9 are fitted to the underside. You don’t need to fit CON1, CON4, CON5, CON7, CON10, CON13, CON14 or the headers for the real-time clock. But if your kit comes with those parts, it won’t hurt to install them anyway. We do recommend that you fit JP1 as it will aid in testing. If you haven’t used a pre-programmed PIC32 then the next step is to program it using a PICkit 3 or 4 (or similar) in-circuit serial programming (ICSP) tool. This is done via 6-pin header CON3. Then we suggest you test the board to make sure it’s working before fitting the LCD panel. The easiest way to do this is to connect a USB/serial adaptor to CON6 and then open a terminal emulator, set to the default baud rate of 38,400. Make sure the correct COM port for your USB/serial adaptor is selected and then wire up its TX, RX and GND pins to the appropriate pins on CON6, making sure to wire TX to RX and vice versa. To power the unit, if your USB/serial adaptor has a 5V output, you can wire this to the bottom-most terminal of JP1 (if fitted). Alternatively, fit a jumper to JP1 and plug a mini USB cable from your PC to CON2. As soon as the unit has powered up, you should see the Micromite’s banner appear on your terminal emulator. If you don’t, disconnect power and re-check your wiring and COM port selection. Typical power consumption for the Explore 100 sans screen is around 100mA (at 5V). If yours is significantly under or over this, something is wrong, so check the PCB carefully for soldering defects and misplaced components. Assuming you’ve had success, remove power and plug the LCD screen into CON10, attaching it with four 12mm tapped spacers and eight machine screws. You will then need to power it up and run the following commands on the console, to set up and test the LCD. Power consumption should jump to several hundred milliamps. OPTION LCDPANEL SSD1963_5, LANDSCAPE, 48 OPTION TOUCH 1, 40, 39 OPTION SDCARD 52, 53, 17 GUI TEST LCDPANEL You should now see coloured circles Building and testing the Explore 100 The circuit details of the Explore 100 module were published in the September 2016 issue of Silicon Chip while the construction details were given in the October 2016 issue (see siliconchip.com.au/Series/304). We won’t repeat them here; however, if you don’t have that issue, the process is relatively straightforward. Briefly, you need to fit SMDs IC1 and Q1 first, being very careful to orientate and align them correctly and ensure that all the fillets are properly formed and no pins are shorted. siliconchip.com.au As an example, here’s one source of 5-inch LCD Touchscreens we found on AliExpress. They’re also available on ebay and from several other suppliers. Australia’s electronics magazine February 2019  81 TX1 47pF 47nF 4.7 F 4.7 F 1 1 33pF 18nH 10nF 1 F TVS3 120nH FB3 T1 10k (ANTENNA 1 ) L4 120nH /FM) CON6 EXTERNAL AM LOOP ANTENNA CON4 100nF X2 12MHz CON7 (DAB+ IC3 1 F 8.2pF 8.2pF 47pF 4.7 F 47nF 47 47pF CON9 SCK MISO MOSI MMpin33 COM2T COM2R COM3T COM3R IC1 Si4689 22nH 10 F 4 2 6 8 10 10 F 1 9 7 5 3 15 13 33 31 35 11 12 14 16 20 18 24 28 26 22 1 15pF 10 F TOSLINK OUT SC 220 IC7 74HC14 CON1 15pF IC2 WM8804 110 30 34 6.8nF 100nF S/PDIF OUT 32 36 38 40 680 CON8 +5V +3.3V MMpin34 MMpin35 AGND EXTRAUD EXTLAUD DGND VHF INPUT X1 19.2MHz 47nF 47pF 21.5T 2.2k 1 F 100k 12pF 12pF IC4 PAM8407 2.2k 47 47 F 47 F 150pF 10k FB1 FB2 4.7 F 47nF 10k 2.2k 2.2k 1k 2.2k 10k 680 IC6 74HC4052 47 47 F 10k 1 F REG1 10 F 5.5T D1 2.2k Q2 Components inside dotted box are optional – see text IC5 OPA1679 Q1 + 6.8nF Q4 1k 2.2k 150pF 3.3 3.3 10 F 47 100 F + D2 FB4 TVS2 TVS1 R 100 F REG4 LM2663 15pF Q3 47 L 150pF CON2 4.7 100k 10k 47 10k 1M 47k 150pF RIGHT LINE OUT T 1 F 2.2k 2.2k 1 F 2.2k LEFT LINE OUT Q5 S CON3 (other side) REG 2 100 270k 100nF HEADPHONES OUT 1 F 4.7 R 39 IRR1 CON5 37 IRR1 100nF 100nF R+ 1 F R– L+ 1 F SPEAKERS L– 100nF 100nF 10 F 20 1 9 06112181 Rev.B CON10 +5V Components inside dotted box are optional – see text Fig.2: the component overlay for the receiver. It’s a double-sided PCB but all components are fitted to the “top” side, with the exception of CON3. This overlay is also printed on the PCB, as shown at right – between the PCB itself, this diagram and the photo opposite you should be able to complete the board construction without too much difficulty. being drawn on the screen. Press Enter in your terminal emulator to stop, then run this command to calibrate the touch sensor GUI CALIBRATE You will then need to use a thin object that will not scratch the screen, like a toothpick, to carefully press and hold in the centre of the targets which appear in each corner of the screen. Hopefully, you will get a message on the console that says “Done. No errors”. Otherwise, try calibrating it again. That completes the initial set-up of the Explore 100 module. Main PCB assembly Use the main PCB overlay diagram, Fig.2, (and the photo opposite) as a guide to the following assembly steps. The main radio board is built on a double-sided PCB coded 06112181, which measures 134 x 84.5mm (the same size as the Explore 100 module). The first job is to install IC1, the Si4689 radio IC, which comes in a 48-pad QFN package. If you have purchased a PCB with this chip already fitted then skip to the next section. There are two reasons why soldering this chip is tricky: the central ground pad on the bottom of the chip, which is inaccessible once it has been 82 Silicon Chip placed on the board, and the fact that the other 48 pads on the underside of the chip are barely visible or accessible. You have two main options for soldering this chip at home: the first is via a hot-air reflow or reflow via direct heating of the PCB with a hotplate. There are other reflow possibilities, such as infrared reflow, but you need expensive, specialised equipment. All you need to perform the hotair reflow is an SMD hot air station (which can be purchased for around $50 online), some fresh solder paste and a wooden clothes peg (not plastic!) or similar clamping device. While it isn’t an easy job, it is certainly achievable with some patience. The second option requires some slightly more expensive and specialised equipment, namely, a temperature-controlled hotplate (such as the type often used for chemistry), howev- Errors in last month’s circuit diagram Some of the connector numbers shown in the circuit diagram (Fig.1) last month were wrong. The two 8-pin expansion headers were labelled CON7 and CON8 but they should be CON8 and CON9. And the auxiliary 5V power connector near IC4 (which is in the lower right-hand corner) should be CON10, not CON9. Australia’s electronics magazine er, we have heard stories that a cheap hotplate intended for cooking food could be used. But that is a bit of a hit-and-miss affair, so we prefer using the scientific hotplate. They can be purchased for a few hundred dollars and you can keep this in your arsenal forever (and you’d better believe SMDs requiring this type of equipment will only become “the norm” in future). The main advantage of the hotplate method is that the alignment of the chip is not critical; solder surface tension will pull it into the correct position as long as it is reasonably close. However, that is not generally possible when using hot air because the airflow tends to blow the chip out of position unless it is clamped down. The preparation for both methods is similar. Start by applying a very thin smear of solder paste along all the 48 small pads on the PCB, as well as a thin smear in the middle of the central pad. By the way, you should use solderpaste that comes in a syringe (ie with a plunger). But you shouldn’t use leadfree solder paste as its melting point is too high and you risk damaging either the PCB or components or both! If you apply too much solder paste at this stage, you will have a lot of excess solder to remove later, so make siliconchip.com.au This same-size photo shows no ferrite rod antenna fitted – we’ve found that it picks up a lot of digital noise from the rest of the receiver and therefore is not recommended – you’re much better off with an external AM loop antenna if you want to listen to AM. Similarly, no sockets/headers are shown for CON8 or CON9 – these may be used further down the track! sure you use a minimal amount. You can’t apply solder paste to the 48 pads individually as they’re too small. Smearing it along the length of each set of 12 pads is adequate. When it melts, surface tension will pull it off the fibreglass/solder mask and onto the copper pads (and the lands on the bottom of the IC package). With solder paste applied to the board, drop the IC down in position. Make sure its pin 1 dot is orientated as shown in Fig.2. Using a good light and a magnifier, check the alignment of the lands and the PCB pads. The lands should be just visible from the side of the IC as they “wrap around” the bottom edge slightly. This step is more critical when using hot air reflow; make sure the lands are accurately aligned on all four sides of the IC and then carefully clamp the chip to the board using a clothes peg (not plastic, or it will melt). Recheck the alignment to ensure it hasn’t changed. For the hot plate method, it’s best to get the chip reasonably close to the correct alignment – and you definitely need to get the pin 1 dot in the right location – but the alignment doesn’t have to be exactly right. Now start heating the board. If using hot air, set the airflow low but not to the minimum and the temperature high (close to maximum). The trick is to keep moving the nozzle; don’t let it dwell on one spot on the board or chip too long or it might damage it. For those without 20/20 vision (and perhaps for those who do!) here are enlargements of two of the sections of the board with closely-spaced SMD components. At left is the area around IC5 & 6, at right IC7 & 2. siliconchip.com.au Australia’s electronics magazine February 2019  83 Move the hot air around, heating the board area around the chip and also the chip itself, then concentrate more on the chip over time until you see the solder melt and start to re-flow. Make sure all the solder paste melts to ensure all the joints have been made correctly. You may see a little flux smoke come out from under the IC as the solder paste on the central pad reflows. The amount of time it takes reflow all the solder will depend on whether you have preheated the board (a good idea if you can) and what type of solder paste you are using. Remove the heat and let the board cool down. If using a hot plate, you basically just ramp up the temperature of the board and chip together until the solder melts. It should all melt more or less at once. Make sure the hot plate is level and don’t bump it. When the solder paste does melt, you should see the chip move slightly as surface tension pulls it into perfect alignment. Switch off the hot plate and let the board cool down. Regardless of the method you used, clean off any excess flux using an alcohol solution or specialised flux remover and then carefully examine the edges of the chip under magnification. Make sure that there is no solder bridging the lands on the outside edge of the chip. If there is, add some flux paste and carefully use fine solder wick and a regular iron to remove it. Then clean off the flux paste and re-examine the chip, repeating until you are happy that there are no solder bridges. Soldering the parts around IC1 These parts are smaller than most of the others on the board and their proximity to IC1 and each other (necessary for good RF performance) makes fitting them a little tricky. But with a steady hand, they are not too difficult to solder. If your board came with these parts already fitted, skip this section. Start with the 12 smaller components surrounding IC1. These are in metric 1608 (imperial 0603) size packages, which measure 1.6 x 0.8mm. There are seven capacitors, three inductors, one resistor and a TVS. Since these components are so small, it’s best to handle them with very fine-tipped tweezers. But be especially careful when picking them up since if you drop them (or they flick out of the tweezer tips). Murphy’s law almost guarantees you will not be able to find them! The capacitors and resistors are the easiest. You can place a small amount of solder on one of the pads, slide the part into place while heating it, then solder the opposite end. It’s then a good idea to wait a few seconds or so for the solder to solidify, add a little flux paste on top of the original solder joint and re-heat it to ensure that both ends are soldered properly. The inductors are more tricky because the ones we used can only be soldered if they are the right way up; it’s easy to put them on the board upside-down and then they will not take any solder. The trick is to make sure the blue side of the inductors is facing up before soldering them – see the closeup photo (below left) to see how ours were mounted. Once you have them orientated correctly, you can solder them in the same manner as the capacitors and resistors. For the small components, that just leaves the TVS, which is similar in construction to the inductors. Its orange side faces up (also visible in the close-up photo). The enlargement at left is of the area of the PCB, around IC1, which could prove the most challenging. Follow closely the steps outlined in the text when constructing this section. Also in this photo T1 is shown very clearly – this will probably be required if you want to listen to hifi AM. Above is shown an area which you’ll only need if you want to connect speakers to your receiver. IC4 is the audio amplifier; it, along with the four 100nF capacitors, two 1µF capacitors, CON4 and CON9 can be left off if not needed (ie, you will use headphones or output to an external amplifier). 84 Silicon Chip Australia’s electronics magazine siliconchip.com.au Parts list – DAB+/FM/AM Radio Receiver 1 Micromite Plus Explore 100 module with 5-inch touchscreen (see September & October 2016 issues) [SILICON CHIP ONLINE SHOP short form kit (no LCD) Cat SC3834] 1 USB Type-A to Mini-B cable or USB/serial adaptor [eg, SILICON CHIP Online Shop Cat SC3437] 1 double-sided PCB, code 06112181, 134 x 84.5mm 1 extendable VHF whip antenna with SMA connector [SILICON CHIP ONLINE SHOP Cat SC4847] 1 AM loop antenna (Jaycar Cat LT3001) 1 small ferrite rod antenna (optional; not recommended) [Jaycar LF1020] 3 small Nylon cable ties 1 22nH 0603 low-loss inductor (L1) [Murata LQW18AN22NG00D] 2 120nH 0603 low-loss inductors (L2,L4) [Murata LQW18ANR12G8ZD] 1 18nH 0603 low-loss inductor (L3) [Murata LQW18AN18NG00D] 4 0603 ferrite beads (FB1-FB4) [Taiyo Yuden BK1608LL680-T] 1 small ferrite balun core (T1) [Jaycar LF1222] 1 1m length 0.25mm diameter enamelled copper wire (T1) [Jaycar WW4012] 1 19.2MHz crystal, HC49-US (SMD), 18pF load capacitance [TXC 9C-19.200MAAJ-T, Digi-Key 887-1062-1-ND] (X1) 1 12MHz crystal, HC49-US (SMD), 18pF load capacitance [TXC 9C-12.000MEEJ-T, Digi-Key 887-1268-1-ND] (X2) 3 XGD10603NR SMD polymer transient voltage suppressors (TVS1-TVS3) 1 black switched PCB-mount RCA socket (CON1) 1 horizontal PCB-mount white/red RCA socket pair (CON2) [eg, Digi-Key RCJ-2112-ND] 1 20x2 female header socket (CON3) 1 20x2 long pin female header socket [Jaycar Cat HM3228] 1 4-way pluggable terminal block and socket, 5/5.08mm pin spacing (CON4) 1 3.5mm stereo switched PCB-mount jack socket (CON5) 1 2-way pluggable terminal block and socket, 5/5.08mm pin spacing (CON6) 1 PCB-mount right-angle SMA socket (CON7) OR 1 PCB-mount right-angle PAL socket (CON7) [SILICON CHIP ONLINE SHOP Cat SC4848] 2 8-pin female sockets (CON8,CON9) 1 2-way mini terminal block (CON10; optional) 1 8Mbit TOSLINK transmitter [Altronics Z1601] (TX1) 1 infrared receiver (IRR1) Case and assembly hardware 1 set of case pieces laser-cut from 3mm clear acrylic [SILICON CHIP ONLINE SHOP Cat SC4849] 4 M3 x 25mm panhead machine screws 4 M3 x 32mm panhead machine screws 4 M3 Nylon hex nuts 4 9mm long M3 tapped Nylon spacers 4 12mm long M3 tapped Nylon spacers 8 15mm long M3 tapped Nylon spacers Semiconductors 1 Si4689 digital radio IC, QFN-48 (IC1) [Digi-Key 336-4159-ND] 1 WM8804 digital audio transceiver, SSOP-20 (IC2) 1 AT25SF321 32Mbit 104MHz serial flash, SOIC-8 (IC3) 1 PAM8407 stereo 5V amplifier IC, SOIC-16 (IC4) 1 OPA1679IDR quad rail-to-rail op amp, SOIC-14 (IC5) 1 74HC4052 dual 4-channel analog multiplexer, SOIC-16 (IC6) 1 74HC14 hex schmitt trigger inverter, SOIC-14 (IC7) 2 MCP1700T-1802E/TT 1.8V LDO regulators, SOT-23 (REG1,REG2) 1 LM2663MX switched capacitor voltage inverter, SOIC-8 (REG4) 2 BC817 NPN transistors, SOT-23 (Q1,Q3) 3 BC807 PNP transistors, SOT-23 (Q2,Q4,Q5) 2 BAV99 dual series diodes, SOT-23 (D1,D2) Capacitors 2 100µF 6.3V electrolytic (through-hole or SMD) 3 47µF X5R 1206 6 10µF X5R 0805 7 4.7µF X5R 0805 9 1µF X7R 0805 4 100nF MKT 3 100nF X7R 0805 4 47nF NP0 0805 1 10nF X7R 0805 2 6.8nF NP0 0805 4 150pF NP0 0805 4 47pF NP0 0603, low-ESL [Johanson 251R14S470GV4T] 1 33pF NP0 0603, low-ESL [Johanson 251R14S330JV4T] 3 15pF NP0 0805 2 12pF NP0 0805 2 8.2pF NP0 0603, low-ESL [Johanson 251R14S8R2CV4T] Resistors (all 1% SMD 0805 apart from one 47) 1 1MW 1 270kW 2 100kW 1 47kW 7 10kW 10 2.2kW 2 1kW 2 680W 1 220W 1 110W 1 100W 5 47W 1 47W (0603) 2 4.7W 1 3.3W Sourcing the critical Si4689 radio receiver IC One of the reasons we chose the Si4689 over some of the other Silicon Labs chips (eg, the slightly cheaper Si4685) was, at the time, due to its better availability. Over the months we have been working on this design, Digi-Key has consistently had several hundred in stock. But some time in late December, their stock level dropped very low. We purchased the last remaining parts in stock to ensure that we could supply at least some pre-populated PCB. The manufacturer’s lead time on this component is not siliconchip.com.au particularly long (around six weeks) and we have already requested that some of the next delivery be sent to us for fitment to our radio boards. Hopefully, by the time this article appears, the stock situation will have improved and Digi-Key will have some chips in stock, ready to order. If you want to mount this chip yourself but find that it is out of stock, we suggest that you order it anyway. As far as we can tell, you should receive it within a few weeks. Australia’s electronics magazine February 2019  85 The completed receiver, housed in its customdesigned acrylic case*. The upper PCB is the Micromite Plus Explore 100 board with its colour touch screen plugged in; the DAB+/ FM/AM radio receiver PCB is the green-edged board at the bottom. It too connects directly to the Explore 100 via a multi-way header plug and socket. There are very few external connections – visible are the external DAB+/ FM antenna socket and the connectors for an AM loop antenna and audio (speaker) output. The opposite end has the stereo audio, headphones, S/PDIF and TOSlink outputs. *Available from the SILICON CHIP ONLINE SHOP Now you can move on to the larger components around IC1. There are three ferrite beads and 11 larger capacitors immediately surrounding it. These can be soldered using the same basic technique. The only difference is that it’s easier since the components are much larger and easier to see. Remaining components near IC1 In terms of the components surrounding IC1, except for the VHF input connector (CON6), which we’ll leave until later, all that remains is regulators REG1 & REG2, crystal X1, flash memory chip IC3 and seven associated passive components, comprising six capacitors and one 3.3 resistor. It’s best to start with REG1 and REG2, both 1.8V regulators. These can be soldered similarly to the passives, by tacking the central pin, checking that the other two pins are lined up over their pads, soldering them, then refreshing the first solder joint with a dab of flux paste. Then you can fit the remaining resistor and capacitors mentioned above. Finally, solder flash chip IC3 in place, ensuring that its pin 1 notch or dot faces the top of the board, as shown in Fig.2. The pins are relatively widely spaced so you can solder them individually. If you accidentally get a solder bridge between adjacent pins, clean it up with some flux paste and solder wick. Now solder 19.2MHz crystal X1 in place. It is not polarised, so its orientation is not critical. It is a two-pin device; the third pad underneath it which it partially overlaps is provided to allow for grounding the crystal case. But we have not 86 Silicon Chip found that to be necessary. Building outside the box Having completed the critical radio receiver section, move on to the remaining components on the board. There are a couple of optional sections so you will need to decide whether to fit them. IC4 and the components that surround it, in the lower-right corner of the board, are only needed if you plan to drive an external speaker or speakers directly from the unit. These are shown inside a dotted box on Fig.2. IC2, IC7 and X2 at lower left, plus the nearby passives and connector CON1 and optical transmitter TX1 are only needed if you require a digital audio output. These are also shown inside a dotted box on Fig.2. As we said last month, the Si4689 firmware does not appear to support digital audio output in DAB+ mode so keep that in mind. Having decided which components to fit, start by soldering the remaining ICs in place. If you are fitting IC2 (WM8804), do that next as it’s in a fine-pitched SSOP package. The remaining ICs are much easier to solder. For IC2, the simplest technique is to spread a thin smear of flux paste on all of its pads, then tack down one corner pin and check that all the other pins are aligned with their pads. Also make sure that its pin 1 dot is facing towards the bottom of the board, as shown in Fig.2. Once you’ve verified that, tack solder the opposite corner pin in place, then load some solder onto the iron and gently drag it along the edge of the pads on one side of the chip. The flux should cause the solder to wick along the pad and onto the pin, formAustralia’s electronics magazine ing perfect joints. Repeat on the other side. Add extra solder to any pins which do not appear to have a good fillet and use flux paste and solder wick to carefully clean up any bridges. Proceed to solder IC4-IC7 and REG4, all in larger SOIC packages, using either a similar technique or soldering each pin individually. Once again, with all these chips, take care to ensure that the pin 1 dot or notch is orientated as shown in Fig.2. If your chip lacks both markings, check for a bevelled edge. This will indicate the pin 1 side of the chip. Transistors and diodes Now mount diodes D1-D2 and transistors Q1-Q5, all in SOT-23 packages and all in the upper-left corner of the PCB. D1 and D2 are identical but Q1Q5 consist of two different types so don’t get them mixed up. Once that’s done, solder the remaining SMD passive components (mainly resistors and capacitors) in place where shown in Fig.2. That also includes the remaining ferrite bead, FB4, near the top edge of the board and SMD inductor, L4, which goes to the right of IC2 and may be left off if you are not fitting IC2. Now is a good time to fit transient voltage suppressors TVS1 & TVS2, just to the left of where transformer T1 will go later. Use the same technique as before, again with the orange side mounted facing up. 12MHz crystal X2 can then be soldered in place. It is not needed if IC2 has not been installed. Winding the transformer The Si4689’s AM antenna input is impedance-matched to a ferrite rod antenna, which has a typical inductance of around 180-450µH. A loop siliconchip.com.au antenna has much better performance (and can be mounted away from sources of interference) but typically has a lower inductance, around 10-20µH, due to the lack of a ferrite core. So a matching transformer is required for the AM loop antenna. This can be easily wound on a small ferrite balun core (see parts list) using 0.25mm diameter enamelled copper wire. Cut a 500mm length of this wire and then wind 21.5 turns onto the balun core, leaving 25mm free at the start. The end of the winding should come out on the same side of the balun but out from the other hole. Trim the longer end to the same length and then strip the enamel off both ends. Make a mark on the end of the core so you know which end has the terminations for the larger winding. Now cut a 200mm length of that same wire and wind five and a half turns onto the same core, starting from the opposite end. Again, leave 25mm spare at the start and cut the end to the same length. Strip the insulation from those wires, too. Your transformer is complete and ready to be mounted. Note that our prototype transformer was wound with the terminations all at one end. This works as well but makes it harder to mount. And it’s easier to get the windings mixed up. Through-hole components Start by fitting the two 100µF electrolytic capacitors, with the longer (positive) leads through the holes towards the top edge of the board, as shown in Fig.2. Surface-mounting electrolytic capacitors can also be used. Next, fit the four 100nF MKT capacitors at lower right, assuming that you have already fitted IC4. These are not polarised. You can now mount your transformer (T1) to the board using a cable tie, with the 21.5 turn winding (marked earlier) towards TVS1 and TVS2. Pull the cable tie tight and cut off the excess, then solder the four wires in place where shown. CON4 and CON6 are pluggable terminal blocks so solder them in place now, with the socket side sticking out over the side of the PCB. Two-way horizontal RCA connector CON2 will probably have a triangular mounting bracket on the top, siliconchip.com.au which we don’t need. It will get in the way of the case later, so we suggest that you cut it off with a hacksaw (flush with the top of the rectangular socket moulding) and then file the top smooth. You can then push the socket down onto the PCB fully and solder its four pins in place. Next, install jack socket CON5, making sure that it is aligned with the edge of the PCB – you may need to twist it a little to get it lined up. Remove the nut from its shaft before soldering it in place. Now is also a good time to install the single RCA socket (CON1) and TOSLINK transmitter (TX1), if you have fitted IC2. Now fit SMA socket CON7. It’s just a matter of pushing it all the way down onto the PCB, with the barrel projecting out over the edge, and soldering the five pins. But note that the body of the connector is a large piece of metal and it has large pins, so you will need a hot iron to form satisfactory solder joints. You could use a PAL socket but these are hard to source, and these days more and more antennas are using F-type or SMA connectors instead. We have added some PCB-mounting PAL connectors to our online shop (see the parts list), so you can purchase one of those and fit it to the board instead of the SMA connector if you prefer to do so. Note that if you do this you will need to enlarge the corresponding hole in the case when the time comes to assemble it. The two eight-pin female headers (CON8 & CON9) are for possible future expansion. You can solder them in place now, or you can leave them off until we publish details of a future expansion board which will plug into those sockets and mount them then. CON10 is an optional two-way terminal block which connects directly to the 5V supply for the audio amplifier IC (IC4). We’ve designed the board so that 5V power is supplied to it via the Explore 100. But since the audio amplifier can draw significant current, and that current must flow through a single pin on the 40-way header, to get maximum power from the speaker outputs, you should feed the 5V supply in via CON10 instead. If you plan to use that option, fit the CON10 terminal block now and then you can wire it up to a Australia’s electronics magazine chassis socket later. Solder the infrared receiver, IRR1, with its leads bent so that it sticks out the top edge of the board, as shown in Fig.2. Make sure that its lens bump faces away from you, when looking at the board as shown in the overlay diagram. If you’re unsure, check our photos. Bend the leads so that the bottom of the receiver package is just about resting on the edge of the board. The last component to mount on the board is the two-row 40-pin header socket. We’ve left it until last because it mounts on the back of the board. Make sure it’s sitting flush on the PCB and solder all 40 pins, taking care not to apply too much heat, which could deform the plastic. You will notice that we have not mentioned fitting the ferrite rod antenna. You can do so if you wish; it’s shown dotted on the overlay diagram in the correct (horizontal) position. You then just need to connect the wires with green and red markings to the pads shown. The reason we have left it out is that we’ve found that it picks up a lot of digital noise from the control circuitry and as a result, AM reception performance with the ferrite rod is not good. If you connect an AM loop antenna without the ferrite rod in place, you will get much better AM reception than if the ferrite rod is mounted on the board. We are currently experimenting with possible shielding solutions and also software changes to mitigate this interference issue and if we come up with a good solution, it will be incorporated into the construction process in next months’ issue. But for now, the safest thing to do is leave the ferrite rod off the board. You can always fit it later. It’s held in place using two cable ties which loop through holes on the PCB, as shown in Fig.2. Coming next month You should now have a fully assembled and working Explore 100 module plus a completed radio PCB. Next month we will have the details on how to put them together, build the case, load the software, test it and get it up and running. We’ll also have more screen grabs and details on how to use the radio. SC February 2019  87