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A WORLD-FIRST DIY PROJECT FROM SILICON CHIP!   TUNER with FM & AM and a touchscreen interface! By Duraid Madina and Nicholas Vinen We believe this is the first build-it-yourself digital radio published in any magazine – certainly here in Australia, if not the world. It receives, as you would expect, DAB+. It also receives FM (mono/ stereo). But (again to our knowledge!) this is THE FIRST to also receive AM radio (not many, if any, commercial DAB+ receivers can do that!) It’s simple to use, thanks to the 5-inch touchscreen and graphical interface provided by the powerful Explore 100 processor. It has many audio output options and offers outstanding sound quality, too. T his is, without doubt, the most capable build-it-yourself radio design ever published – anywhere! It can receive DAB+ digital radio in stereo, FM in mono or stereo and 28 Silicon Chip AM in mono. It also has a really intuitive colour touchscreen graphical user interface (GUI) and lots of other great options such as remote control, headphone and speaker outputs, digital audio outputs and more. Australia’s electronics magazine You just need to glance at the features and specifications to get an idea of how comprehensive this design is. We’ve tried to take advantage of all the features of the digital radio receiver IC that we’ve used, as well as the GUI siliconchip.com.au capabilities of the Explore 100 module, to make the user interface experience as smooth as possible. The radio incorporates an onboard headphone amplifier with digital volume control, so you can plug headphones or earbuds straight in. There’s also a small onboard stereo power amplifier, with decent sound quality, allowing a pair of passive speakers to be driven at up to two watts per channel. In AM and FM modes, you also have the option of using one of the digital outputs (S/PDIF or TOSLINK) to feed audio to a hifi receiver or DAC. The radio incorporates a ferrite rod antenna for AM but an external AM loop antenna can also be used, for better reception. To receive FM and DAB+ broadcasts, an antenna is connected to the SMA socket. This can be a proper roofmounted VHF antenna, or a telescopic whip attached directly to the side of the radio. As well as using the intuitive touchscreen interface, you can also control major functions such as changing channels, modes and volume via an infrared remote control. You can easily enter station frequencies if you use a remote control with a numeric keypad. The whole thing is powered off 5V, so you can use a standard plugpack. You can even use a USB power bank, making the radio fully portable. We’ve also made the design upgradeable in future, so that internet radio could potentially be added using a WiFi “daughter board”. The whole thing is housed in a custom laser-cut acrylic case. Design challenges We’ve been working on this radio design for more than six months. There are several reasons that it has taken so long, besides the fact that it is an ambitious project. For example, there is little publicly available information on the main chip, the Si4689 radio receiver IC. And some of the information that we found turned out to be incorrect. We bought a development kit to get the chip up and running initially, which included the firmware needed for that chip to operate, along with information on how to configure it. We then had to develop MMBasic software to drive that chip, along with other parts of the circuit such as siliconchip.com.au the serial flash (which is used to store firmware), the digital audio transceiver and so on. We had hoped to produce a radio which could also receive Digital Radio Mondiale (DRM), the long-range digital radio broadcasting standard. This is not yet available in Australia but there are DRM stations in New Zealand and we figured that one day, we would get it too. Unfortunately, while the Si4689 supports DRM in theory, the firmware supplied does not have a DRM mode. The hardware as presented supports DRM reception but we don’t know if or when firmware will be released to enable it. For more information on DRM, see our articles in the November 2013 (siliconchip.com.au/Article/5448) and September 2017 (siliconchip.com.au/ Article/10798) issues. Another unfortunate limitation has to do with the Si4689’s digital audio output. Our board has support for converting the digital data to both common consumer formats – S/PDIF and TOSLINK – so you can feed it to a DAC or receiver. But again, the firmware lets us down, as it disables the digital output in DAB+ mode; something not mentioned in any of the documentation. So we can only guarantee that the digital outputs work in the AM and FM modes. That may be fixed in a future firmware update, but we can’t say when that might happen. We are guessing that the digital output is disabled in DAB+ mode due to concerns over users making copies of the audio data. Regardless of those problems, this is still a very capable radio. And it can be easily upgraded in future if any of the above firmware gremlins are resolved. Features • DAB+, FM and AM reception • Eight favourite station presets per mode • 5-inch colour touchscreen interface • SMA socket for external FM/DAB+ (VHF) antenna or telescopic whip • Internal AM antenna (ferrite rod) plus terminals to connect external loop antenna • Stereo line outputs, headphone driver and onboard stereo audio amplifier • Digital audio outputs (S/PDIF and TOSLINK) • Digital volume control, with separate settings for line out/ headphones and speakers • Signal strength reported in all modes • AM/FM modes report signal-tonoise radio (SNR); DAB+ mode reports error count • Optional infrared remote control • Auto-mutes speakers when headphones are plugged in • Stereo amplifier can drive two 4-8speakers at 1W+ each • FM RDS/RBDS decoding • Automatically scans for channels (services) in DAB+ bands • Channel name and currently playing program displayed Surface-mount components • Upgradeable firmware The Si4689 radio chip has many great features and there really aren’t any equivalent chips available, so it’s the obvious choice for this project. But it’s only available in a 48-pin QFN (quad flatpack no leads) package. The “no leads” part of its name may give you a hint that this is not a particularly friendly package for handsoldering. Having said that, we succeeded in soldering two of these chips by hand (out of two that we tried), using two • Possibility for future expansion (eg, WiFi internet radio support) Australia’s electronics magazine • Powered from 5V DC regulated plugpack • “Quiet” mode for AM reduces digital pickup • DAB+ frequencies default to Australian channels • Optional laser-cut acrylic case January 2019  29 1 F FB4 INTB IR TO CON7/8 +3.3V 9 7 5 3 1 (TO & FROM EXPLORE 100) SMODE TO CON8 TO IC6 PIN9 TO IC6 PIN10 +5V CON3 2 5 SO SI 4.7 F 8 Vdd HOLD CS 34 1 1 2 4 48 3 3x 47 4 5 29 6 T1 5t TVS1 21t EXTERNAL 9 H AM LOOP ANTENNA 7 ANT1 XGD10603NR 362 H 10nF FERRITE ROD L1 22nH TVS2 X1 19.2MHz 33pF L3 18nH TVS3 XGD10603NR 2.7pF 8 9 10 11 15 XGD10603NR VHF IN CON7 47pF 47pF 4.7 F 47nF 6 47 CON6 47nF 7 IC3 SCLK AT25SF3 AT 2 5SF3 2 1 WP 3 Vss MISO MOSI RSTB FLHD SS FLWP FLSO FLSI FLCK FLCS IC2CSB IC2IFM IC2RST IC4DN IC4DP IC4SD SCK 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 39 37 35 33 31 29 27 25 23 21 19 17 15 13 11 TO CON8 REG4SD HPSW 16 13 14 12pF L2 120nH 0 12pF 35 47pF 12 37 VA VIO VMEM VCORE NVSSB NC 46 NVMOSI 45 44 NC 43 NVMISO NC 47nF 4.7 F NC NVSCLK NC INTB DBYP RSTB DOUT SMODE MISO SSB MOSI SCK IC1 Si4 6 8 9 LOOP_N LOOP_P LOUT ROUT RFREF DCLK RFREF DFS VHFI DACREF VHFSW NC XTALI NC NC XTALO NC ABYP NC NC NC PAD NC GNDD GNDD GNDD GNDD 39 40 41 42 38 36 33 32 47 31 30 18 19 27 28 17 26 25 24 23 22 21 20 1 F SCK MOSI MISO IRR1  +3.3V 100 3 1 IR L4 120nH 10 F 1 F 7 2 1 2 3 4 5 6 9 X2 12MHz 10 11 PVdd 15pF 19 DVdd LRCLK SCLK BCLK SWIFMODE DIN SDIN SDOUT IC2 WM 8804 CSB RESETB DOUT MCLK 15 14 13 12 16 IC7f CLKOUT XOP TXO XIN RXO PGND 15pF 10 F 8 17 13 20 14 12 7 DGND 18 100nF IC4SD IC4DP IC4DN SC 20 1 8 DAB+/FM/AM DIGITAL RADIO RECEIVER Fig.1: at the heart of this radio board is IC1, the Si4689 digital radio receiver IC. Its crystal oscillator timebase and antenna matching components are shown to its left, with the analog audio switching and filtering parts to its right. The digital audio processing chip (IC2), expansion headers and audio amplifier (IC4) are arrayed along the bottom of the diagram, with the serial flash chip (IC3) and power supply components along the top. 30 Silicon Chip Australia’s electronics magazine siliconchip.com.au REG4SD REG4SD HPSW HPSW +5V +5V +1.8V1 REG1 MCP1700–1.8 REG2 MCP1700–1.8 FB3 +1.8V2 OUT 47 F 8 1 IN GND 10 F IN GND 10 F OUT 3.3 7 10 F 6 10 F V+ REG4 LM2663 OSC 2 CAP+ 47 F 4 CAP– LV –5V 5 Vout SD 1 F EXT_AUD_L 4 Yb3 150pF 10k 2 Yb2 2.2k Zb 3 5 Yb1 680 1 Yb0 FB1 +5V 6 6.8nF 2.2k IC5b 5 2.2k 14 Ya1 680 12 Ya0 8 100 F 10k E S0 Vee Vss 7 8 TO CON3 PIN31 10 14 IC5d 12 TO CON3 PIN29 3 C 1 Q3 1 F 2 E B 150pF +3.3V Q4 TOSLINK OUT 100nF C 2.2k 2 +3.3V 1 IC7: 74HC14 3 +5V TO CON3 PIN21 TO CON3 PIN19 CON7 CON8 1 1 SCK 2 2 MISO 3 3 MOSI 4 4 5 5 COM2TX 6 6 COM2RX 7 7 COM3TX 8 8 COM3RX 5 9 IC7a IC7b IC7c IC7d 11 2 1 IC7e 220 10 5 1 F 9 100nF RIGHT CH AUDIO 1 2 100nF IC4SD 3 IC4DP 4 TO CON3 PIN37 IC4DN 5 TO CON3 PIN33 LEFT CH AUDIO TO CON3 PIN27 100nF 7 74HC4052/ DG4053E TO CON3 PIN25 EXPANSION HEADERS 8 100nF 16 Vdd Vdd RINP ROUTP RINN SD ROUTN UP DOWN IC4 PAM M8 84 407 07 LOUTP LINP LOUTN LINN GND GND GND 6 12 13 8 16 110 1 F AUX. 5V 6 15 SPEAKERS 4 14 10 7 B 14 1 BAV99, CM1213A C E MCP1700 3 1 2 GND OUT R– 2 L+ L– IRR1 2 AT25SF321 IN R+ 3 11 1 BC807, BC817 CON4 1 1 74HC14 S/PDIF OUT +5V + TO CON3 PIN23 CON1 100nF CON9 8 TX1 3 –5V 2.2k +3.3V 4.7 E 1k 2x 10k CON5 B 11 –5V EXT_AUD_L +5V D2 BAV99 13 9 4.7 2.2k 2.2k 6 HEADPHONES –5V Q1, Q3: BC817 Q2, Q4, Q5: BC807 FB2 S1 C 2.2k IC5c 10 Q5 C 100k Q2 2.2k 9 6.8nF 15pF 270k E B 150pF 10k Za 13 47k E 1 F 1M RIGHT LINE OUT B Q1 B 1 2 1k E C IC5: OPA1679IDR 11 Ya3 15 Ya2 3 1 IC5a 150pF 8.2pF D1 BAV99 4 3 IC6 74HC4052/ DG4052E EXT_AUD_R EXT_AUD_R 2.2k 7 2 8.2pF CON2b 47 16 Vdd LEFT LINE OUT 47 F +5V –5V 100 F CON2a 47 GND 3 10k TO CON7 PIN1 100k +3.3V +3.3V 8 LM2663 4 1 3 8 4 1 Be sure to read next month’s article on the DAB+/FM/AM Radio for construction details, as well as a special offer. We will be producing a limited run of radio PCBs with the tricky parts (IC1 and some associated components) pre-soldered, making the assembly substantially easier for you. siliconchip.com.au Australia’s electronics magazine January 2019  31 Specifications • Power supply: 5VDC (regulated) <at> 2A • AM tuning range: 520-1710kHz • FM tuning range: 76-108MHz • DAB+ tuning range: 168-240MHz (suits Australia, New Zealand and rest of world using DAB+ standard) • Line level outputs: 2 x 775mV RMS (~11dBm) • Headphone output power: ~20mW into 32, ~40mW into 16, ~80mW into 8 (can be increased) • Speaker output power: 1-2W (depending on speaker impedance and power supply) different techniques. So it isn’t as difficult as you might think But you will definitely have a better chance of success if you already have some SMD soldering experience. Since the key part is an SMD, and since the Explore 100 which we’re using to drive the radio also involves a few SMDs, we figured that the remainder of the parts might as well be surface-mounting types too. Actually, for the critical parts required by the Si4689 IC, we really don’t have a choice since through-hole parts would be too large to get close enough to the radio chip for good RF performance, and many of those parts would not be available in through-hole packages anyway. The good news is that where possible, we’ve used larger and easier-tosolder parts, meaning that once you’ve gotten the Si4689 and its surrounding components in place, the remainder of the board is not too difficult to assemble. We’ll give detailed instructions on how to successfully solder the tricky parts in this project in a future article. We are also planning to get the more difficult parts pre-soldered to a batch of PCBs and then make these available to our readers who would prefer to avoid the trickier parts of the build – more details next month! Circuit description The full circuit of the radio, except for the components mounted on the Explore 100 module, is shown in Fig.1. It’s based around IC1, a Si4689 digital radio receiver IC. 32 Silicon Chip The board containing all the components shown on Fig.1 piggybacks on the Explore 100 module and the two are connected via 2x20 header CON3. This carries both control signals from the Explore 100 and also power for the radio circuitry. IC1 requires relatively few components to operate and these can be broken down into a few categories: antennas and matching networks, a crystal oscillator, supply bypass capacitors, a serial flash chip used to store its firmware and audio filter circuitry. Antennas & matching networks AM signals are picked up either by an external loop antenna connected across terminal block CON6, or via an onboard ferrite rod antenna. The external antenna (if fitted) is connected in parallel with the ferrite rod via a small 1:6 turns ratio transformer wound on a ferrite core. This is necessary since the external antenna will typically have an inductance in the range of 10-20µH while IC1 expects an inductance in the range of 180-450µH, as is typical for a ferrite rod. We couldn’t find any source of prewound transformers but found it was quite easy to wind one using standard parts. The instructions for doing so will be in a subsequent article. Ideally, you should use an external antenna for AM since the ferrite rod, being relatively close to the digital circuitry, inevitably picks up some noise and will only work well if you have a strong signal. Transient voltage suppressors TVS1 & TVS2 are low-capacitance devices that do not affect the RF signal but will conduct to protect IC1 from electro-static discharge and lightninginduced energy. That is provided that the lightning strike is not too close; it certainly will not do much if there is a direct strike on the antenna! The Silicon Labs literature suggested using a single CM1213 dual diode clamp rather than TVS1 & TVS2 but we found that these reduced the received RF signal strength whereas the XGDseries polymer clamps do not. The AM antennas are both connected between the AM dedicated pins on IC1, LOOP_N and LOOP_P. FM and DAB+ reception use a different, VHF antenna. This is connected Australia’s electronics magazine via CON7, which can be either an SMA connector (as on our prototype) or a PAL connector, which is difficult to find these days, but we have a source. You can use an extendable whiptype antenna, a rooftop antenna, or any other antenna suitable for the relevant frequency range, ie, 88-206MHz. The same transient voltage suppressor device is fitted to CON7, again for ESD and lightning protection of the main chip. The recommended CM1213 had an even more drastic affect on FM/DAB+ signal strength so again, we have used a polymer clamp ,TVS3. The signal is fed into IC1 via a matching and tuning network (mostly as per the data sheet), to the VHFI pin on IC1 (pin 10). While developing this circuit, we ran into some differences between the recommendations in the SiLabs literature and their actual implementation of the circuit, in the form of the demonstration/development board. One of the differences is that the 2.7pF capacitor is recommended in the literature but not fitted on the demo board. We left it out of our final prototype, with no apparent ill effects. Hence the dotted connections shown in the circuit diagram. We suggest that constructors leave this part out, but we left its pads on the PCB in case it is needed. The VHFSW pin (pin 11) of IC1 is pulled to ground when the radio is in DAB+ mode. This connects 22nH inductor L1 in parallel with the 120nH inductor, re-tuning the matching network to better suit the higher DAB+ frequencies (203-206MHz), compared to FM (88-108MHz). All of the FM/DAB+ matching components are carefully chosen small SMDs placed close to IC1 and in a line between it and CON7. This minimises signal loss from parasitic effects such as PCB track capacitance and inductance. Crystal oscillator A high-precision 19.2MHz crystal is connected between pins 15 and 16 of IC1 and this is used both for tuning and to provide clock signals for the internal digital circuitry in IC1. The crystal we’re using has a specified load capacitance of 18pF but we are using two 12pF load capacitors, since IC1 also has software-programmable load siliconchip.com.au capacitance on those two pins. By using lower-than-specified value load capacitors, we were then able to program the tuning capacitors within IC1 to get the crystal frequency very close to nominal. Bypass capacitors IC1 has four supply pins: VIO, which defines the external I/O pin voltage levels, VCORE, which powers its digital circuitry, VMEM, which powers its internal memory and VA which powers its analog RF circuitry. All of these are designed to run at 1.8V but VIO can go as high as 3.3V. Since the Explore 100 has 3.3V I/Os, we decided to run VIO at 3.3V too, allowing the two chips to communicate without signal level translation. All four rails have three bypass capacitors each, ranging in value from 47pF to 4.7µF. The 47pF capacitors are physically smaller than the others and located right up near the IC. The reason for this is that low-value, physically small capacitors have a very high resonant frequency and keeping them close to the IC minimises the parasitic inductance of the tracks. Therefore, these small capacitors are very effective at bypassing very highfrequency signals, while the larger capacitors provide bulk bypassing at lower frequencies. The combination gives each supply rail a very low impedance from DC up to around 4GHz. This is important since IC1 contains a PLL (phase-locked loop) which includes a VCO (voltage-controlled oscillator) that runs at between 2.88GHz and 3.84GHz. Good bypassing on the supply pins is essential both for proper operation of the VCO and other internal circuitry, and to prevent this VCO from “leaking out” of the chip and being radiated into the surrounding environment (and possibly also interfering with radio reception). This is also why we have four ferrite beads in the circuit. FB1 and FB2 (along with the 8.2pF capacitors from the audio outputs to ground) shunt any VCO signals present at the audio outputs to ground, so that these signals cannot be radiated from the tracks and audio circuitry. Similarly, FB3 and FB4 prevent leakage of any high-frequency signals which may make their way back out of the supply pins from getting very far away from the IC, where the supply tracks may become antennas. Again, these ferrite beads have been carefully selected to be effective at suppressing the range of frequencies that we’re concerned about. Serial flash chip IC3 is a 32Mbit serial flash chip which runs from a 3.3V supply and can operate at up to 104MHz. IC1 requires a 512KB firmware image to be loaded into the chip for each operating mode, ie, AM, FM or DAB+. So we need to provide it with a minimum of 1.5MB (12Mbit) of firmware for the radio. This firmware can come from the Explore 100 but loading it this way is quite slow – it takes a few seconds. Since it’s annoying to have to wait several seconds to change radio modes, we instead use the Explore 100 to load the firmware into IC3 before the radio is first used. It’s read off the SD card and then fed to serial flash chip IC3 via a dedicated SPI interface on pins 8, 10, 12 and 14 of CON3. These are not connected to either of the Explore 100’s hardware SPI ports, so they are con- Simple l Economical I Great Performance Shockline 1-Port Vector Network Analyzer TM Simplify your testing while capitalizing on performance with a 1-Port USB VNA. The MS46121B Shockline VNA from Anritsu provides price, performance and space saving advantages when testing passive devices up to 6GHz. NOW AVAILABLE from $3,995 + GST# (laptop not included) # Exclusive special price for SILICON CHIP readers. Valid to Feb 28, 2019 Web: www.anritsu.com/en-AU/ Email: AU-sales<at>anritsu.com siliconchip.com.au Australia’s electronics magazine January 2019  33 trolled via software. We have written a CFUNCTION to communicate over those pins using SPI since it was too slow in MMBasic. While IC1 supports programming flash chips, its support is quite limited and IC3 has write-protect features which IC1 cannot handle. Hence, we have to program it separately in this manner. The SPI bus on the aforementioned pins of CON3 also connects directly to IC1, to pins 1, 2, 47 and 48, so it can read the firmware off the flash chip; hence we only drive those pins while IC1 is in reset, before it has started operating. Once the firmware is stored in IC3, IC1 can load it very quickly on request, so it takes less than one second to change radio modes. And unless you want to upgrade the firmware in future, you only need to load it into the flash chip once. The only extra component required for IC3 is its 1µF supply bypass capacitor. It has two extra control pins: WP (pin 3), which can be used to prevent modification of the contents of the flash, and HOLD (pin 7), which is used to pause SPI communications temporarily. We don’t really need these functions but the pins are connected to the Explore 100 header anyway (at pins 16 and 20 respectively). The Explore 100 can then set its digital outputs to a high level to disable these functions. This gives us the flexibility to modify the software to use them in future if it ever becomes necessary. After all, the Explore 100 has plenty of free I/O pins and it’s easier to program these pin states in software, rather than to tie them to GND or 3.3V and then have to re-make the board if we make a mistake Audio switching and filtering Analog audio from radio chip IC1 appears on pins 18 (left channel output) and 19 (right channel output). As recommended in the Silicon Labs literature, we have 8.2pF filter capacitors connected between these pins and ground, plus series ferrite beads close to the chip. This is necessary because signals from the high-frequency internal PLL may “bleed out” through these pins and radiate back to the antenna(s) and radio input circuitry. This filter34 Silicon Chip ing does not affect audio signals but eliminates any RF components which may be present. The Si4689 has internal volume control, so we don’t need to provide an external volume control for the line outputs or headphone amplifiers. The audio signals are AC-coupled via two 100µF electrolytic capacitors to remove the half-supply DC bias which is present, then fed to the input pins of IC6, a dual four-way analog multiplexer. It’s controlled by the Explore 100, via pins 29 and 31 at CON3. Its default state, set by the pulldown resistors on the S0 and S1 pins, is for the left and right channel audio sources to come from pins 1 and 12. These are connected to ground, so by default, the analog output is muted. The Explore 100 must drive one of the S0/S1 pins high for the audio from IC1 to be fed through, and if S1 is high, the left and right channels are swapped. If both S0 and S1 are driven high, the audio source instead comes from expansion header CON7. So if we later develop, say, an internet radio module that plugs into CON7/CON8, the Explore 100 can be programmed to feed its audio through to the outputs when it is activated. The audio signals which have been selected by IC6 are fed through to op amps IC5b and IC5c, which provide 23.35dB of gain (14.7 times) and also operate as third-order low-pass filters to remove any supersonic DAC switching artefacts from the audio signals. The gain is quite high because the audio signals eminating from IC1 are low in level – only about 50mV RMS. The filter’s -3dB point is 33.6kHz, resulting in a loss of less than 0.5dB at 20kHz, the upper threshold of human hearing. It’s a Butterworth type, for a flat passband, hence the minimal loss within the audible frequency range but it’s still good at eliminating supersonic signals. We’ve used a multiple-feedback type filter, rather than a Sallen-Key type because it is more effective at filtering out signal frequencies well above the bandwidth of the op amp being used. It is therefore more suitable for getting rid of the very highfrequency artefacts which are typical of delta-sigma type audio DACs. You can get frequency response, Bode plots and other data on this Australia’s electronics magazine type of filter at the following website: http://sim.okawa-denshi.jp/en/ OPtazyuLowkeisan.htm We’ve kept the impedances of the components in the filter relatively low, to reduce the chance of any digital interference being picked up there. Headphone drivers The filtered audio signals are fed to dual line output RCA socket CON2 via 47 isolating resistors, to prevent any capacitance on these outputs from destabilising the op amps. They’re also fed to the other two op amps in the quad package, IC5a and IC5d, via 2.2kresistors. These operate as headphone drivers, in conjunction with transistors Q1-Q4, which boost the output current capability. The outputs of IC5a and IC5d (pins 1 & 14) are connected to the headphone socket directly via 1kresistors, which helps to linearise the headphone amplifier, but these outputs also drive transistors Q1-Q4 via dual diodes D1 and D2. The purpose of these diodes is to bias the output transistors into conduction, so that there is always some current flowing through both (the quiescent current). In the case of D1, Q1 and Q2, current flows from the +5V rail, through a 2.2k resistor, then both diodes in D1, through another 2.2k resistor and then to the -5V rail. The current through these diodes is approximately 2mA, resulting in a forward voltage of around 600mV per diode, or 1.2V total. That 1.2V is also across the bases of Q1 and Q2, so they are biased with around 600mV each. That’s enough to bring them into conduction, resulting in a flow of around 5mA per transistor pair from the +5V to the -5V supply. Therefore, there is little to no “crossover distortion” as the audio signal crosses the 0V point. A 1µF capacitor between the transistor bases keeps this bias more constant as the output swings away from 0V, despite the changing current through the bias resistors. The headphones are driven through 4.7 resistors, again to isolate any capacitance at the output from the op amps. The values are lower because headphone impedances tend to be low, so higher value resistors would reduce the volume and also reduce siliconchip.com.au To whet your whistles, here is the completed DAB/FM/AM receiver prototype PCB shown very close to life size (actual board size is 135 x 84mm). For such a huge circuit diagram, there is virtually nothing on the rear side of the PCB except the Explore 100 connector socket seen top centre. the damping factor, leading to increased distortion. Like the audio filter, the headphone amplifier section is inverting. This means that the line outputs and headphone outputs are 180° out of phase but it’s difficult to think of any reason why that would cause any problems. 150pF capacitors connected across the 2.2kfeedback resistors help to stabilise the headphone drivers despite the extra phase shift from the buffer transistors. While the headphone driver section has no gain, most headphones/ earphones are sensitive enough that a few hundred millivolts is all that’s required. If you have headphones which need a much higher voltage swing, you could increase the 2.2k feedback resistor values to say 4.7k (to get 2.1 times gain) or to 10k(to get 4.5 times gain). We don’t suggest you go any higher than 10ksince maximum volume with 4.5 times gain would be approaching the maximum swing of around ±4V (2.8V RMS) that the circuit is capable of. Headphone plug insertion detection The extra components connected to the headphone socket, including Q5, are to detect when headphones/ earphones are plugged into the socket, so that the speaker outputs will be automatically muted. These are necessary because the connector only switches the signal pins when a plug is inserted, so we need to be a bit tricky to sense the plug siliconchip.com.au insertion. When a plug is absent, the switched pin is connected to the ring, which carries the right channel audio. Since the audio signal voltage is normally well below the 5V supply, that means that current can flow from the base of PNP transistor Q5 and through the 270k resistor to the switch contact and so transistor Q5 is switched on. Current can therefore flow from the +5V supply, into its emitter and out of its collector, and through a 47k/100k voltage divider, producing a ~3.3V signal. This goes to pin 39 on CON3 (“HPSW”), and on to the Explore 100, to be interpreted as a high level, indicating that the headphones are not plugged in. When headphones are plugged in, this connection is broken and so Q5’s base is pulled up to the +5V supply by the 1M resistor, switching Q5 off. The HPSW signal is pulled down to 0V by the 100k resistor, and this is sensed as a low level by the Explore 100. The 15pF capacitor prevents RF or EMI pick-up across the 1M baseemitter resistor from switching Q5 on in this condition. The reason for the use of relatively high resistor values is to prevent this circuit from loading the headphone amplifier and introducing distortion. Audio amplifier The same filtered audio signals that are fed to the line outputs and Australia’s electronics magazine headphone amplifier also go to power amplifier IC4. This device (PAM8407) runs off 5V and can deliver about 2W to a pair of 4speakers, or about 1W to 8speakers, with reasonably low distortion (below 0.1%). It also has an internal digital volume control, activated via pulses delivered to pins 4 (UP) and 5 (DOWN), plus a shutdown pin at pin 3 which allows the amplifier to be switched off, saving power and muting the speakers. This is activated when headphones are inserted, both to save power and to provide the required muting. This chip was chosen because it is delightfully simple but can deliver reasonable power and without the hassles of a Class-D amplifier (eg, the risk of EMI affecting the radio receiver). It has differential inputs but the inverting inputs are simply terminated to ground with the same value (100nF) coupling capacitors that are used to feed audio to the non-inverting inputs. Its volume/gain is programmable in 32 steps from -80dB to +24dB. This is controlled via pulses fed into pins 3 and 4 from the Explore 100 via CON3 pins 32 and 34, while the shutdown function goes to pin 30 of CON3. It defaults to 12dB gain on power-up or after the chip it shut down. At power-up or when the headphones are unplugged, the software January 2019  35 divider and 100nF AC-coupling capacitor, to remove the DC bias from the signal. This arrangement is used to obtain the correct output voltage (about 0.5V peak-to-peak) and source impedance (about 75) to suit the S/PDIF standard. A 75 coaxial cable can be wired from CON1 to the S/PDIF input on a DAC or home theatre receiver, which will internally terminate the signal with a 75 load. You need to keep that in mind when calculating the voltage at CON1. This photo shows how the PCBs are “stacked” within the clear acrylic case, designed to suit the DAB+/FM/AM Radio. The radio board is at the bottom of the stack, upside down. It plugs into the Explore 100 control board, with the LCD touch screen at the top. Construction details, along with the parts list, will be presented next month. running on the Explore 100 mutes the audio (using IC6) and then sends however many up/down pulses are required to set IC4’s volume to the desired level, before unmuting the audio. There are two advantages to IC4 having its own volume control, separate from IC1. One is that it makes it easy for the user to choose comfortable volume levels for both the headphones and speakers. As soon as you unplug the headphones, IC4’s volume is set to the desired speaker volume level before audio is applied to the speakers. And when you plug the headphones back in, the volume on the socket is already suitable for headphone listening; IC4 is simply shut down as soon as the software notices that the headphones have been plugged back in. The second advantage is that IC4 provides enough gain to get plenty of volume from the speakers (assuming they have reasonable efficiency) even if the audio signals from IC1 are relatively quiet. However, there is a limit to how low you can set the headphone volume before the maximum speaker volume is reduced. So it’s a bit of a balancing act. We didn’t fit any speakers in the Radio itself, to keep it compact; instead, it has a 4-way pluggable terminal block (CON4), which is externally accessible, to make it easy to wire speakers up and plug/unplug them as required. Digital audio interface In some modes, the Si4689 can be programmed to produce I2S format digital audio data as well as analog audio. This digital data appears in se36 Silicon Chip rial format at the DOUT pin (33), which is fed to digital audio transceiver IC2, along with the corresponding clock signal (DCLK, pin 27) and framing signal (DFS, pin 28). IC2 is a Wolfson WM8804 which converts between I2S and S/PDIF formats. It’s controlled by the Explore 100 over the same SPI bus as the Si4689, except that it has a separate chip select line (CSB, pin 5) which is driven from I/O pin 40 on CON3. IC2 also has a reset pin, pin 6, which is driven from pin 36 of CON3, plus a mode control pin (SWIFMODE, pin 2), wired to pin 38 of CON3. The Explore 100 sets up the interface mode pin during its start-up sequence, then releases reset and sends SPI signals to IC2 to configure it to operate as an I2S to S/PDIF translator. IC2 uses a separate 12MHz crystal for its timebase. It can use a variety of frequencies but we decided to use the frequency suggested in the data sheet. When valid I2S data is being produced by IC1, an S/PDIF data stream appears at pin 17 of IC2 (TXO) and this is buffered by schmitt trigger inverter IC7f. Its output pin 12 then feeds the signal to the input of another inverter, IC7e (pin 11) and its output feeds the input of TOSLINK transmitter TX1. This provides the optical digital audio output. The same signal from IC7f is also inverted by the four remaining inverters in the hex package, IC7a-d, which are wired in parallel to increase drive strength. That’s because these inverters drive the S/PDIF coaxial output, CON1, via a 220/110 resistive Australia’s electronics magazine Infrared receiver All of the Radio’s functions are controllable using the Explore 100’s 5-inch colour touchscreen, but in case that isn’t enough, we’ve also included an infrared remote control receiver, IRR1. It’s powered from a filtered 3.3V supply rail and its output signals are fed to pin 7 of CON3, the designated infrared receiver input pin of the Explore 100. MMBasic can therefore receive and decode standard infrared protocol commands and pass them on to the BASIC software. As a result, you can use a universal remote to change modes, channels, input frequencies, adjust the volume, mute, switch the radio into and out of standby mode and various other functions. The full list of remote control commands will be described in a later article. Power supply The radio runs off a regulated 5V DC supply which is fed into the Explore 100 board, and then onto the radio board via pin 3 of CON3. This powers the audio amplifier (IC4) directly, as well as op amp IC5 and the headphone amplifier transistors. As mentioned earlier, the Si4689 radio IC (IC1) has four supply pins: VI/O, VMEM, VCORE and VA. For compatibility with the Explore 100, we are using 3.3V for VI/O, which is drawn from pin 5 on CON3 after passing through a ferrite bead to prevent EMI from radiating back from the radio chip into the Explore 100 board. VMEM and VCORE are powered from one 1.8V rail, derived from the 3.3V supply by REG1, a 1.8V low-dropout linear regulator. VA is used to power IC1’s internal PLL and its stability is critical, so this supply is fed from a separate but identical regulator, REG2. siliconchip.com.au All three supplies are extensively decoupled, as recommended by Silicon Labs. A switched capacitor inverter IC (REG4, LM2663) generates a -5V rail from the +5V supply, both of which are fed to the op amps and headphone amplifier. This has three benefits over using a single-ended 5V supply: One, it provides more than double the potential signal swing for driving headphones; Two, it allows us to use op amps with lower distortion and noise; and Three, it means we don’t need to apply a DC bias to the various audio signals in our analog circuitry, eliminating the possibility of supply noise injection. REG4 has a shutdown pin (SD) which is wired to pin 35 of CON3; however, REG4 needs to be kept powered most of the time to prevent DC voltages from appearing at the analog audio outputs. But the SD pin is used, because we found that if REG4 was left enabled at power-up, it could “latch up”, drawing a lot of current without actually producing a -5V output. The solution is to tie the SD pin high, to +5V, via a 100k resistor so that REG4 is shut down initially. Then, after the Explore 100 “boots up”, we wait a short period for the 5V rail to stabilise before activating REG4. That solves the latch-up problem. Like IC1, digital audio transceiver IC2 also has an internal PLL and this is powered from the PVdd pin, but it must be the same voltage as the DVdd pin, which is 3.3V in this case. For stability, we have isolated the two supplies using a low-value inductor and this forms a low-pass filter with the 10µF bypass capacitor, helping to smooth out the PLL operation. Expansion headers We briefly mentioned CON7 and CON8 earlier. We thought that at a later date, it might be a good idea to add internet radio capability using an ESP-01 (or similar) WiFi module and some extra processing circuitry. CON7 and CON8 are provided for such a board to plug into. Power is supplied to the add-on module in the form of 5V DC between pins 1 and 8 of CON7, and 3.3V DC at pin 2 of CON7. The Explore 100 can control the ESP-01 and any other devices on the add-on board using two bidirectional serial ports (pin 5-8 of CON8) and/or SPI (pins 1-3 of CON8, with pin 4 of CON8 or pins 3/4 of CON7 used for chip select). Audio can be fed back into the radio board via pins 6 and 7 of CON7, which are connected to the spare inputs of multiplexer IC6, with a signal ground on pin 5. Finally, pins 3 and 4 of CON7 and pin 4 of CON8 provides some generalpurpose digital I/O lines which may be used to control aspects of the addon board (eg, power up and down); as mentioned earlier, one or more of these may also be used as chip select lines for the SPI interface. Coming next month That’s all we have room for this month. Next month we will present the parts list and PCB overlay diagram, show more photos of the final prototype PCB and describe in detail how to assemble the board. A third article will then describe the software operation, final assembly and how to use the radio. SC AUSTRALIA’S OWN MICROMITE TOUCHSCREEN BACKPACK Since its introduction in February 2016, Geoff Graham’s mighty Micromite BackPack has proved to be one of the most versatile, most economical and easiest-to-use systems available – not only here in Australia but around the world! Now there’s the V2 BackPack – it can be plugged straight into a computer USB for easy programming or re-programming – YES, you can use the Micromite over and over again, for published projects, or for you to develop your own masterpiece! The Micromite’s colour touchscreen BackPack can be programmed for any of the following SILICON CHIP projects: Many of the HARD-TOGET PARTS for these projects are available from the SILICON CHIP Online Shop (siliconchip .com.au/ shop) GPS-Synched Frequency Reference (Oct18 – siliconchip.com.au/Series/326) FREE Tariff Super Clock (Jul18 – siliconchip.com.au/Article11137) PROGRAMM ING Altimeter & Weather Station (Dec17 – siliconchip.com.au/Article/10898) Buy either tell us whichV1 or V2 BackPack, Radio IF Alignment (Sep17– siliconchip.com.au/Article/10799) pr oj ec t yo u for and we’ll program it fowant it Deluxe eFuse (Jul17 – siliconchip.com.au/Series/315) r you, FR EE O F CHARGE! DDS Signal Generator (Apr17 – siliconchip.com.au/Article/10616) Voltage/Current Reference (Oct16 – siliconchip.com.au/Series/305) Energy Meter (Aug16 – siliconchip.com.au/Series/302) Micromite Super Clock (Jul16 – siliconchip.com.au/Article/9887) V2 BackPack: Boat Computer (Apr16 – siliconchip.com.au/Article/9977) * Ultrasonic Parking Assistant (Mar16 – siliconchip.com.au/Article/9848) JUST $7000 See May 2017 (Article 10652) P&P: Flat $10 PER ORDER (within Australia) *Price is for the Micromite BackPack only; not for the projects listed. siliconchip.com.au Australia’s electronics magazine January 2019  37