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Single-Chip Silicon Labs FM/AM/SW Digital Radio Receiver By Charles Kosina The ultimate in FM/AM radio reception technology is the single-chip solution. All you have to do is connect some antennas to pins on an IC, send it some serial commands, and stereo audio comes out the other end. As a result, these Silicon Labs chips make building a capable radio receiver a doddle. It’s straightforward to set up and use, fits in a compact case and runs from a simple AC plugpack. I was happy with my AM/FM/SW Receiver design from the December 2021 issue, at least in terms of how easy it is to build, ease of use and coverage of multiple radio bands. But I still felt that its overall performance left a little to be desired. I was also not happy that I didn’t have enough information for full digital control of the BK1198 radio chip. While that radio design was relatively straightforward as radios go, it would have been a lot simpler if I could have gotten the digital control working. In the last few years, several new chips have appeared that greatly ease radio receiver design. Many of these are from Silicon Labs – the offer about 34 varieties of chips in the Si473x family, and you can download the main data sheet from their website: https://bit.ly/pe-jul22-skyw They have a similar architecture to the BK1198 chip I used for the December 2021 design. One major advantage of the Silicon Labs chips is the documentation; whereas information on the BK1198 is sparse, to say the least, the application note for the SiLabs chips runs to 321 pages! (Check it out at: https://bit.ly/pe-jul22-sky2). The board that I have laid out here is suitable for a prebuilt module with the Si4730 chip, or a standalone 16 Si4732 chip. Both are available on AliExpress at quite low prices. The Si4730 only handles the standard AM and FM bands, whereas the Si4732 can be programmed to cover longwave and shortwave. Both can decode FM stereo. The specifications give the bands shown below. But what about the gaps between the bands? I decided to experiment and set frequencies in these gaps. And what a surprise; with the Si4732 chip, I could select any frequency from 153kHz up to 30MHz by sending the appropriate code to the chip. No gaps! Whether there is anything of interest in the gaps is another matter. Thus, I have the AM band Worldwide FM band support ............64–108MHz Worldwide AM band support ...........520–1710kHz programmed from 153kHz to 1730kHz, and the SW SW band support (Si4734/32/35) ....2.3–26.1MHz band from 2MHz to 30MHz. LW band support (Si4734/32/35) .....153–279kHz Fig.1: the radio’s sensitivity across a widened AM band, from 153kHz to 1.7MHz. Except for a dip around 445-455kHz (typical intermediate frequencies), the result is pretty flat. Across the standard AM broadcast band of 550-1720kHz, there is only about 4dB variation. Practical Electronics | July | 2022 These two photos show that the topside of the PCB for the Si4730-based version (top) of this project is barely different from the Si4732 version (bottom). Ignore the additional screws/nuts as those are just for mounting the screen. Performance On the FM band, a short piece of wire inside the box will bring in most of my local (Melbourne, Australia) stations with a good SNR. With an outdoor long-wire antenna connected directly to the AM antenna input, I could get many stations with an SNR of 25dB or better without any ferrite rod. This way, there is not a single inductor required in the circuit. Using a ferrite rod, the weaker stations came through, but there was a lot of hash caused by all the electronics in my lab. I made a plot of sensitivity on the AM band from 153kHz to 1700kHz, shown in Fig.1. Note the sharp dip at 450kHz. I have no idea why this is, but it is near the intermediate frequency of most superhet receivers, so it is of no consequence. On shortwave, the sensitivity is comparable to the AM band (see Fig.2). This is not brilliant, but adequate. There were a few ‘birdies’ on some frequencies – eg, 8MHz, 14MHz and 16MHz – which made SNR measurement difficult. Above 22MHz, the SNR display did not seem to give sensible readings, although performance up to 30MHz seemed the same as at 20MHz. The audio drive capability of the SiLabs chips is not stated in the data sheets. I determined experimentally that the minimum load resistance on the headphone output is 1.6kΩ. Any less, and clipping will occur. Fig.2: a similar ‘frequency response’ plot for the SW range from 2MHz to 22.3MHz. Practical Electronics | July | 2022 The maximum output with this load is 250mV peak-to-peak, or about 88mV RMS for a sinewave, giving less than 1mW. It still works with low-impedance headphones, although at maximum volume there will be some distortion. Sennheiser 60Ω headphones gave an acceptable listening level in a quiet environment. Panasonic noise-reducing headphones with a 330Ω input resistance (with the noise reduction turned on) gave a considerably higher sound level. Feeding the signal into external amplified speakers gave good-quality sound. Because of this weak output, I have added an op amp buffer that provides drive capability for low-impedance headphones, while also providing enough voltage swing for insensitive high-impedance ’phones. This is also useful if you are feeding the audio to a preamp or amplifier, as the signal is closer to ‘line level’. When the tuning knob is rotated, each pulse from the shaft encoder sends out six bytes via I2C and then receives seven bytes of status. This takes a significant time, so if you spin the tuning knob too rapidly, the encoder pulses are missed, and you only get a small frequency change. Just slow down the rotation. Circuit description The full schematic is shown in Fig.3. The Si4730 module includes the 32.768kHz crystal and associated capacitors. The FM antenna is connected to the module’s FM input via a 1nF capacitor, while the AM band requires a ferrite rod, typically 400μH. An optional 10nF capacitor joins the two antenna inputs, allowing a single length of wire to provide both FM and AM reception in metropolitan areas. The SEN line is tied high internally on the Si4730 module. The audio output is coupled to header CON4. The drive strength from the radio chip itself is just adequate to drive 60Ω headphones; as hinted above, depending on the ‘phones, the volume level can be a bit low, and distortion can be higher than we’d like. The dual op amp (IC3) in the final version is not present in the prototypes shown. This gives a voltage gain of 4 and low-impedance output, enough to drive just about any headphones or earphones to a decent volume level (even insensitive types), and possibly even very efficient unpowered speakers. Alternatively, an external audio amplifier, such as PC-style speakers can be used, with or without the op amp. If you don’t need the op amp, you can simply bridge pin pairs 1/3 and 5/7, to feed the radio chip’s output to CON4. 17 You will note that the I2C bus is made externally accessible via CON8, together with the +5V supply. This could be useful in future for expansion, or as a debugging aid. The power supply may be 9V AC or 9-12V DC via CON1. If a DC supply is used, it must not be a switching type – its hash can wipe out the AM band. A 7805 regulator supplies the ATmega chip and the LCD module, while a small TO-92 linear regulator provides 3.3V for the SiLabs chip. The Si4732 version differs due to the installation of two 22pF capacitors, a crystal (X2) and the chip itself on the underside of the PCB. CON4 also has +5V and GND pins. This supply might be used for a small amplifier module mounted in the same case, to drive 8Ω speakers. I don’t recommend Class-D amplifiers because they could generate hash which will interfere with radio reception, much the same as a switching regulator. Control is via a standard I2C serial bus and a reset line. I have specified a 32KB ATmega328P chip in a DIL package, although I used the 16KB ATMega168 in my prototype; the program only occupies 68% of its 16KB of Flash, and I have heaps of these chips left over from a previous project. Besides the Flash size, they are essentially identical. The display is a standard 16x2 alphanumeric LCD module. There is provision for an external crystal for the ATmega chip, but I found the internal 8MHz RC oscillator quite adequate. The processor runs from 5V, whereas the SiLabs chip requires 3.3V. This is not a problem for the I2C interface, as the output is open-drain, and the 15kΩ pull-up resistors go to 3.3V. There are also two 1kΩ series current-limiting resistors between the I2C outputs of the micro and the radio module’s inputs as a precaution against incorrect programming of the I2C pins. The typical value of an I2C pull-up resistor is 4.7kΩ, but the SCL and SDA pins on the SiLabs chip have limited drive capabilities. Operation with 4.7kΩ pull-ups could be marginal, especially given the 1kΩ series protection resistors. Hence the use of 15kΩ pull-ups; lower values would give a marginal low voltage with either pin when pulled externally low, via those 1kΩ resistors. I have not found any problems with these higher-value pull-up resistors (eg, sensitivity to EMI). Tuning is by a standard shaft encoder with a pushbutton switch (RE1). The switch cycles through different step sizes on the bands. The 18 external band switch, S3, toggles between AM and FM modes. I used an ON-OFF-ON type switch to provide for three bands. This gives three different voltages which can be read by the analogue-to-digital converter (ADC) input on the ATmega, PC3 (pin 26). If the Si4730 module is used, there is no SW band, so you should use a two-position switch instead. Another ADC input, PC0 (pin 23), monitors the voltage at the wiper of potentiometer VR2 which sets the volume. The reading is scaled and sent via the I2C lines to control the volume of the SiLabs chip. A third ADC input at PC1 (pin 24) reads the position of potentiometer VR3; the reading is scaled and sent to the SiLabs chip to adjust the bandwidth on the AM band. I could have used a multiple position switch, but this is a simpler and cheaper option. The bandwidths that can be selected are 1.0, 1.8, 2.0, 2.5, 3.0, 4.0 and 6.0kHz. The potentiometer that I have used has a centre detent which gives a 2.5kHz bandwidth, but this is optional. There is no bandwidth option for FM. Using the Si4732 chip For those who wish to include SW or LW bands, you can use the Si4732 chip instead of the Si4730 module. This comes in an SOIC SMD package, which is not difficult to solder. There are only slight changes to the circuit, as shown in Fig.4. The SENB pin goes to ground on the Si4732, which gives it a different I2C address to the Si4730. It requires an additional crystal and three capacitors. The Si4730 module I2C addresses are C6 hex for writing, and C7 for reading. With the Si4732 chip, the corresponding addresses are 22 and 23 hex. Don’t load both the Si4730 module and Si4732 chip. Although they have different I2C addresses, the loading on the RF inputs is such that it severely degrades sensitivity. Debugging interface MOSFETs Q1 and Q2 provide a serial debugging interface. This was invaluable for debugging purposes, but not required if you just want to use the radio. It is set up for 38,400bps, eight data bits, one stop bit and no parity. Microcontroller IC2 is programmed via the standard 6-pin header, CON9. A pushbutton switch is provided to reset IC2. Component Selection While I try to make sure that components can be sourced locally, it is not always possible. In this case, several major components have to be sourced from international suppliers. There are a few suppliers of the Si4730-V2.0 module on AliExpress that sell it from £5 to £12. Make sure it’s the version with six connections on each side of the board. There are some with only five connections that will not fit. As with most orders from China, be prepared for a fairly long delivery time. The Si4732 chip is manufactured in the SOIC-16 package. It is available on AliExpress, and also Digi-Key and Mouser, where you can order it along with other parts. Apart from the 1000μF electrolytic and the 2W resistor, all other resistors and capacitors are either 1206 or 0805 (imperial) size SMDs, and there are no fine-lead-pitch devices to worry about. There are various colours of backlighting for the LCD module. We much prefer the white-on-blue version to the old-fashioned yellow/green variety. This type is available from several suppliers on eBay. But if you don’t mind waiting, the LCD module can cost as little as about £1.50 from China. Since we’re using the parallel interface, you won’t need the I2C serial interface board supplied with some of them. The LCD is mounted off the main PCB with standoffs, and connected using the supplied standard header plugging into a low-profile PCB-mounting socket strip. The LCD height above the board means that the two pots and rotary encoder need 25-30mm-long shafts. The parts list shows suggested components. Practical Electronics | July | 2022 Digital AM/FM/SW Radio Receiver Fig.3: there isn’t a lot to the radio circuit thanks to the Si4730 radio module. The antennas at left are simply coupled to the module using capacitors, while the audio outputs on the right-hand side feed into a pair of op amp buffer/gain stages, which are better at driving headphones than the module by itself. IC2 controls the radio over an I2C serial bus while monitoring user input via rotary encoder RE1, and displaying tuning and signal strength information on a two-line LCD. Construction A word of caution. The crystal on the tiny SI4730 module is not firmly attached and can be easily bent to Practical Electronics | July | 2022 one side and damage the board. I can vouch for that from experience! I recommend a spot of superglue to attach it firmly to the board. You might want to consider ordering two of these modules to be on the safe side. The main circuit board (coded CSE210301C) is double-sided with 19 ► Fig.4(a): if you want SW ► reception, all you have to do is leave off the Si4730 module (MOD1) and instead fit IC1, its 100nF supply bypass capacitor, crystal X2 and its two 22pF load capacitors. All the other components shown here were in the original circuit (Fig.3) and are only duplicated to clarify how IC1 is connected to the rest of the circuit. Fig.4(b): how the panel-mount jack socket is wired to CON4. Check your socket’s pinout to determine the tip (T), ring (R) and sleeve (S) connections. components on both sides. It measures 123 x 49.5mm and is available from the PE PCB Service. Both Receiver versions use the same PCB; either you mount the Si4730 module on one side, or the Si4732 chip on the other. Refer to overlay diagrams Figs.5 and 6, and ensure that you either fit the module as shown in Fig.5, or the components in the red oval in Fig.6 – not both. Start by mounting the 16-pin chip. This is the SOIC-16 type with pins spaced widely enough that they can be soldered individually using a finetipped iron. First, apply some flux paste to the pads to reduce the risk of bridging between pins. If bridges do form during soldering, use more flux paste and some solder wick to remove it. Next, fit the SMD capacitors on the underside of the board. Note that the two 22pF capacitors (values in parentheses) are only needed if you wish to use a crystal oscillator for the ATmega168/ATmega328 chip. If not, we suggest you leave them off. The other side of the board has most of the components. Install the remaining surface-mount parts next. If you are using the Si4730 module, make sure it’s placed accurately. It needs plenty of solder flowing into the ‘half holes’ either side of the module PCB. Ensure that the 10μF and 100μF tantalum capacitors are placed with the correct polarity. The striped end is positive, so face the striped ends towards the ‘+’ symbols on the PCB. Then add the through-hole components, possibly including the optional 8MHz crystal. There is also provision for an SMA socket, CON6, although I didn’t use it. This is an alternative input for the AM, LW and SW bands. Ideally, I prefer the LCD module to be removable; hence, I plugged it into a socket strip. The matching headers are not that easy to find, but the parts list mentions suppliers. The LCD is then attached using 9mm untapped spacers (Jaycar HP0862 or Altronics H21362) and M2.5 x 15mm screws and nuts. The last components to attach are the two potentiometers (VR2 and VR3) and rotary encoder RE1 on the LCD side. Finally, give the board a good wash on both sides with circuit board cleaner. Figs.5 and 6: most of the components mount on the top side of the PCB; apart from a few SMDs, the only parts on the bottom are the two pots, the rotary encoder and crystal X2 (if IC1 is fitted). It’s best to fit all the SMDs on the underside, then the SMDs on the top, then the through-hole parts on the top, then the underside. Ensure the polarised parts like the radio module, all the ICs, the aluminium and tantalum electrolytic capacitors, bridge rectifier BR1, diode D1 and trimpot VR1 are orientated as shown. Errata: if using the specified part, REG2 should be mounted upside down relative to the overlay. Otherwise you can mount it on the underside of the PCB, ensuring it doesn’t foul the front panel. This is due to the input and output pins being swapped on the PCB footprint. 20 Practical Electronics | July | 2022 Figs.8 and 9: (shown 85% full size) these panel labels are available to download from the July 2022 page of the PE website, so you can print them, cut them out and attach them to the inside (or outside) of the box lid. Preparing the enclosure I encased the radio prototypes in a Hammond RP1175C box, which has a clear lid. This avoids having to make a rectangular cutout for the LCD, so you can drill all the holes. It is available from Mouser, Digi-Key, Rapid and others. You could use a larger case that’s locally available, but that would make the radio a bit less convenient to use. This is how I wired up the prototype Si4730-based radio. Practical Electronics | July | 2022 You can place the power input connector, headphone jack and BNC antenna connector on any convenient surface. I chose the righthand side of the box. The headphone jack presents something of a problem. The case thickness is too much for easily obtainable 3.5mm stereo jacks. The simplest solution is to use a 6.35mm jack, and if necessary, a 3.5mm adaptor (eg, Jaycar PA3590). The drilling details are shown in Fig.7; use this as an initial template to locate the circuit board mounting holes (D) and the toggle switch holes (B). As accuracy is required, the blank circuit board can then be used as a template for drilling the mounting holes. Use a countersinking tool so that the screw heads will be flush with the front panel. You will note that there is a small hole in the centre of the encoder and two potentiometers. Once the four mounting holes (D) are drilled, attach the board to the panel with 3mm screws and drill 1mm holes through the centre of the two potentiometers and encoder positions, to accurately mark the centres of the 8mm holes (A). I printed the 139 x 76mm front panel label on heavy photographic paper, and it fits neatly in the slot on the transparent panel. Fig.8 is the panel label for the Si4730 module-based version, while Fig.9 shows the label for the Si4732-based version. The only difference is in the labelling for the band change switch, adding the SW option for the Si4732 chip. You can download these labels from the July 2022 page of the PE website and print them out. Use a sharp blade to cut out the slot for the LCD and the five holes for potentiometers, encoder and switches, then cut out the panel and slot it into the inside of the clear lid. It should be a neat fit. Attach the circuit board to the back of the front panel using 12mm-long M3 countersunk head screws at the front and M3 x 6mm screws at the back. 18mm-long spacers are needed, which can be made from a 12mm threaded spacer plus an untapped 6mm spacer stacked. You can use other spacer combinations to give the required 18mm. The potentiometer and encoder shafts are 6mm in diameter. Be careful if you are using metric knobs, as some might not be suitable for the shafts. Choose the types with a grub screw 21 Fig.7: if you use a box with a clear lid, then you only have to drill round holes, as shown here. You can stick masking tape on your panel, measure and mark the hole dimensions, or simply copy/print this diagram, cut it out and use it as a template. For the neatest result, countersink the holes marked D on the outside of the panel. since these will fit a wide variety of shaft types. There remains the internal wiring to the various switches and connectors on the enclosure. This is relatively straightforward, and shown in the photographs (refer to Figs.3-6). Programming the micro I wrote the control software using BASCOM, a BASIC compiler for AVR micros. Having the application and programming notes provided by SiLabs made the code fairly straightforward. Both the .BAS source code and .HEX firmware file are available for download from the July 2022 page of the PE website. Note that you might need a paid version of BASCOM to compile the .BAS file – see the box below. The program header on the board is designed for an AVRISP Mk2 programmer. This can be used in conjunction with the free Atmel (now Microchip) Studio program available for download from: www.microchip.com Control of the SiLabs chip is via I2C serial commands, and believe me there are heaps of them. There are all sorts of features, such as scanning, that could be incorporated into the design, but I decided to ‘keep it simple, stupid’ (KISS). Others might wish to expand on what I have done. As mentioned above, the pushbutton switch integrated into the tuning encoder toggles through steps to allow fine selection or quick tuning across the band. On the AM band, the step is 1kHz, 9kHz or 100kHz. The FM band is 87MHz to 108MHz and has a step of 100kHz or 1MHz. On the SW band (if used), the step is 1kHz, 10kHz, 100kHz or 1MHz. About half a second after a frequency is selected, it and the step size are stored in EEPROM. This means that on the next power-up, the EEPROM values are read and that frequency selected. The top line of the 16x2 LCD shows the frequency, and on the AM and SW bands, it also shows the bandwidth. The second line shows the step size and the signal-to-noise ratio (SNR). The Si chip is sampled once a second to update the SNR figure. However, the Si4730 module does not give SNR readings on the FM band. Weaker signals give mono rather than stereo output as expected. Radio source code Similarly, an example of the wiring for the Si4732 version of this project. 22 We will make the source code available for this project, along with the HEX file. The firmware was written in BASCOMAVR, a version of the BASIC language that compiles to native Atmel AVR code. So it is quite easy to modify. BASCOM is commercial software; there is a free demo version available which can produce binaries up to 4KB in size, but the radio software is larger than that. A full license for the software costs around £100 (it’s available from a few different online shops). But, do note this is only relevant for those who wish to edit the software. You do not need it to load the HEX file into the chip. There are plenty of free software packages that can do that, like MPLAB IPE and AVR Studio. Practical Electronics | July | 2022 Parts List – Silicon Labs AM/FM/SW Radio 1 double-sided PCB coded CSE210301C, 123 x 49.5mm available from the PE PCB Service 1 9V AC plugpack with 2.1/2.5mm ID barrel plug 1 plastic box with clear lid [eg, Altronics H0326, Hammond RP1175C: Digi-Key; Mouser] 1 panel label, to suit version being built 1 16x2 alphanumeric LCD module with blue backlight (LCD1) 1 28-pin narrow DIL IC socket 3 2-pin polarised headers with matching plugs and pins (CON1-3) [Jaycar HM3412/02, Altronics P5492/72 + 2x P5470A] 1 5-pin polarised header with matching plugs and pins (CON4) [Jaycar HM3415/05, Altronics P5495/75 + 5x P5470A] 2 3-pin polarised headers with matching plugs and pins (CON5, CON7) [Jaycar HM3413/03, Altronics P5493/73 + 3x P5470A] 1 4-pin polarised headers with matching plugs / pins (CON8; optional) [Jaycar HM3414/04, Altronics P5494/74] 1 panel-mount BNC socket [Jaycar PS0658, Altronics P0516A] 1 PCB-mount DC barrel socket, 2.1/2.5mm ID, to suit plugpack [eg, Jaycar PS0522/4, Altronics P0620/1A] 1 panel-mount stereo 6.35mm jack socket [eg, Jaycar PS0182, Altronics P0065] 1 16 pi lo pro le ac i e pi ea er trip it matching socket strip (for LCD) * 1 10kΩ multi-turn trimpot (VR1) 2 10kΩ 9mm vertical potentiometers with D-shafts (VR2, VR3) [eg, Bourns PTV09A-4030F-B103-ND; or use Altronics R1946 with a fluted shaft] 1 vertical rotary encoder with D-shaft and integrated pushbutton switch (RE1) [eg, Bourns PEC11R-4225F-S0024] 3 small or medium-size knobs to suit VR2, VR3 and RE1 1 PCB-mounting small tactile pushbutton switch (S1) [eg, Jaycar SP0601 or Altronics S1120] 1 SPDT miniature toggle switch with solder tags (S2) [eg, Jaycar ST0335] 1 00μ errite ro a te a 1 [eg, Jaycar LF1020] 4 9mm untapped spacers (for LCD mounting) [Jaycar HP0862, Altronics H1362] 4 9-10mm-long M3 panhead machine screws and nuts (for REG1) 4 12mm-long M3 countersunk head machine screws 4 6mm-long M3 panhead machine screws 4 12mm-long M3 tapped spacers 4 6mm-long untapped spacers, 3.25mm inner diameter 4 15mm-long M2.5 panhead machine screws and nuts (for LCD mounting) various lengths of medium-duty hookup wire various short lengths of heatshrink tubing to suit wire size * some options include Semtronics SBU400Z (header) + MH1S19-140 (socket), Mouser 200-BBL116GF (header) + Mouser 200-SL116T10 (socket), element14 1667454 (header) + Jaycar PI6470 (socket) or Altronics P5400 (socket) Initial setup I did not want to have a separate control program for the Si4730 and Si4732 chips, so the chip type is automatically identified on power up. You don’t need to do anything. When I built a second unit, I discovered that the tuning was backwards. Clockwise decreased the frequency! It appears that shaft encoders differ. So I came up with a method to select the correct tuning direction using the Practical Electronics | July | 2022 Semiconductors 1 ATmega168 or ATmega328 8-bit microcontroller pro ra e it 10 01 1 5V rail-to-rail op amp, SOIC-8 (IC3) [eg, LME49721, available from Digi-Key, Mouser, eBay, AliExpress] 1 7805 5V 1A linear regulator, TO-220 (REG1) 1 LM2936-3.3 3.3V low-dropout linear regulator, TO-92 (REG2) 2 2N7002 small-signal N-channel MOSFETs, SMD SOT-23 package (Q1,Q2) 1 10 ri e recti er 1 [Jaycar ZR1308] 1 LL4148 small-signal diode, SMD DO-80 MELF (D1) [Jaycar ZR1103] Capacitors (all SMD M2012/0805 size unless otherwise stated) 1 1000μF 16V t ro ole ra ial electrolytic 100 F 0V cera ic 100μF 6V ta tal i e 10 F 0V cera ic 10μF 6V ta tal i e 1 1 F 0V cera ic 0 F 0V cera ic 2 100pF 50V C0G/NP0 ceramic 0 F 0V cera ic 1 47pF 50V C0G/NP0 ceramic Resistors (all 1% SMD M3216/1206 size unless otherwise stated) 4 100kΩ 3 1kΩ 2 33kΩ 2 22kΩ 1 100Ω 5% 2W axial 7 15kΩ Additional parts for the Si4732-based version 1 Si4732 IC, SOIC-16 (IC1) [AliExpress, eBay] 1 on-off-on (centre off) miniature toggle switch with solder tags (S3) [eg, Jaycar ST0336] 1 6 atc cry tal 1 100 F 0V cera ic capacitor 01 0 0 i e 2 22pF 50V C0G/NP0 ceramic capacitors, SMD M2012/0805 size Additional parts for Si4730 module-based version 1 Si4730 module, surface-mounting, with six pads on either side of module’s PCB (MOD1) [AliExpress, eBay] 1 SPDT miniature toggle switch with solder tags (S3) [eg, Jaycar ST0335] Optional parts 1 vertical SMA socket (CON6) (external AM antenna input) 1 2x3 pin header (CON9) (for in-circuit programming of IC2) 1 cry tal 1 ee te t 2 22pF 50V C0G/NP0 ceramic capacitors, SMD M2012/0805 size UK/EU/US... readers You don’t need to use the exact Altronics/Jaycar parts listed here – they are quoted so you can find local alternatives from the detailed specs provided online by these sellers. existing radio interface. If you find that your encoder action is reversed, then follow these steps: 1. Turn the Bandwidth knob fully clockwise. 2. Tune the AM band to 500kHz. The display will show ‘Toggle Direction’ on the top line, and ‘Direction 1’ or ‘Direction 2’ on the lower line. You don’t need to press the button, as it automatically selects the alternative direction when you access this display. 3. Tune to another frequency – make sure the tuning direction is correct. This setup only needs to be done once, as the parameters are stored in EEPROM and restored on power-up. Reproduced by arrangement with SILICON CHIP magazine 2022. www.siliconchip.com.au 23