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PicoSDR Shortwave Receiver by Charles Kosina, VK3BAR My SSB Shortwave Receiver, published in June & July last year, is a classic superheterodyne receiver (siliconchip.au/Series/441). This one is quite different – it uses a Raspberry Pi Pico to implement a software-defined radio (SDR). This is the first standalone SDR published in Silicon Chip. Tuning range: 3-30MHz Minimum tuning step: 10Hz Modulation support: AM, AM-Sync, LSB, USB, FM, CW AGC: adjustable speed & gain Power supply: 7-9V DC from plugpack or internal battery SNR/sensitivity: 10dB for 1μV input over 3-10MHz; 5μV <at> 30MHz Display: OLED with optional external TFT LCD screen T here is nothing wrong with the classic superhet design, but with advances in digital technology, you won’t find too many radio receivers built that way anymore. The various analog circuits have been largely displaced by programs running on high-speed processors. As a result, this receiver is quite a bit simpler and easier to build while being more capable. About three years ago, I bought a couple of Raspberry Pi Pico modules with the intention of doing ‘something’ with them. After some half-hearted siliconchip.com.au Bandwidth: adjustable Audio output: internal speaker or headphones Squelch: optional & adjustable attempts at designing something with them, I put them back in their box. Recently, though, I came across a GitHub project using the Pico as the basis of an SDR (siliconchip.au/link/ ac9m). This was the first of what turned out to be a four-part series written by Jon Dawson. To quote Jon: The receiver covers frequencies up to 30MHz, including commercial broadcasts on Longwave, Medium Wave, Shortwave, and the HF amateur radio bands. What’s great about this design is that it’s completely Australia's electronics magazine Antenna connector: BNC standalone—it doesn’t need a PC or sound card and can run for hours on just three AAA batteries. I decided to design a receiver based on his articles, but with some enhancements. I recommend that you read all the articles as the design is quite complex. The mathematics written to decode all the modes is extremely advanced. Jon does describe everything in the articles, but to fully understand what he’s doing, you need a good knowledge of communications theory at an advanced tertiary level, and C++ April 2026  35 programming. My electronics engineering degree was of some help, but goes back many years and predated many of the current techniques. I will not attempt to reproduce the mathematics in those articles; it makes for interesting reading, but a full understanding is not required to build this Receiver. The starting point of SDR designs is producing the in-phase (I) and quadrature (Q) signals from the input signal. This is achieved by multiplying the signal by local oscillators 90° out of phase. It is important that the amplitudes and phases of the signals are accurate. The traditional way in the past was to use double-balanced diode mixers, like the circuit shown in Fig.2. However, this requires close matching of all components and is an expensive way of doing things. Dan Tayloe published a paper titled “Quadrature Sampling Detector”, where the same multiplication is performed with an analog switch: www. norcalqrp.org/files/Tayloe_mixer_ x3a.pdf The incoming signal is mixed with a two-phase local oscillator, with 90° phase shift between them. We get 36 Silicon Chip four outputs, each containing the sum and difference between the input frequency and the local oscillator, but with phase differences of 0°, 90°, 180° and 270°. We are only interested in the difference frequency; a simple RC lowpass filter eliminates the sum. If the local oscillator and input frequencies are the same, we recover the two baseband signals with different phase shifts. Following the quadrature detector, we have two low-noise, highgain op amps that give us the amplified I and Q signals, 90° out of phase. As it is described below, the local oscillator signal does not have to be at the same frequency as the incoming signal; instead, it is offset to give an intermediate frequency (IF) output. Now that we have the I and Q signals, digital processing takes over. They are sampled with analog-to-­ digital converters (ADC) at a very high sampling rate. The Nyquist criterion means the sampling rate needs to be at least double the required bandwidth. Two-phase local oscillator In many designs, the two-phase local oscillator is generated using the Silicon Labs Si5351A clock-­generator chip. This is extremely cheap and Australia's electronics magazine is ubiquitous in lots of commercial radios, signal generators and spectrum analysers. But the RP2040 chip on the Pico module is extremely powerful and fast, so it is possible to use it to generate the quadrature signals. It has a somewhat novel feature: a programmable state machine that can offload IO functions from the software and generate the quadrature outputs on two I/O pins (I write “somewhat” because some newer PICs have similar hardware). Once this is configured, it runs autonomously without software intervention. But it isn’t quite that simple. Because this is a digital oscillator that’s timed using the chip’s global phase-locked loop (PLL), the output resolution is not nearly enough, and as the frequency increases, so does the step size. The step size is as large as 8kHz. To achieve a step size of 1Hz, the software implements a second, very high-resolution numerically controlled oscillator (NCO) that shifts our IF to baseband. The IF is typically 4.5kHz, but it is varied slightly in conjunction with the NCO and gives a theoretical resolution of 0.0001Hz, far more than required for a 1Hz step size. siliconchip.com.au Fig.1: the RF board for this radio bears some similarity to the SSB Shortwave Receiver published last year, but it’s considerably simpler since most of the processing after the tuning and RF gain stage is performed digitally. IC2 is a digitally controlled analog multiplexer chip that, under the Pico’s control, mixes the RF signal with a local oscillator and produces the I/Q signals to feed back to the Pico for audio extraction. If this sounds complicated, that’s because it is, and requires very clever coding to generate our local oscillator. Processing the I and Q signals Processing of the signals to decode amplitude modulation (AM), frequency modulation (FM) and single sideband (SSB) is beyond the scope of this article. If you’re interested in how it works, I recommend you read the articles written by Jon Dawson at the link above. Choosing a Pico module The Pico 2 module using the RP2350A processor is considerably more powerful than the original Pico module that uses the RP2040 chip. The RP2350A also fixed a bug in the analog-­ to-digital converter (ADC) module within the chip, although the amount of averaging applied and noise present in this circuit means that bug does not currently affect its performance. So both modules can be used in this radio, with no real difference in the user experience. The advantage of using the Pico 2, which costs a couple of dollars more, is future-proofing it. While the Pico can handle the processing load at the moment, features may siliconchip.com.au be added in the future that require the Pico 2 to work, or at least to work well. So, we suggest you spend the extra couple of dollars and get the Pico 2 module, but there will be no immediate benefit. You need to load the right firmware – there are separate files for the Pico and Pico 2. For more details on this subject, see siliconchip.au/link/ac9n We recommend that you purchase a genuine, original Pico or Pico 2. There are clones in existence, and they work, but our testing shows that they are not directly compatible and will not work in this project without significant modifications to the board. So stick with the original. RF board circuit Like my SSB Receiver design, this receiver uses two circuit boards, a control board and an RF board. The circuit of the RF board is shown in Fig.1. The left-hand side is very similar to the July/August 2025 SSB Shortwave Receiver front-end (siliconchip.au/ Series/441), but it has some improvements incorporated. Two schottky diodes on the antenna input limit the input voltage to a safe level. Australia's electronics magazine Fig.2: an old-fashioned doublebalanced mixer uses two transformers and four diodes as shown here to mix the local oscillator (LO) and tuned antenna (RF) signals. The signal at the IF OUT terminal includes several new signals, one of which is the desired intermediate frequency (IF) signal. To control the tuning, there are three digital lines from the Pico: BAND0, BAND1 and BAND2. They go to a three-to-eight decoder IC (IC1) that selects filters on the antenna input. They could be fixed bandpass filters, but that would require numerous components. I opted for a similar technique to my SSB receiver and used two high-Q toroidal inductors. The 74HC238 chip selects different-­ value capacitors to roughly tune the radio to the centre of eight different bands. A BB201 dual varicap diode is then used to make the antenna circuit resonate at the input frequency; only half of it is used. This has a capacitance of 118pF at 0.5V and 27pF at 8V. The minimum capacitance, including stray/parasitic capacitance, is about 35pF. This requires an inductor value of 0.8µH at 30MHz. At 10MHz, we need 7µH. See Table 1 overleaf for the frequencies of the eight different ranges. These are chosen so that they are within range of the varicap tuning and the fixed capacitors across the input inductors. The changeover between the two toroids is 10.2-10.3MHz; you will hear the relay click on this transition. There are no capacitors switched in for Band 8, so there is no setting. Seven NPN transistors are used to select the capacitors (not eight, as the highest frequency uses just the varicap). These are BFR92P devices chosen for their very low collector-to-base and emitter capacitances. The relay is switched by the BAND2 signal, buffered by two N-Channel Mosfets, April 2026  37 The top and bottom of the Control Board for the PicoSDR Receiver. Q8 and Q9. A diode across the relay absorbs the switch-off transient. The BF998 dual-gate Mosfet (Q10) gives about 20dB of RF gain and also improves the noise figure. The gain is varied by a front-panel potentiometer that adjusts the gate 2 voltage, avoiding overload on strong signals. A wide-bandwidth Coilcraft transformer (T3) is used in the drain circuit. This has a 4:1 turns ratio, which gives a 16:1 impedance ratio. One problem with receiver design is the rejection of strong signals at other frequencies that may overload the front end. There is no easy solution to this, and various filters are used to reduce such interference. The Tayloe mixer uses half of a 74CBTLV3253 dual 4-way analog multiplexer chip. The input is DC-biased half the supply voltage of 3.3V, which gives midpoint bias to the following op amps. The requirements for the op amps are low noise, wide bandwidth and rail-to-rail operation with a 3.3V supply. The combination of this multiplexer (mux), the two op amps and the way the mux is controlled via the LOI and LOQ digital lines results in the extraction of the I/Q signals (RXI & RXQ) from the tuned RF signal. These are fed to the control board via CON2 for processing. Table 1 – tuning bands Toroid Band Centre frequency Low 1 3625kHz Low 2 4375kHz Low 3 5625kHz Low 4 10250kHz High 5 10625kHz High 6 11250kHz High 7 14500kHz 38 Silicon Chip The MCP6022 op amp is recommended for IC3, having a gain-bandwidth (GBW) of 10MHz and 8.7nV/√Hz of noise while running from 2.5-5.5V. With the resistor values used, the voltage gain is 683 times (57dB), giving a -3dB bandwidth of 14.6kHz (10MHz ÷ 683). A two-pole low-pass filter is provided using 56nF capacitors on the output of the ‘3253 and 220pF capacitors across the 56kW resistors. Control board circuit The control board sits behind the front panel. Its circuit is shown in Fig.3; it is based around a Raspberry Pi Pico or Pico 2 module (MOD1). There is not much connected to the Pico. A standard rotary shaft encoder is used for tuning and selecting items in the menu, with two extra pushbutton switches for display options and choosing menu items. The OLED screen is a standard SSD1306 module, with a resolution of 128×64 pixels. The Pico handles RF signal demodulation and produces an audio output generated by filtering a pulse-width modulated (PWM) signal from digital output pin GP16. The 100W/470nF low-pass filter removes most of the high-frequency switching components of the signal, and potentiometer VR1 provides volume control. The menu system does include a digital volume control, but this requires several pushbutton presses and encoder rotations, which is not very convenient. The original design fed headphones and could, in a pinch, drive a small speaker. In the final version, I have included an LM386 audio amplifier. The external 8W loudspeaker is connected via a headphone jack, so it is automatically disconnected when the ‘phones are plugged in. A resistor in series with the headphone connection Australia's electronics magazine limits the power to a safe level. The two-phase local oscillator required by the RF board is produced at the GP0 and GP1 pins of the Pico. The I & Q signals coming back from the RF board go to the GP26 and GP27 pins via anti-aliasing low-pass filters made of 5.6kW resistors and 2.2nF capacitors. It is from these signals that the modulated audio is recovered by software in MOD1. There are three connectors on the control board; CON4 & CON6 connect to the RF board, while CON8 goes to an external socket for an optional TFT LCD screen. A 16-pin connector is used for an IDC cable to the RF board, siliconchip.com.au On the left side of the RF board are the antenna tuning components: two transformers, the relay and a series of tuning capacitors. The mux is in the centre and dual op amp on the right. plus a 4-pin connector for selecting the input tuning and a 6-pin connector for the optional LCD screen. Power comes from a 9V DC source to CON9, which can be a plugpack. At least 8V is required to give sufficient range for the varicap fine-tuning. While a plugpack can be used, the best performance is with a battery supply, so that is what I’ve shown. Two lithium-ion rechargeable cells connected in series provide up to 8.4V when fully charged. A two-cell AA holder is adequate, but I opted for a three-AA battery holder and added a 1.2V NiMH rechargeable cell, which has a capacity of 1500mAh and results in a total supply voltage of up to 9.6V. 14500 (AA-size) Li-ion cells have a capacity of about 1200mAh, and with a total current drain of 100mA, will last up to 12 hours. Make sure to buy good-quality cells as cheap Li-ion cells carry a fire risk (see Mailbag, January 2026). The Pico requires a supply voltage of about 5V, so an LM1117 low-­dropout regulator is used. The Pico module has an on-chip 3.3V supply, available on one of its pins, which is used by the RF board. The 3.3V supply could also be used to power the OLED screen, but it is an I2C device and there is switching noise when it is being accessed. Coupling of this noise into the main supply is reduced by running it off the 5V supply instead, through a diode and using a 100µF filter capacitor. The series diode is not strictly necessary, but is included as a precaution and helps to isolate its supply from the other components. Construction Start by assembling the control board, which is coded CSE251101 and measures 96.5 × 53.5mm. Begin by soldering all the SMD components – refer to the overlay diagram, Fig.4. There are no fine-pitch devices on the board, and only one SOIC-8 chip, the LM386. Next, solder the connectors on the back of the board, including the two 20-pin socket strips for the Pico module. This module is plugged in rather than soldered; otherwise, replacing a faulty Pico module would be very difficult. Make sure that the 16-pin box header has its notch orientated correctly. There is provision for an Si5351A module socket on the back of the board. This was added as it is supported by the firmware as an option. You may experiment with it if you Fig.3: the control board is built around MOD1, a Raspberry Pi Pico or Pico 2. It produces the local oscillator signal, performs audio demodulation, controls the tuning circuitry, updates the screen(s) and feeds the audio signal to amplifier chip IC4. The user controls are volume (VR1), RF gain (VR2), fine tuning (VR3) plus the rotary encoder and three pushbuttons to drive the menu system. siliconchip.com.au Australia's electronics magazine April 2026  39 Fig.4: the control board has the Pico and connectors on the back (plus the electrolytic capacitor) and the user controls and other parts on the front. The Pico is plugged into a pair of header strips so it can be removed if necessary. When finished, D1 and the 100μF capacitor are hidden under the OLED screen. wish, but it is not required. The male header should be installed with the pins pointing up from the top side. If you are not using that module, you don’t need to fit JP1 or JP2. The front side of the board has the on/off toggle switch, three potentiometers, the rotary shaft encoder, two pushbutton switches and the socket for the OLED screen. To ensure that the components are aligned correctly, slip the front panel over the controls before soldering. Don’t forget the two components that will be hidden under the OLED screen (D1 & the 100μF ceramic capacitor). Once all the components are mounted on the front, flip the board over and add the five connectors on the back (CON4CON6, CON8 & CON9), orientated as shown, plus the two 20-way header sockets for the Pico module. Finally, add the electrolytic capacitor, with its longer + lead towards the closest edge of the board. Once the board has been cleaned, inspect it for any short circuits or dry joints. Use an ohmmeter (eg, a DMM) to check that the 5V and 3.3V lines are not shorted to ground. Before plugging in the Pico module, connect the power supply and measure the voltage on the output of the voltage regulator (REG1) to ensure that it is close to +5V. 40 Silicon Chip At this stage, it is worthwhile programming the Pico and checking the operation of the program. Programming is very simple – use a USB cable to connect to a PC. Hold the BOOTSEL button down when plugging the cable in. It will then appear as a removable disk drive. For the original Pico, the file to be programmed is “picorx.uf2”, or for the Pico 2, it is “pico2rx.uf2” (you can download both from siliconchip.au/ Shop/6/3579). Just transfer this file to the Pico’s “drive” and it will be written to its flash memory. Do not press the button again. Unplug the Pico module from your computer and connect it to the control board, with the USB connector at the top. Connect your power supply or battery and switch it on. You should immediately see a PicoRX splash screen followed by a picture on the OLED, which is a schematic of a crystal set! This stays up for a couple of seconds. This is followed by a complex menu system, to be described later. RF board construction The RF board is coded CSE251102 and measures 82.5 × 53mm; its overlay diagram is shown in Fig.5. All the components on this board mount on the same side. There are three integrated Australia's electronics magazine circuits, which you should solder first. All three must be orientated with the pin 1 locator placed as shown in the diagram. The 16-pin 74CBTLV3253 comes in a fine-pitch package and needs great care. Position it very carefully so that it is accurately on all the pads, then apply a small blob of solder on opposite corners. Run some flux paste on both sides and, using a fine-tip soldering iron, move it slowly across the pins. Editor’s note: I prefer a medium conical or chisel tip for better heat transfer; when using good flux paste, you don’t need a very fine tip. You may end up with shorted (bridged) pins, in which case the excess solder can be removed with some copper braid. It may take a couple of attempts with extra flux to get clean joints with no shorts between them. Note that the 74HC238 is mounted in the opposite orientation to the other chips. This was done to make the layout easier. Make sure orientation is correct; the circuit will definitely not work if any IC is reversed. While fixing them after soldering is possible, it is a real pain, especially if you don’t have a hot air rework station! All the transistors except the BF998 have SOT-23 footprints (the BF998 is in a similar package but with an extra, wider pin, which must be placed as shown). This also applies to the BB201 dual varicap diode, so ensure you don’t confuse it for a transistor. The wideband Coilcraft RF transformer (T3) is also fitted ‘upside down’, in the same orientation as the 74HC328. The remaining resistors and capacitors can be fitted now. They are all in M2012/0805 SMD packages (2.0 × 1.2mm) and are not polarised. The resistors will be marked with codes indicating their values, but the capacitors won’t, so solder them in place as soon as you remove them from their packaging to avoid confusion. Winding toroidal transformers can be tedious, but take your time and keep them neat and wound in the correct direction so they correspond to the termination pads on the PCB. The low-band toroid (T1) requires 37 turns of 0.3mm diameter enamelled copper wire, closely spaced (connected between points C & D on the PCB). This leaves enough room for the siliconchip.com.au four-turn primary winding, also using 0.3mm diameter wire, connected between points A & B. The high-band toroid (T2) uses 13 turns of 0.6mm diameter wire, which should be spread out around the toroid to connect between points G & H, again leaving room for the two-turn primary (also using 0.6mm diameter wire), connected between points E & F. There are only a few through-hole components remaining to be mounted: the relay (RLY1) and connectors CON1-CON3. Make sure the notch on the 16-pin box header is aligned as shown in Fig.5. Fig.5: the RF board is considerably smaller and easier to build than the one for the SSB Shortwave Receiver thanks to the digital processing. The only tricky part to solder if IC2, as it is a finepitch IC, but it isn’t too difficult if you have decent light, good flux paste and a magnifier. Preparing the cables The main connection between the control and RF boards is a 16-wire flat ribbon cable with 16-way IDC connectors at either end. Cut a piece about 80mm long and use a vice or IDC crimping tool to clamp the cable on the connectors. Make sure the cable is exactly square onto the connector and that the pin 1 notches are facing the same way at each end before clamping them. The other cable required is 120mm long with four wires. Crimp pins on each end for the four-way polarised connectors and push them into the blocks, ensuring that the wire order is the same at each end. You could strip out a length of 4-wire ribbon cable to make this, or use individual wires twisted together or held within tubing for neatness. If you want to use the optional external TFT LCD screen, this requires a 6-way shielded cable. The ground wire and shield wire should be crimped onto the same pin. A round 6-pin connector on the back panel is used to connect this screen, as per Fig.6. The shield is needed to reduce RF radiation that would induce noise into the RF board. Keep this cable away from the RF section. Even with the best arrangement, there will still be a pulsating noise at low signal levels. The external screen can be switched off in the HW Config → TFT Settings menu to remove this source of noise. Two-way connectors are used for the input DC power and speaker connections. As there is no room for the headphone socket on the front panel, it is on the back panel. Wire it in such a way that the speaker is disconnected with headphones plugged in (if in siliconchip.com.au The wiring is straightforward, as shown here. Ensure pin 1 on both connections between the boards are the same at each end. doubt, refer to Fig.3). A 100W ¼W resistor mounted on the headphone socket (also shown in that diagram) limits the headphone power to avoid hearing damage. Case assembly Attach the 50mm speaker to the front panel using four 10mm-long M3 machine screws, washers and nuts. The control board is attached to the front panel by M2.5 × 16mm threaded spacers and M2.5 × 6mm screws. I used black screws on the front panel for the best appearance. The RF board should be mounted on the bottom plate to line up the 16-pin headers. Use the board as a template for the holes in the base. The RF board is attached by four M2.5 × 10mm threaded spacers and eight M2.5 × 6mm screws. Next, mount the connectors on the back panel. The antenna connector is a 15cm-long coax cable with an SMA plug on one end and a panel-­mounting BNC socket on the other. This is a Australia's electronics magazine ready-made item available from AliExpress (see the parts list). If an external DC supply is used, include a suitable connector (eg, a chassis-mounting barrel type). As mentioned above, the headphone jack socket is also on the back panel. See the photos for a suggested layout. The two pushbuttons pass through 3mm holes on the front panel. If you have access to a 3D printer, their appearance and ease of use can be improved by making caps to fit over the buttons. The caps are a push-fit on the switch. This will require drilling out Fig.6: if using the optional larger external LCD screen, wire it up to the circular plug like this. April 2026  41 Parts List – PicoSDR Reciever 1 assembled control board (see below) 1 assembled RF board (see below) 1 black front panel PCB coded CSE251103, 159 x 64.5mm 1 170 × 75 × 130mm vented metal enclosure [AliExpress 1005007496723103] 1 50mm 4W or 8W 10W loudspeaker [AliExpress 1005006957225238] 1 100mm length of 16-way flat ribbon cable 2 16-way IDC line sockets [Jaycar PS0985] 1 3.5mm jack socket, 5-pin type (CON7) [AliExpress 1005006501723152] 1 6-pin circular connector with matching plug (optional; for external TFT LCD screen) [AliExpress 1005004645761532] 1 150mm-long SMA male to panel-mount BNC female coaxial cable [AliExpress 1005001385620859] 1 2-cell or 3-cell AA battery holder with flying leads (see text) 2 AA-size (14500) Li-ion rechargeable cells 1 AA-size (14500) NiMH rechargeable cell (optional; see text) 4 M3 × 16mm black panhead machine screws 4 M3 flat washers 4 M3 hex nuts 16 M2.5 × 6mm black panhead or countersunk head machine screws 4 M2.5 × 16mm tapped spacers 4 M2.5 × 10mm tapped spacers Control Board 1 double-sided PCB coded CSE251101, 96.5 × 53.5mm 1 Raspberry Pi Pico or Pico 2 module (MOD1) 1 128×64-pixel 0.96in 4-pin OLED screen with SSD1306 controller (OLED1) [Silicon Chip SC6176] 1 3.5in LCD module with ILI9488 controller (optional) [Silicon Chip SC5062] 1 SPDT solder tag toggle switch (S1) 2 PCB-mounting 4-pin tactile pushbuttons with 15mm-long actuators (18mm total height) (S2, S3) [AliExpress 1005001629305461] 1 rotary encoder with integrated pushbutton and 20mm-long D-shaped shaft (RE1) [AliExpress 1005006690469571] 1 10kW 9mm logarithmic taper vertical potentiometer with 20mm-long D-shaped shaft (VR1) [AliExpress 1005008648801832] 2 10kW 9mm linear taper vertical potentiometers with 20mmlong D-shaped shafts (VR2-VR3) [AliExpress 1005006029199652] 1 medium/large knob to suit RE1 3 small knobs to suit VR1-VR3 [AliExpress 1005006637211404] 1 4-pin vertical polarised header (CON4) 2 2-pin vertical polarised headers (CON5, CON9) 1 2×8-pin keyed box header (CON6) 1 6-pin vertical polarised header (CON8) 2 20-pin header strips (for mounting MOD1) 2 20-pin female headers (for mounting MOD1) 1 4-pin female header (for mounting OLED1) 2 11mm-long untapped (or tapped) spacers, 2.5mm inner diameter (for mounting OLED1) 2 M2 × 16mm panhead machine screws (for mounting OLED1) 2 M2 hex nuts (for mounting OLED1) 42 Silicon Chip Semiconductors 1 LM386M audio amplifier IC, SOIC-8 (IC4) 1 LM1117(I)MP(X)-5.0 5V LDO linear regulator, SOT-223 (REG1) 1 LL4148 100V 200mA signal diode, SOD-80 (D1) Capacitors (all SMD M2012/0805 size 50V X7R unless noted) 1 470μF 16V electrolytic 1 100μF M3216/1206 size 10V X7R 6 10μF 16V 1 470nF 1 100nF 1 47nF 2 2.2nF Resistors (all SMD M2012/0805 1% unless noted) 1 6.8kW 1 220W 1 100W ¼W axial resistor 2 5.6kW 1 100W 1 47W 1 680W RF Board 1 double-sided PCB coded CSE251102, 82.5 × 53mm 1 vertical PCB-mounting female SMA connector (CON1) 1 2×8-pin keyed box header (CON2) 1 4-pin vertical polarised header (CON3) 1 HFD4-5 DPDT 5V DC coil telecom relay (RLY1) 2 Micrometals T50-6 Carbonyl toroidal cores, 12.8 × 7.5 × 4.95mm (T1, T2) [www.minikits.com.au/T50-6] 1 200mm length of 0.3mm diameter enamelled copper wire (T1) 1 50mm length of 0.6mm diameter enamelled copper wire (T2) 1 Coilcraft PWB-16-ALC 80MHz 1:16 SMD signal transformer (T3) [Mouser 994-PWB-16-ALC] Semiconductors 1 74HC238D/74HC238M 3-to-8 decoder IC, narrow SOIC-16 (IC1) 1 74CBTLV3253PW dual 4-way analog multiplexer, TSSOP-16 (IC2) 1 MCP6022(T)-I/SN or MCP6022(T)-E/SN dual 2.7V low-noise 10MHz op amp, SOIC-8 (IC3) 7 BFR92P low-noise 15V 5GHz NPN transistors, SOT-23 (Q1-Q7) 2 2N7002 60V 115mA N-channel logic-level Mosfets, SOT-23 (Q8, Q9) 1 BF998 12V 1GHz dual-gate Mosfet, SOT-143 (Q10) 1 BB201 dual varicap diode, SOT-23 (VD1) 1 LL4148 100V 200mA signal diode, SOD-80 (D2) 2 1N5711 70V 15mA axial schottky diodes (D3, D4) Capacitors (all SMD M2012/0805 size 50V X7R unless noted) 2 10μF 16V 9 100nF 4 56nF 1 1nF 1 330pF NP0/C0G 3 220pF NP0/C0G 2 180pF NP0/C0G 1 100pF NP0/C0G 1 68pF NP0/C0G 1 4.7pF NP0/C0G Resistors (all SMD M2012/0805 1%) 1 470kW 8 12kW 1 220W 1 100kW 2 10kW 4 82W 2 56kW 1 1kW Australia's electronics magazine siliconchip.com.au the front panel holes to 5mm. The file for this is “button_caps_V02_CK.stl”. Thanks to Andrew Woodfield for the design of these caps. Initial setup The menu system is quite overwhelming, and it reminds me somewhat of menus in digital cameras. It has a branching tree system to adjust many different parameters and settings. The menu items are chosen using the two pushbutton switches, plus the rotary encoder with its integrated pushbutton switch. As with digital cameras, some of the settings are of little importance and are best left alone. But there are some initial setup parameters that are important. The first of these is Encoder Direction. Press the ▲ button and the display will show Menu on the top line and Frequency on the second line. Rotate the encoder knob one click right or left and, depending on the shaft encoder direction, it will show HW Config. If HW Config comes up with a clockwise rotation, you need to change the direction of the tuning knob. With HW Config on the second line, Press the ▲ button and the display will show HW Config on the top line and Tuning Options below. Rotate the encoder knob to navigate to Reverse Encoder. Press the ▲ button and the display will show Reverse on the top line and Encoder on the second line. Rotate the encoder knob to select On. Finally, press the ▲ button and the display will go back to Menu on the top line and Frequency on the second line. Press the ▲ button to return to the opening screen, then press the ▼ button several times to select Viewing Option. There are about 25 different parameters that can be set by first pressing the ▲ button and then rotating the knob to select different options. One of them is Volume, which can be adjusted from 0 to 9. This is why I have added the volume control on the front panel, to avoid going through several steps to get to such a basic control. The following are some of the more important parameters: • Mode: AM, AM-Sync, LSB, USB, FM, CW • AGC: Manual, Fast, Normal, Slow, Very Slow • AGC Gain: with maximum gain, the background noise is high. Changing this to 30dB reduces it significantly, siliconchip.com.au An external display can be added to the PicoSDR if you need a bigger screen with more information. without affecting the sensitivity. • Squelch: this silences the receiver until a signal is strong enough. I found that S5 will completely silence it, but a 1μV signal will open up the receiver on most frequencies. Experiment with this setting to find the optimum value. • Bandwidth: Normal, Wide, Very Wide, Very Narrow, Narrow • Freq Step: 10Hz, 50Hz, 100Hz, 500Hz, 1kHz, 5kHz, 6.25kHz, 9kHz, 10kHz, 12.5kHz, 25kHz, 50kHz or 100kHz. In the HW Config menu, there are 22 different hardware parameters that can be adjusted! Many of them can be safely ignored. I won’t go through all the possible menu settings; you can look through them if you want to. The ▼ button selects what appears on the OLED. The photos show some of the possible displays. Australia's electronics magazine As with all receivers, there are some spurious signals and ‘birdies’ due to harmonic mixing. If they happen to be on a frequency that you are tuned to, there is a simple way of removing them. Navigate to IF Frequency and change it from the nominal 4.5kHz slightly. The received frequency is identical, but the birdie has moved. Conclusion This receiver is an example of what can be done with a software approach to design. Jon Dawson has done an incredible job in writing the highly complex code to make it possible. It could not be classed as a first-class ‘communications receiver’, but it does have creditable performance. There are regular updates to the software on his site, so it’s worthwhile SC looking at it from time to time. April 2026  43