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Project by Tim Blythman We have updated the Pico Audio Analyser design from November 2023 to use the Pico 2, which has improved its performance in some areas. A followup article also examines how the Pico 2 would work in some of our other Pico projects and some other hints for using the Pico 2. 2 PICO Audio Analyser T he Pico Analyser project from November 2023 (siliconchip. au/Article/16011) is a compact handheld device that offers many useful features for analysing audio frequency signals. It includes a signal generator, oscilloscope and spectrum displays and can perform harmonic and sweep frequency response analyses. The Pico Analyser is by no means a high-end device, but it was let down somewhat by a defect in the RP2040 chip used on the original Pico. We discussed this in detail in a panel in that earlier article. To sum it up, the 12-bit ADC (analog-to-­digital converter) on the RP2040 chip has errors in the tiny capacitors used to perform the conversion. This means that the ENOB (effective number of bits) of the ADC is only eight; less than the nine or so that would be expected. This affects the accuracy of measurements and in particular limits the THD (total harmonic distortion) measurements to no better than around 0.4%. We were able to apply some Features & Specifications > Audio signal generator (up to 3V peak-to-peak/1.06V RMS) with selectable frequency > Sine, square, triangle, sawtooth and white noise waveforms > Audio signal input with switchable 3.6V and 34V peak-to-peak ranges (1.27/12V RMS) > Oscilloscope and spectrum displays > Harmonic analysis with THD measured down to 0.2% (1.2V RMS, 1.2kHz) > Can measure and monitor mains distortion with a suitable plugpack > Sweep analysis with frequency response display > RCA sockets for input and output > Runs from USB power or an internal rechargeable battery > Uses 128×64 OLED display and pushbutton controls > Compact and portable > Controllable from a virtual USB serial port > Typical current draw around 50mA > Operates for around 12 hours with a fully charged 600mAh battery 82 Silicon Chip Australia's electronics magazine compensation to the ADC readings, improving it to 0.3%. The Pico 2 uses an RP2350 microcontroller instead of the RP2040, and the RP2350 data sheet notes that the spikes in differential nonlinearity should not be present in the newer part. It claims an ENOB of 9.2, which should theoretically allow total harmonic distortion (THD) to be measured below 0.2%. So it’s clearly worthwhile to update the Pico Audio Analyser with the Pico 2. We’ll also look at whether the Pico 2’s increased flash memory, increased RAM or faster processor clock will provide any other opportunities for improvement. A straightforward update The Pico Analyser was intended to be simple and inexpensive, so we have not made any radical changes to the circuit. In fact, the only change in the Pico 2 Audio Analyser hardware is substituting a Pico 2 for the Pico. Fig.1 is the circuit for the Pico Analyser with this small change. To briefly recap, the Pico 2 generates a PWM (pulse-width modulated) audio signal on GP16 that has its higher frequency components attenuated by a pair of 2.2kW/1nF low-pass filters. The signal is then buffered by the op amp, AC-coupled and biased to circuit ground before being delivered to CON2. siliconchip.com.au Fig.1: the circuit for the Pico 2 Analyser has not changed much from the original Analyser, with the exception of a Pico 2 now being used for MOD1. The other half of the op amp is arranged to provide a mid-rail 1.65V reference. The audio input at CON1 is filtered to remove ultrasonic components before being AC-coupled and biased to the 1.65V rail to centre it within the ADC’s input range. The processed input voltage is sampled at the Pico 2’s GP26 pin. The 510W resistor switched in by S6 can be used to attenuate the incoming signal, allowing for input voltages up to 34V peak-to-peak. That’s ideal for using something like an isolated 9V AC (RMS) mains transformer to check mains power distortion. IC2 and its associated components form a charging circuit for a rechargeable lithium battery, with LED1 providing a status display. The Pico 2 is powered either from its USB socket or the battery if S5 is closed. The Pico 2 connects to four tactile switches (S1-S4) for user input and MOD2, an I2C OLED display. The 22kW/22kW voltage divider allows siliconchip.com.au the Pico 2 to also monitor the battery’s voltage at its GP28 analog input. Software features The software has numerous modes and means to set some calibration parameters. Much of the calibration is done automatically once a multimeter is connected externally to set the output level correctly. A WAVE OUTPUT screen allows the frequency, amplitude and waveform (eg sine, square, triangle, sawtooth or white noise) to be set. Most of the remaining screens provide analysis of the input signal. SCOPE and SPECTRUM screens provide displays of the input waveform. A HARMONIC ANALYSIS screen determines the fundamental frequency of the input and the amplitude of the fundamental and its harmonics, as well as reporting a THD figure. Finally, a SWEEP page drives the output with a sinewave at varying frequencies and measures the received response back at the input. These last three screens make use of a fast Fourier transform (FFT) to extract frequency information about the waveform at the input connector. There was no need to update the Analyser PCB, so it looks the same as the original. Australia's electronics magazine March 2025  83 The fully assembled PCB of the Pico 2 Analyser looks much the same as its predecessor, with the Pico 2 silkscreen on MOD1 being the only visible difference. Note the unusual mounting arrangements for the LED and OLED. If you’d like to read about the circuit and software operation in greater depth, we recommend reading the original Pico Analyser article from November 2023. That article also contains the detailed construction notes for the Pico Analyser. The construction of the Pico 2 Analyser is the same, with the proviso that a new binary file (0410723B.UF2) is needed to program the RP2350 processor on the Pico 2. In any case, the Pico 2 should ignore a binary file for a different processor (such as one prepared for the Pico), so there is little chance of damage, even if the wrong file is inadvertently used. If you’re more interested in simply building the Pico 2 Analyser, you can follow the instructions in the older article with those two minor amendments to the build process. Hardware differences The Pico 2 offers slightly different hardware features to the Pico, so we have investigated what can be improved by using these. The ADC is the first of these to address. While the Pico 2’s RP2350 corrects the erratum present in the Pico’s RP2040, there is otherwise not much difference in the peripherals that are used in the Analyser. Both parts are capable of 500kS/s ADC operation at 12 bits of sampling depth and can use the DMA (direct memory access) peripheral to capture samples without bogging down the processor. In the Pico Analyser, the ADC is run at 490kS/s, taking 12-bit samples using DMA, and we have done the same for the Pico 2 Analyser. So both parts are run very close to their respective limits in that regard; we cannot do much to improve the effective sampling rate. The software binary for the Pico Analyser required less than 10% of the Pico’s flash memory, so the extra flash memory doesn’t help here. The ARM Cortex M33 processor on the Pico 2 can run at up to 150MHz, about 10% faster than the 133MHz of the Pico’s ARM Cortex M0+. While the Pico Analyser was not constrained by processing speed, this provides one advantage in that the processor on the Pico 2 can generate the audio samples at a higher rate. The PWM outputs now run at around 73kHz instead of 64kHz, so there is an improvement in the attenuation of higher-frequency PWM artefacts by the low-pass filters. This shaves about 0.05% from the final THD reading when the signal is looped back into the Analyser. It’s a small but tangible improvement. To test the impact of the different ADC, we fed in a sinewave from an Audio Precision System One Audio Analyser. It typically deals with THD levels below 0.001%, so its output can be considered close enough to pure for the purposes of testing the Pico 2 Analyser. Under the same conditions as our tests on the Pico Analyser (a 1.2V sinewave at 1.2kHz), the Pico 2 Analyser reported a THD of 0.20%, better than the 0.30% that we saw with the Pico Here are the internals from the Pico 2 Analyser, just before the case is closed up. Note the mounting of the LED and OLED. You should apply some glue or sealant wherever the wires meet the PCB; this will help to prevent them from coming loose if a solder joint breaks. 84 Silicon Chip Australia's electronics magazine siliconchip.com.au Analyser. Note that this is almost, but not quite, what we expected based on the figures provided in the data sheet and is a definite improvement. Of course, our tests on the Pico 2 Analyser required disabling the code we added that corrects the Pico’s ADC readings for the error in the RP2040 silicon. We’ve also changed the initial splash screen to help tell the two apart. Porting the code Our Pico 2 Review in the December issue (siliconchip.au/Article/17316) noted that much of our existing code for projects based on the Pico required little more than recompiling to work with the Pico 2. The RP2350 in the Pico 2 is from a different family of ARM processors, so the two are not ‘binary compatible’. We found that the same was true for the Pico Analyser code. We used the Arduino IDE and the arduino-pico board profile (https://github.com/ earlephilhower/arduino-pico) to compile the code for the Analyser. Since the Pico and Pico 2 are easy to program using their USB flash drive bootloader, if you just want to use the compiled UF2 binary file, then you don’t need to worry about the steps involved in compiling the software, and you can jump to the next section. The first step in porting the code is to update the board profile. We are using version 4.1.1 of the arduino-pico board profile, which is the latest at the time of writing. v4.1.0 version was the first to provide the option to compile the code to use the RISC-V processor cores. If you have not installed the board profile previously, the process is to add the appropriate Additional Boards Manager URL (noted in the We have changed the splash screen for the Pico Audio Analyser Mk2, so that you can tell it apart from the older version. GitHub repository above) to the Preferences menu of the Arduino IDE and use the Boards Manager to install the package. The external libraries can be installed from the versions we included with the software download or via the Library Manager. We found that the existing code compiled without changes, but as we noted above, we needed to disable the ADC corrections needed for the RP2040, and we also took the opportunity to increase the PWM frequency for audio generation. Since writing the original Pico Analyser article, it has become clear that the RP2040 chip used in the Pico is capable of being overclocked, that is, operated at a frequency above its specified maximum. There are reports that the Pico 2 is similarly overclockable. We tried compiling the code at higher processor speeds to see if this could improve the output audio further, but the gains were negligible. So we opted to run the Pico 2 Analyser at its maximum design speed of 150MHz for the sake of stability; it is much newer and so has not been as thoroughly tested as its predecessor. Some parameters were tied to the 133MHz processor clock, so they needed adjusting to work at 150MHz. However, running the Pico 2 at 133MHz was sufficient to get the same code working without changes. We also tracked down a minor bug that was giving odd readings when no signal was applied to the Pico 2 Analyser. It was present in the Pico Analyser, but for reasons we could not determine, did not result in spurious readings. We suspect it is due to differences in the underlying library code. We’ve also updated the splash screen graphic shown when the Pico 2 Analyser starts up. While it might appear purely decorative, it also gives time for the internal biases to settle. Construction Construction of the Pico 2 Analyser is much the same as for the Pico Analyser. While we won’t give the full details here for brevity, experienced constructors should be able to work from the overlay diagram reproduced here as Fig.2. As well as using a Pico 2 instead of a Pico, the firmware image is different. Otherwise, assembly and operation are much the same. First, fit the surface-mounting parts (excluding the six switches) to the PCB in the usual fashion. Before fitting the switches, clean off any excess flux. Note that the reverse-mount tactile switches can benefit from having their leads splayed slightly before soldering. The Pico 2 (MOD1) and OLED (MOD2) modules are each fitted in a non-standard way. MOD2 is attached first, with its front side visible through the large hole in the front of the PCB. Don’t forget to remove the screen’s protective film! Four wires are used to connect the GND, VCC, SDA and SCL Fig.2: this overlay diagram shows the locations of the parts on the Pico 2 Analyser PCB. If you need detailed assembly instructions, refer to the original Pico Analyser article. siliconchip.com.au Australia's electronics magazine March 2025  85 Silicon Chip PDFs on USB ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. ¯ Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). EACH BLOCK OF ISSUES COSTS $100 NOVEMBER 1987 – DECEMBER 1994 JANUARY 1995 – DECEMBER 1999 Parts List – Pico 2 Audio Analyser 1 double-sided PCB coded 04107231, 83 × 50mm, with black solder mask 1 UB5 Jiffy box (83 × 53 × 30mm) 2 chassis-mount RCA sockets (CON1, CON2) [Altronics P0161] 1 single AA cell holder with flying leads 1 14500 (AA-sized) Li-ion rechargeable cell with nipple 1 Raspberry Pi Pico 2 board, programmed with 0410723B.UF2 (MOD1) 1 1.3-inch (33mm) OLED module (MOD2) [Silicon Chip SC5026] 4 reverse-mount SMD tactile switches (S1-S4) [Adafruit 5410] 2 SPDT SMD slide switches (S5-S6) 4 M3 washers, 1.5mm thick 2 20cm lengths of hookup wire (eg, white and black) 1 4cm length of fine bare wire (eg, lead offcuts from LED1) 1 small tube of neutral-cure silicone sealant 1 short RCA-RCA cable (for testing & calibration) Semiconductors 1 MCP6002 or MCP6L2 rail-to-rail dual op amp, SOIC-8 (IC1) 1 MCP73831-2ACI/OT Li-ion charge regulator, SOT-23-5 (IC2) 1 bi-colour red/green 3mm LED (LED1) 1 SS34 40V 3A schottky diode, DO-214 (D1) Capacitors (all M3216/1206 size, X7R ceramic) 6 10μF 16V+ 3 1nF 50V Resistors (all M3216/1206 size, 1% 1/8W) 4 100kW 2 2.2kW 2 22kW 2 1kW Use this photo as a 3 10kW 1 510W guide to fitting the 3 4.7kW smaller components. This stage of assembly is a good point to clean off any excess flux in preparation for adding the final components like the switches, LED, Pico 2 and OLED. Pico 2 Audio Analyser Kit SC6772 ($50): includes the PCB and everything that mounts directly on it. The Pico 2 is supplied blank and will need to be programmed using a computer and USB cable. A loopback cable like this can be used to test and calibrate the Pico 2 Analyser. JANUARY 2000 – DECEMBER 2004 JANUARY 2005 – DECEMBER 2009 JANUARY 2010 – DECEMBER 2014 JANUARY 2015 – DECEMBER 2019 OUR NEWEST BLOCK COSTS $150 JANUARY 2020 – DECEMBER 2024 OR PAY $650 FOR THEM ALL (+ POST) WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS 86 Silicon Chip Fig.3: the UB5 case needs holes for the USB socket and RCA sockets, as well as notches for the slide switches. Australia's electronics magazine siliconchip.com.au pads, with two extra wires providing mechanical support. Fig.3 shows the case cutting diagram. We recommend you prepare the case before fitting the Pico 2, since it will allow you to check the switch slots and also that the Pico 2 is aligned correctly with the hole for its USB socket. MOD1 is mounted on its edge, using only pins 21-40. At this stage, you can program the Pico 2 using the 0410723B.UF2 file, and you should see a display on the OLED screen if all is working well. You can refer to the earlier photo to see the state of the board after these steps. The LED has its leads bent 180° to allow it to point downwards at the hole in the PCB solder mask, while the battery holder and RCA sockets are soldered to the PCB via flying leads. Use glue to help secure the battery wires to the PCB and affix the battery holder to the case. Once the glue has cured, the cell can be fitted to the holder. The Analyser should start up when S5 is closed. If all is well, close up the case using the Nylon washers to space the lid off the pillars slightly. Ideal Bridge Rectifiers Choose from six Ideal Diode Bridge Rectifier kits to build: siliconchip. com.au/Shop/?article=16043 28mm spade (SC6850, $30) Screen 1: pressing OK on the WAVE OUTPUT screen cycles between the parameters, while UP and DOWN modifies them. The USB serial port can also control the output waveform. 21mm square pin (SC6851, $30) Screen 2: the SPECTRUM display uses UP and DOWN to change the horizontal scaling, while OK toggles the vertical scale between peak and total energy. siliconchip.com.au 5mm pitch SIL (SC6852, $30) mini SOT-23 (SC6853, $25) Screen 3: the SCOPE display also uses UP and DOWN to change the horizontal scaling. The OK button changes between dot and line displays. Width of W02/W04 2A continuous, 40V Connectors: solder pins 5mm apart at either end IC1 package: MSOP-12 Mosfets: SI2318DS-GE3 (SOT-23) D2PAK standalone (SC6854, $35) Screen 4: HARMONIC ANALYSIS provides information about the harmonic content of a waveform. Connecting the input to the output is a good way to check this feature. Conclusion The Audio Analyser wasn’t the only Pico-based project we had a go at updating. In fact, we tested all our Pico code on the Pico 2 and also decided to look into taking advantage of some of the Pico 2’s new features, like the RISC-V cores. The following article explains what we found and gives a few hints to those keen to use the Pico 2. SC Compatible with PB1004 10A continuous (20A peak), 72V Connectors: solder pins on a 14mm grid (can be bent to a 13mm grid) IC1 package: MSOP-12 Mosfets: TK6R9P08QM,RQ Compatible with KBL604 10A continuous (20A peak), 72V Connectors: solder pins at 5mm pitch IC1 package: MSOP-12 Mosfets: TK6R9P08QM,RQ Calibration and use Cycle through the screens using the MODE button and press OK to enter calibration mode. Follow the instructions on the screen to complete the calibration. For the OUTPUT LEVEL, you will need a true RMS voltmeter to trim the output from CON2. You will also need an RCA-RCA cable (connected between CON1 and CON2) to complete the INPUT LEVEL calibrations, since the Analyser reads back its own output to establish that its input is correct. Ensure that the calibration values are saved before using the Analyser. You can check its operation by running a SWEEP with the RCA-RCA cable connected; it should be flat at 0dB with slight dips at each end. Compatible with KBPC3504 10A continuous (20A peak), 72V Connectors: 6.3mm spade lugs, 18mm tall IC1 package: MSOP-12 (SMD) Mosfets: TK6R9P08QM,RQ (DPAK) Screen 5: in this display, the UP and DOWN buttons change the vertical scaling; the unlabelled horizontal line is the -3dB point compared to the set level at the output. Australia's electronics magazine 20A continuous, 72V Connectors: 5mm screw terminals at each end IC1 package: MSOP-12 Mosfets: IPB057N06NATMA1 (D2PAK) TO-220 standalone (SC6855, $45) 40A continuous, 72V Connectors: 6.3mm spade lugs, 18mm tall IC1 package: DIP-8 Mosfets: TK5R3E08QM,S1X (TO-220) See our article in the December 2023 issue for more details: siliconchip.au/Article/16043 March 2025  87