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Part 2 by Richard Palmer WiFi DDS Function Generator This flexible function generator, introduced last month, has seven different output modes and numerous other useful settings like burst and sweep modes. It can be controlled via an onboard touchscreen, a remote web interface via WiFi, or SCPI commands via WiFi from a computer. D espite its substantial feature set, the LCD touchscreen interface makes it simple to use. The unit can also be controlled from a computer, tablet or mobile phone via its web browser interface. This second and final part of this series of articles focuses on constructing, commissioning and operating the unit. As with the other test bench instruments I have designed (Bench Supply, Programmable Load and ‘Swiss Army Knife’), SCPI commands are also supported. The device fits neatly into a snap-­ together instrument enclosure, with a single PCB accommodating all the components, LCD screen, controls and connectors. The Raspberry Pi Pico W microcontroller has a much simpler ‘drag and drop’ programming method than the ESP32 processors I used in the earlier instruments in this series, making programming simple. Construction Because a generous PCB is required to accommodate the switches, rotary 82 Silicon Chip encoder and various connectors, there is ample space to use through-hole components almost exclusively in this project. As shown in Fig.8, the Pico and PCM5102A modules mount on one side of the PCB, with all the passives, while the LCD, LEDs and switches are on the other. Two footprints are provided for the PCM5102A module, to suit the two most common versions available online. It is best to start by fitting all of the parts on the Pico side of the PCB first, doing some testing, then moving to the other side of the board. That’s because the LCD screen obscures the pads of several components. The screen is mounted on 6mm spacers to align its face with the front panel, rotary encoder and pushbuttons. Refer to the overlay diagram, Fig.8, as you mount the parts on the PCB. You can also check the PCB photos (from part one) to see how it should look. Start with the only surface-mounting device, diode D1. Tack-solder one lead Australia's electronics magazine to its pad (making sure the leads bend down towards the PCB, not up in the air like a dead bug), then check its alignment with the other pads. If it’s misaligned, remelt the solder and nudge it gently into position, then solder the other leads and refresh the first one. You can do that by adding a little extra solder or, even better, adding a tiny bit of flux paste and then heating it with a clean soldering iron tip. Follow with the resistors. Ideally, you should check each batch with a multimeter to verify they have the correct resistance (the colour-coded bands can sometimes be hard to distinguish). After that, fit diode D2, the only through-hole (axial) diode, with its cathode stripe to the left as shown in Fig.8. If you are using IC sockets, mount them so that the notched ends face in the correct directions (IC2 faces down, the others face up), then plug REG3 into its socket, with pin 1 at upper left. If not using sockets, solder REG3 in place, also being careful siliconchip.com.au 4 37 MOD2b 5 36 35 7 RASPBERRY 34 PI Pico W 33 10 11 12 29 13 28 14 27 15 26 16 25 18 WIFI MODULE 23 22 20 21 OUT A 5.6kW 2.2kW PCM5102A MOD2 24 19 220pF 100nF 2.2kW 31 30 220pF 5.6kW 32 RP2040 MCU 2.2kW 2.2kW L IC2 24C256 10kW 10kW 10kW 9 100nF 2.2kW 2.2kW 2.2kW 6 100nF 220mF IC1 NE5532 100nF 17 100nF 10W 38 220pF 220pF G R G 10kW 2.2kW 1kW 2.2kW + 100nF 39 3 8 2.2kW 40 10W D1 BAT54S SCK BCK DIN LRCK GND VIN 100nF 1 2 MICRO USB–B PORT 5819 10W 10mF 10mF K 100nF 220mF D2 MOD2a + MOD1 100pF CON5 10W 4.7kW 4.7kW REG3 MAX1044 220mF CON4 CON1 + 100nF REG1 7809 O UT B TRIG IN TRIG OUT CON3 CON2 + 100nF +12V + REG2 7805 PCM5102_MOD 4.7kW 100nF 4.7kW 2GER 5087 LED1 LEDW A K 3GER 4401XAM S1 LED2 LEDT K A LED4 LEDA LED3 LEDB K K A A 3.5" SPI TOUCH SCREEN LCD MODULE WITH 480 x 320 PIXEL RESOLUTION (ILI9488 CONTROLLER, LCD1) ROTARY ENCODER S5 S4 S3 S2 A ON B ON L BUT R BUT siliconchip.com.au Australia's electronics magazine Figs.8 & 9: fit the components on both sides of the PCB as shown here. It’s best to solder the top side components first (starting with the sole SMD, then the axial components) and only fit the switches, LCD screen etc to the underside once all the components on the other side have been mounted and tested. Errors on the PCB cause Button A to start channel B and Button B to have no effect, while LED T/Trig Out is shorted to ground. The two tracks currently going to pins 22 and 23 (GP17 and GND) of MOD1 should be cut and re-routed to pins 21 & 22 (GP16 and GP17), respectively. Also, both tracks currently going to pin 33 (AGND) need to be re-routed to pin 32 (GP27). June 2024  83 Screen 1: the Function Generator provides this web page so it can be controlled remotely via WiFi. to orientate it correctly. Leave IC1 and IC2 off for now. After that, mount REG1 and REG2. While they do not generate substantial amounts of heat, it is worth mounting them with a thin smear of thermal paste between the tabs and PCB. Start by bending their leads down by 90° just after the end of the thick part, insert them into their pads, attach the tab with a machine screw and nut, then solder and trim the leads. Don’t get REG1 & REG2 mixed up, as they have different output voltages but come in the same package type. Now solder all the ceramic capacitors in place. They are not polarised, so their orientations are not critical. Many are 100nF types, but there are other values, so don’t confuse them. Follow with the electrolytic capacitors, which are polarised; in each case, the longer lead should be inserted into the pad nearest the + symbol on the PCB. Fit the DC socket, ensuring it is pushed down fully before soldering its tabs, and you are ready for initial testing. Apply 12V DC to the input and use a DMM set to measure DC volts to check the +5V, +9V and -9V rails. You can use one of the regulator tabs as a convenient ground (negative) reference and probe the Pico’s pin 40 pad (+5V), IC1’s pin 8 (+9V) and IC1’s pin 4 (-9V). Each should be within half a volt of the expected reading. If not, switch off the power and check for incorrectly placed, orientated or poorly soldered components. Assuming all is well, solder or plug in IC1 and IC2, ensuring that pin 1 is 84 Silicon Chip in the correct location in each case. Next, solder in the sockets for the Pico W and PCM5102A modules. The 20-pin sockets for the Pico W and the 6-pin socket for the DAC module may be available pre-made. If not, you can cut them down from longer sockets. The 9-pin socket for the DAC module will probably have to be cut from a socket with at least 10 pins. Cut in the middle of a pin to ensure a clean break. The four RCA connectors are the final components to mount on this side of the board. Ensure they are fully pushed down before soldering their pins. Now move on to the other side of the PCB. Mount the switches and encoder on the rear of the board, as shown in Fig.9. The switches need to have the flats orientated as shown, or they might not work. We will add the LEDs and LCD screen at a later stage. Programming the Pico W Loading software to the Raspberry Pi Pico W is very straightforward. It does not need to be mounted on the PCB for this process. Plug it into any computer (Windows, Linux or Mac) using a suitable USB cable. It will appear as a virtual drive on the system called “RPI-RP2”. If the virtual drive doesn’t appear, unplug the Pico and hold down the white BOOTSEL button while plugging it back in. Copy the 0410421A.uf2 binary file (download at siliconchip.au/ Shop/6/398) onto that drive using the computer’s regular file management tool. The Pico will automatically Australia's electronics magazine reboot and run the uploaded code as soon as the file is transferred. After programming has finished, the Pico will reboot and the drive on your computer will disconnect, at which point you can unplug it. Uploading that file actually did two things: it loaded the software onto the Pico and also some files that are used to generate the web page for remote control (stored in a ‘LittleFS’ file system). We have combined them into a single file to make programming as easy as possible. There is a file in the download package linked earlier called “Pico Production Programming.pdf” that explains how the files can be loaded separately if you are interested. Further testing The main functions can now be tested by plugging the programmed Pico W and PCM5102A module into the board and powering it up. Solder the headers to them if they are not already attached; you can use the sockets on the main PCB as a jig to hold them in place while you do so. Clicking the channel A and B switches should start the Generator producing a 1kHz sinewave at 1V peak-to-peak on channel A and 500Hz at 1V peak-to-peak on channel B. Both signals should have no significant DC offset. A 3.3V 1kHz square wave should also appear at Trig Out. The LCD screen can now be mounted on 6mm spacers. While I used tapped metal spacers in the prototype, plastic or untapped spacers can be used with 12mm countersunk head machine screws and nuts. If your LCD screen has a four-pin header mounted at the SD card holder end of the module, cut the pins off flush with the plastic retaining strip to prevent them from binding on the PCB and RCA sockets. The LCD screen’s pins are only just long enough to reach the PCB pads, so they should be soldered on both sides of the board to ensure good connections. Powering up the unit should now produce the operating display on the LCD. If the screen orientation isn’t correct or it responds to touches erratically, use the touchscreen calibration process described in the PDF manual included in the download package. That should correct any screen rotation and touchscreen alignment problems. siliconchip.com.au Setting up WiFi If desired, the following steps to enable the WiFi functions can be performed later. Edit your WiFi credentials using the touchscreen interface (see Screens 6 & 7) and click the AC button to enable WiFi. When a connection is made to the WiFi LAN, the red LED will change from flashing to constantly on. Don’t switch off the unit for 30 seconds after setting the WiFi credentials to ensure they have been saved to EEPROM. The unit may now be accessed from a web browser at http://dds.local If the firmware program and files have been loaded correctly, the display should look like Screen 1, and the values should update to match those on the LCD screen after a second or so. If not, try a hard reload of the web page by holding down the Shift key while refreshing the page. Apart from the optional calibration step, the unit should now be fully functional. Preparing the case The main depression on the underside of the case is slightly larger than the one on the top, and clearance around the LCD screen is at a premium. So, we use the case upside down, with the four small circular dimples beside the rounded rectangular depression on top. Fig.10 shows the case drilling details; it is also available as a PDF download (siliconchip.au/Shop/11/400). If you print that PDF, ensuring that you do it at “actual size” or 1:1 (not “shrink” or “fit to page”), you can use it as a drilling template. Carefully trim the templates to size, but don’t cut out any holes. Lay the top template on top of the case and prick through the centre of the four LEDs, four switches, four PCB mounting holes, the encoder mounting hole and the corners of the LCD cutout. Next, drill all the holes: 3.5mm diameter for the LEDs and PCB mounting holes and 10mm diameter for all others. After that, make the LCD cutout. Probably the easiest way to do that is to drill a series of small (~3.5mm) holes around the inside of the perimeter, knock the centre piece out, then file the edges smooth. The LCD cutout is intentionally a millimetre or so larger all-round than the actual screen; the decal will cover any gaps. siliconchip.com.au Fig.10: you can mark the case using the dimensions shown on this drilling diagram, or print/copy it at actual size and use it as a template that can be temporarily attached to the case. If using it as a template, prick or drill small holes through the centres of each hole to locate them before drilling. Fig.11: you can download the artwork for these labels from our website, print them at ‘actual size’, laminate them, cut them out and stick them to the case. Australia's electronics magazine June 2024  85 Countersink the four PCB mounting holes so that the tops of the screw heads are flush with the surface of the case. Test-mount the PCB on 10mm spacers. If required, ream out the switch holes in the top of the case to stop them from binding. Once the cover fits neatly with a little clearance around the switches, encoder and LCD, colour around and inside the switch and LED holes, plus the LCD cutout with a black permanent marker. That will stop the grey plastic from being visible through the holes in the decal. Assemble the PCB to the top cover on 10mm spacers. If the LCD mounting screws bind on the inside top of the cover, either drill clearance holes in the cover or gently countersink the screw holes in the LCD’s PCB. Test the LEDs against the inside top of the case. Their tops should protrude by about 1mm. It may be necessary to lightly countersink the backs of the holes in the top of the case if the LEDs don’t protrude far enough. Insert but do not solder the LEDs. The two white LEDs fit above the channel A and B buttons, the blue one (trigger) above them and the red one (WiFi) near the 5V regulator. Mount the PCB into the top of the case and solder in the LEDs. ensuring the flats on the lenses face as shown in Fig.9. If you choose different coloured LEDs, the current limiting resistor values may need to be changed to equalise their brightness. In development, 2.2kW resistors provided adequate brightness for the red and blue LEDs, but the white LEDs needed 4.7kW resistors to reduce their brightness to match the others. Print and laminate the decals (Fig.11), also available as a download at the link above, again ensuring that they are printed at 1:1 scale. Carefully trim their outsides to size. Cut a hole in the main decal for the encoder. A 10mm wad punch does the job neatly. The LED holes can be cut with a 3mm plier punch. Cut the switch holes with an 8mm wad punch to allow for adjustments if the switches are not perfectly centred. You can use a sharp hobby knife if you don’t have punches. Lay the decal in position and check that all the holes align. Make any switch centring adjustments on the decal and punch/cut them to 10mm. If the LED holes in the decal are slightly out of position, make the hole in the case top marginally larger. The decal will cover any scars. Finally, check the LCD screen alignment by feeling for its corners through the decal. Prick the corners of the LCD screen on the decal and remove the unwanted section with a sharp knife. Repeat the process for the rear panel, noting that the exact height of the RCA connectors will vary slightly depending on which version you have used. Prick all the holes through the decal and drill 3mm holes for the power socket and one RCA socket. Loosely fit the back shell and place the unit on the bench, then slide the connector panel up to the RCA connectors and check the alignment of the pilot holes. Make any adjustments to their positions and drill all five holes to 10mm, allowing adequate clearance for the RCA plug shells and the coaxial power plug. Now lay the trimmed decal in its cutout on the rear panel. Holding the assembly up to the light should enable you to establish the correct position of the holes in the decal. Punch the holes with a 10mm wad punch. Colour in and around the holes in the connector cover with a black marker to hide any grey plastic behind the decal. The decals may now be affixed with thin double-sided tape and the encoder knob attached. Stick the small rubber feet onto the bottom of the case. If using the optional acrylic stand, assemble it and place the Generator into the stand to ensure everything is square. Turn the assembly over and Screen 2: a sample of the Function Generator’s display on the 3.5in LCD touchscreen. This one sets the Pulse waveform output parameters. Screen 3: the sweep menu is accessed via the “Swp” button on the main screen. Screen 4: the burst menu lets you set up a channel to switch its signal output on and off at intervals, or have the signal switch between the two channels (the “B alt A” setting). 86 Silicon Chip Australia's electronics magazine siliconchip.com.au Scope 9: when “B alt A” is enabled for burst waveforms, the signal alternates between channels B and A. The idle value for the currently inactive channel is the DC offset for sinewaves, or V Low for other waveforms. put a small drop of superglue at each join. Stick the rubber feet to the crossing points of the stand. The unit is now complete. Operation Basic operation is very straightforward: supply power to the unit and click the channel on/off button to start generating the selected waveform. The white status LED lights when a channel is active. Changing settings is achieved by Screen 5: the Control menu lets you set the phase difference between channels, enable the external trigger input and set the trigger input/output signal polarities. siliconchip.com.au Scope 10: channel A’s signal is inverted in channel B (blue trace) when coupling is enabled and the phase lag is set to more than 0° for step and pulse waveforms. touching the value on the screen and winding the encoder knob. The highlighted digit is changed with the white ‘number position’ buttons under the knob. The left button will move the highlight to a more significant digit, and the right button to a less significant digit. Channel A and B settings are accessed by touching the A or B at the top of the screen (see Screen 2). The selected channel button is highlighted. To change the waveform, touch the waveform label at the top of the screen and select the required function from the drop-down list. Due to its computation requirements, the IMD waveform is only available on channel A. It is possible to set some parameter combinations that are not legitimate; for instance, a sinewave with an amplitude of 10V and a DC offset of +5V. Erroneous parameter combinations are flagged at the bottom of the LCD and web page. Where the combination will cause the unit to clip or Screen 6: the Settings menu lets you calibrate the output levels and provides access to the touchscreen calibration and WiFi settings screens. Screen 7: the communications settings menu (“COMMS”) is accessed via the settings menu by pressing the SET button on the main screen. Australia's electronics magazine June 2024  87 otherwise generate a distorted waveform, the software ensures that the settings are compatible. In the case above, the sinewave’s amplitude value is automatically reduced to prevent clipping. Further details of the handling of problematic setting combinations are provided in the PDF user manual included in the software download package. Across the bottom of the screen are the menu buttons that give access to the sub-menus shown in Screens 3-7. In the sweep (Swp) menu (Screen 3), setting the V/F/D value determines whether channel A waveform’s amplitude, frequency or duty cycle is swept. The Initial and Final values of the swept parameter can then be set. Sweeps can be one-shot or continuously repeated and have linear or logarithmic steps. Logarithms can only be calculated for positive values, so for log sweeps, a value of 0.01 is used when the initial value is set to zero or less. Touching the Sweep button at the top of the LCD screen or clicking the encoder button will start the sequence, as will an external trigger pulse if that function has been enabled in the control menu. Sweep parameters are stored separately for each waveform and V/F/D combination. The “X” button at the bottom right exits the menu and returns to the channel A waveform display. For bursts (Screen 4), set the number of cycles for the channel A signal to be active and idle. Clicking on the Burst button at the top of the LCD screen or clicking the encoder knob will start the burst sequence. One-shot or continuous burst cycles can be generated. Channel B can also be set to alternate with channel A. When the “B alt A” feature is selected, channel B uses channel A’s waveform settings (Scope 9). This setting overrides the value of the Control menu B=A setting. The Con (Control) menu (Screen 5) sets B-to-A channel signal coupling, phase, and trigger input and output functions. For most waveforms, channel B’s output can be set to follow channel A using the B=A setting in the Control menu. For sine, square and triangle pulses, a phase offset from 0-359.99° can be set. For step and pulse waveforms, any phase setting above 0° results in an inverted waveform on channel B (Scope 10). The Set (settings) menu (Screen 6) provides output voltage and touch screen calibration, communication settings, and a factory reset button. To set your WiFi parameters, enter the Com (communications) sub-menu (Screen 7). Replace “mySSID” and “myPass” with your WiFi network’s credentials using the on-screen keyboard and click the AC button to enable the unit to auto-connect to your local WiFi network. The connection process can take several seconds, during which the WiFi LED will flash. Multiple WiFi networks can be stored – instructions for doing that are in the PDF user manual. All parameters are saved to EEPROM within 30 seconds of the last value change. The red WiFi LED will change state for two seconds to indicate an EEPROM save has occurred. accessible via http://dds.local once your WiFi credentials have been entered and activated in the Com submenu. Both channels, the Control and the Burst/Sweep menus are all displayed side by side on the screen. Operation is similar to the LCD screen: click on the value to be changed and wind the virtual knob. The radio buttons below the knob indicate which digit will be changed. Changing true/false or +/- parameters is best accomplished with the units radio button selected (just to the left of the decimal point). More detailed information on the web interface is in the PDF user manual. SCPI remote Control Screen 8: adding the Function Generator to the TestController software is straightforward; select the unit from the drop-down list and add its hostname. Almost all settings and functions can be set and read using SCPI commands. The results of the power on self-test (POST) and the last error message can also be read remotely via the Pico’s serial interface or TCP port 5025 using http://dds.local as the address. The user manual explains the SCPI commands, parameters and results in detail. Using a program such as TestController (siliconchip.au/link/abev) enables automated and repeated testing using one or more remotely controllable instruments. While more complete instructions are available in the user manual, connecting the Function Generator to TestController is as simple as copying two files from the download pack and registering the device on the TestController Load Devices menu (Screen 8). To illustrate the power of automated tests, I have included the script used to test the frequency response of the DAC’s sinewave (Listing 1). It cycles through the DDS frequency range at a set output voltage. After waiting several seconds for the reading to settle at each point, the script reads the value from my Bluetooth-­enabled Owon B41T multimeter and XDS3000 digital oscilloscope, puts the frequency and voltage values into the logging table and proceeds to the next value. The table of readings was exported to Excel for analysis, though it could also have been performed in TestController. These values were used to produce the Fig.3 frequency response plot published last month (after correcting for the B41T’s frequency response). Australia's electronics magazine siliconchip.com.au 88 Silicon Chip Web interface The web interface (Screen 1) is The finished WiFi DDS Function Generator. The touchscreen is used to select functions, while the knobs and buttons let you set values and turn the channels on or off independently. After each tweaking of the settings, the automated tests ran in the background, saving hours manually adjusting the frequency and jotting down the results. With a little more effort, I could have used the ‘math’ functions in TestController to plot the final response curve. Further information on using TestController can be found in my April 2023 article on that software, see: siliconchip.au/Article/15740 Calibration Uncalibrated, the unit’s output voltages should be accurate to within 1%, with any error due to resistor tolerances in the buffer amp. If greater accuracy is required, set both channels to PULSE mode and set both V High and V Low to 5.00V. At least one of the time values should have a nonzero value. Start both channels and enter the LCD touch screen Set menu (Screen 6). Enter the voltages measured on the output pins in the respective fields, then touch Save and restart each channel’s output. The output voltages will now reflect the new calibration settings. Wait 30 seconds before turning siliconchip.com.au the unit off to ensure the settings are permanently saved. Conclusion The use of modules simplified the design and construction of what could otherwise been a substantially more challenging project. The PCM5102A module avoids soldering the DAC chip’s finely spaced pins and allows optimum component placement around the main DAC chip. Similarly, the Raspberry Pi Pico W is an inexpensive, highly functional WiFi-capable microcontroller that is much simpler to program than the ESP32 used in earlier instruments in this series. Using these two modules allowed the project to almost avoid soldering surface-mounting components altogether. This may bring the project within reach of those who don’t have easy access to, or confidence with, SMD components. Providing remote control capability extends the usefulness of the unit where access to the LCD screen controls is difficult. Importantly, it also allows it to be teamed up with other test instruments for automated SC testing. TestController sinewave frequency response script ; DDS to B41T multimeter and DSO =var sVal=20 ; create a control variable #log 4 ; log readings every 4 seconds #while (sVal<70000) PlatyDDS:::SINE:FREQ (sVal) #hasLogged ; wait for the log delay to expire =sVal=(sVal*1.2) ; exponential frequency increment #endwhile #log 0 ; stop logging Listing 1: this TestController script geometrically steps the unit’s output frequency from 20Hz to 70kHz while logging the output levels via separate instruments. Australia's electronics magazine June 2024  89