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Pt.2: By Jim Rowe Arduino-based Multifunction 24-Bit Measuring System Last month, we introduced our new Arduino-based Multifunction Meter (MFM) and gave the circuit details. This month, we describe how to install the software and firmware that’s needed to control it from a desktop or laptop PC. We then explain how to get it going, how to calibrate the various ranges (if you have the facilities) and how to use the finished unit. A S MENTIONED in Pt.1, a couple of pieces of software need to be installed on your PC in order to use the MFM. In addition, a “sketch” (Arduino-speak for firmware program) has to be uploaded into the flash memory of the Arduino to enable it to carry out its tasks. This is detailed in the software block diagram of Fig.7. The large box at left represents a PC (desktop or laptop) running Windows XP/SP3 or later (you’re not still using Windows XP, are you?). The MFM is shown over on the right, linked to the PC via a USB cable. The MFM Control and Display Application (upper left) needs to be 82  Silicon Chip installed on the PC, together with a virtual COM port driver (lower right in the PC box) to allow it to communicate with the Arduino in the sampler. The Arduino IDE (integrated development environment) also needs to be installed in your PC, at least temporarily, in order to upload the MFM sketch to the Arduino module. Here’s the step-by-step procedure: Step 1: download and install the Arduino IDE from the main Arduino website at https://www.arduino.cc/ en/Main/Software. We’ve been using the 1.6.5-r2-windows.exe version but there may be a later version available by the time you read this. There’s also a zipped-up version. The Arduino IDE comes with a USB virtual COM port driver to suit the Arduino Uno and this is installed in the “Drivers” folder of the IDE installation. As a result, if you are using an Arduino Uno in your sampler, you’ll already have its matching USB port driver. Alternatively, if you’re using a Freetronics Eleven, go to www.freetronics.com.au and download their USB driver. At the time of writing, this was in a zip file called FreetronicsUSBDrivers_v2.2.zip. Unzip this and make a note of where the files have been extracted. Step 2: plug the cable from your MFM siliconchip.com.au DC INPUT SOCKETS SILICON CHIP MULTIFUNCTION METER CONTROL & DISPLAY APPLICATION +HV ARDUINO IDE (NEEDED TO UPLOAD MFM SKETCH FIRMWARE TO THE ARDUINO) +LV MULTIFUNCTION METER SHIELD (PCB MODULE) – (+5V) WINDOWS OPERATING SYSTEM AND GUI (GRAPHICAL USER INTERFACE) ARDUINO USB VIRTUAL COM PORT DRIVER ARDUINO UNO, FREETRONICS ELEVEN OR DUINOTECH CLASSIC (WITH MFM SKETCH IN FLASH RAM) (USB CABLE) DESKTOP OR LAPTOP PC AF INPUT SOCKET FROM RF SENSING HEAD MULTIFUNCTION METER Fig.7: the software block diagram for the MFM system. The MFM connects to a Windows PC via a USB cable and is controlled by an application (app) running on the PC. The Arduino IDE software is required to upload the MFM “sketch” firmware to the Arduino module in the MFM. into one of your PC’s USB ports. The MFM’s power LED should immediately turn on but Windows may not successfully install the driver right away. Step 3: open up the Windows Device Manager. If you see a yellow error icon alongside an “Unknown device” listing, the driver has not installed automatically. If you then right-click the device and select Properties, it will be shown as either not working properly or not installed. To fix this, select “Update Driver” in the Properties dialog and then “Browse my Computer for Driver Software”. Then browse to either the Drivers folder of your Arduino IDE installation (to get Arduino’s Uno driver) or, alternatively, to the folder where you unzipped the Freetronics driver software. In either case, you should be able to find the .inf file that Windows needs to install the driver. Once it’s installed, the Device Manager should then indicate that the device is working properly. Step 4: go to the SILICON CHIP website (www.siliconchip.com.au) and download both the Windows software for the MFM Control & Display App (SiliconChipMFM.zip) and the matching Arduino firmware sketch (ArduinoMFMSketch.ino). The firmware sketch should be saved in a sub-folder called “Arduino sketches” in your PC’s Documents folder. That done, launch the Arduino IDE, direct it to that folder to find the sketch, open it, compile it and then upload it to the flash RAM in your MFM’s Arduino. You will find that this is all quite straightforward. siliconchip.com.au Step 5: unzip the SiliconChipMFM.zip file to get the install package (SiliconChipMFM.msi), then run it to install the “Windows MFM Control and Display” application. You should then be ready to roll with your new 24-bit MultiFunction Meter. Using the Windows app Apart from the range selection (done via rotary switch S1), all functions on the MFM are controlled using the MFM Control and Display application. This is easy to use because when you fire it up, a GUI (Graphical User Interface) window appears (see Fig.8) which provides combo-boxes along the top so you can set the configuration of the MFM-PC serial link; ie, the (virtual) COM port to which it’s connected and the baud rate (115,200). There’s also a third combo-box which allows you to select the external load resistance you will be using, if you intend using either the AF or RF level and power ranges. However, you don’t have to worry about doing this if you simply intend measuring DC voltages. Just below this top row of combo boxes is a box labelled “Sampling interval:”. This allows you to select the sampling rate to be used when taking continuous or repetitive samples. You can choose from 13 different sampling intervals, ranging from 200ms (five samples per second) to 60s (one sample per minute). Immediately to the right is a small check box with the label “Live reading”. Clicking on this check box allows you to view MFM measurements in real time, at the same rate as that selected for continuous sampling. When “Live reading” is enabled, the measurements are displayed just to the right of the “Live reading” label itself, with each successive reading replacing the previous one. By the way, “Live reading” may be enabled at the same time as continuous sampling, with the data for each measurement being displayed at top right as well as on the next available line in the main text box. Further down the GUI window, you’ll find two rectangular control buttons with red borders and red text labels reading “Take a Sample” and “Start Sampling” respectively. These allow you to either take a single measurement sample or to begin taking a series of samples at a rate corresponding to the selected sampling interval. To the right of these two control buttons is a “Range Selected:” label, followed a text box which will initially be blank. However, when you begin taking measurement samples, this text box will show the MFM range that’s been selected via range switch S1 on the MFM itself. The lower portion of the GUI window is taken up by a text box which displays the measurement samples as they are made. Each sample is on a separate line and is preceded by the date and time at which it was taken. In addition, when you first start sampling, the application displays a header line at the top showing not only the date and time but also the virtual COM port and serial data rate being used, plus the sampling interval May 2016  83 LT1019ACS8-2.5 has an initial accuracy of ±0.05%. Next, launch the Arduino IDE in your PC, open up the MFM firmware sketch (ArduinoMFMSketch.ino) and move down the sketch listing until you get to the start of the main loop; ie, a line which reads: void loop() { The next line should read: const float Vref = 2.50000f; //Vref = the ADC reference voltage Fig.8: the MFM Control & Display application lets you set the virtual COM port and baud rate for the MFM-PC serial link. It also allows you to select the load impedance and the sampling interval and to either take a single sample or a series of samples at the selected rate. The app is shown here displaying Vrms and dBm readings on the AF Level & Power range. selected. This is to make the MFM and its control application more suited for measurement data logging. Making measurements To take a measurement or a series of measurements, all you need to do is click on either the “Take a Sample” button or the “Start Sampling” button. The data then appears in the main textbox. If you click the “Start Sampling” button, the unit will continue to take further measurements at the selected time interval. During this time, the button label also changes to read “Stop Sampling” and you can stop the sampling simply by clicking this button again. So that’s how simple it is to use our MFM Control and Display app for taking measurements. But what about saving the measurements? Easy – just click the “File” drop-down menu at top left on the window and you’ll see a number of handy options for saving, reopening or printing out your current collection of measurement samples. There’s also an option called “New”, for “clearing the slate” before taking another set of samples, plus the usual option of closing the application itself at the end of the job. These all work the same way as for other Windows applications, so you shouldn’t have any problems. MFM set-up & calibration There are no trimpots or other hard84  Silicon Chip ware items to adjust in order to calibrate the MFM’s DC voltage ranges. That’s because we’re taking advantage of the accuracy built into the LTC2400 24-bit ADC, its companion LT1019ACS8 voltage reference and the 0.1% tolerance resistors used in the input dividers. These will give you an accuracy of ±0.06% (±1.25mV) on the basic 2.5V range and ±0.5% on the three higher ranges (ie, ±125mV on the 25V range, ±1.25V on the 250V range and ±5V on the 1000V range), without any adjustments. However, if you happen to have access to a high-accuracy DC voltmeter, it can be used to measure the exact reference voltage being provided by the LT1019ACS8 voltage reference in the MFM. You can then make a single change in the MFM Arduino’s firmware sketch to improve the accuracy of its basic 2.500V DC range (and of all the other ranges by default). If you want to do this, first remove the control knob and the lid of the MFM, then plug its USB cable into your PC and use the high-accuracy DC voltmeter to measure the voltage from REF1, the LT1019ACS8. The easiest way to do this is to place your meter’s test probes across ZD1, the 3.9V zener diode located just behind REF1. Be sure to write the measured reference voltage down carefully, using as many significant digits as your voltmeter provides. The reading should be somewhere between 2.49875V and 2.50125V, because the Replace “2.50000” in the above line with your own measured reference voltage, then use the Arduino IDE software to recompile the sketch and upload it to the Arduino in your MFM. This will make its 2.5V DC range and all of the other ranges as accurate as possible without access to a full-scale calibration lab. Audio Level & Power range Unlike the DC voltage ranges, the Audio Level & Power range does require some hardware set-up and adjustment. To do this, you need an audiofrequency sinewave signal of at least 10VAC (RMS) and some way of accurately measuring its level. One possibility is to use an audio signal generator with an output meter and attenuator, or you could use an uncalibrated audio oscillator with a separate audio level meter to monitor its output. Another possibility is to use an AC plugpack (ie, one with a conventional iron-core transformer) with a 9-24VAC secondary to provide the sinewave signal, together with an RMS-reading AC voltmeter (to measure the output voltage). If you use this latter approach, you’ll be doing the set-up at 50Hz but that’s OK since 50Hz is well inside the audio range. On the other hand, if you are using an audio signal generator, it’s probably a good idea to set its frequency to around 1kHz. Once you have a suitable signal source, just follow this step-by-step procedure to set up and adjust the MFM’s Audio Level and Power range: Step 1: connect the MFM to your PC and check that it’s operating normally. Step 2: launch the MFM Control & Display app and set it up to communicate with the MFM via the previously installed virtual COM port at 115,200 baud. Step 3: move the MFM’s range switch siliconchip.com.au RF Level & Power range This range is a bit simpler to calisiliconchip.com.au POWER 2.50V DC USB LINK TO PC 25.0V DC RF LEVEL & POWER 250V DC 1000V DC AUDIO LEVEL & POWER SILICON CHIP RF HEAD END AUDIO INPUT USB LINKED MULTIFUNCTION 24-BIT MEASURING SYSTEM DC VOLTAGE INPUTS – +2.50V/25.0V +250V/1000V BNC CABLE ENDING IN TEST CLIPS AUDIO LOAD (8 Ω, 16 Ω, 32 Ω OR 600 Ω) AUDIO SIGNAL SOURCE (AMPLIFIER, ETC.) LOW LOSS CABLES CONNECTING AMPLIFIER OUTPUT TO LOAD CLIP FROM CABLE SCREENING BRAID CONNECTS TO EARTHY TERMINAL Fig.9: the MFM can be used to measure power levels from an audio amplifier using the configuration shown here. Be sure to use low-loss cable to connect the load and be careful not to short the amplifier output terminals. 50 Ω SMA TERMINATION (OR CABLE TO HIGH POWER TERMINATION) OUTPUT TO MFM SILICON CHIP RF MEASURING HEAD FOR MFM SMA ‘T’ CONNECTOR CABLE TO CON4 OF MFM RF INPUT to the fully anticlockwise “Audio Level & Power” position, then fit a 50Ω termination plug to the Audio Input BNC connector (this will ensure a nominal input of near enough to zero). Step 4: select a short sampling interval (eg, 200ms) and enable the Live Reading display. You’ll probably initially see dBV/Vrms figures that are somewhat higher than they should be with the input terminated in just 50Ω. Step 5: use a small screwdriver to adjust trimpot VR1 (Intercept Adjust) via the small hole in the case immediately to the left of the Audio Input BNC connector to achieve minimum readings. Ideally, you’ll be able to get readings well below -47.5dBV and 4.2mV RMS. Step 6: remove the 50Ω plug from the Audio Input connector and connect your audio signal source in its place. Step 7: make small adjustments to trimpot VR2 (Slope Adjust) via the small hole just to the right of the Audio Input BNC connector. Make the Vrms reading as close as possible to the voltage of the input signal. If you can’t achieve the known Vrms reading, you may need to repeat Steps 3-5 to make further small adjustments to trimpot VR1. For example, if your reading in Step 7 remains stubbornly higher than the known input level, try reducing the “zero input” reading by a small amount using VR1 before coming back to Step 7. Conversely, if you are unable to increase the reading in Step 7 to reach the known input level, increase the “zero input” reading by a small amount. It should then be possible to achieve the known input level reading by adjusting VR2. Once the known input level reading has been achieved, the Audio Level and Power range has been correctly set up and calibrated. Note that its accuracy will depend on the accuracy of your audio signal generator’s level meter (or the accuracy of your AC voltmeter). By the way, we ignored readings other than dBV and Vrms in the previous steps because the other values are calculated from these (taking into account the selected load impedance). That means that it’s only necessary to have the load impedance setting correct if you are actually making dBm and power readings. INTERCEPT ADJUST CABLE FROM RF SOURCE (SIG GEN, TRANSMITTER ETC.) Fig.10: this diagram shows how to connect an RF signal to the MFM RF Measuring Head. Note that the signal is connected to a 50Ω load as well to the relevant input using a “T” connector. brate since it’s only necessary to adjust trimpot VR3 (Intercept Adjust) inside the companion RF Measuring Head. This time, you will need an RF sinewave signal source (preferably unmodulated) and again some way of accurately measuring its level. You can either use an RF signal generator with an output meter and an attenuator or an uncalibrated RF oscillator or transmitter with a separate RF level meter to monitor its output. May 2016  85 calibrating the “RF Level & Power” range, the RF signal source must be terminated at the input to the MFM head end. That’s because if cables carrying RF signals are not terminated with the correct impedance at both ends, there will be reflections. As a result, standing waves can develop in the cable, leading to measurement errors. This isn’t necessary when calibrating the “AF Level & Power” range. In fact, given the amount of power that may be dissipated in the load resistance, you will normally connect the load directly to the Device Under Test (DUT) using heavy cables and run a smaller, shielded cable from the output terminals of the DUT to the MFM input. Fig.11: the display app automatically shows the range selected on the MFM, immediately to the right of the sampling buttons. It’s shown here displaying a number of voltage measurements at 1s sampling intervals. The frequency of the generator/oscillator should be set to around 1MHz, or at least no higher than 10MHz. As before, here’s the procedure you need to follow, step-by-step: Step 1: plug the cable from the RF Measuring Head head into the 3.5mm “RF Head End” socket on the MFM. Step 2: connect the MFM to your PC and check that it’s operating correctly. Step 3: launch the MFM Control & Display app, set it to communicate with the MFM via the listed COM port at 115,200 baud and with a load resistance of 50Ω. Step 4: set the MFM’s range switch S1 to the “RF Level & Power” position. Step 5: connect your RF signal source to the RF Head’s input using an SMA “T” connector, as shown in Fig.10. Also, fit a 50Ω dummy load with an appropriate power rating to the other end of the “T” connector. Step 6: select a short sampling interval and enable Live Reading. As before, the figures you now need to check are for dBV and Vrms. They will probably be either too high or too low, compared to the level set on your RF signal source. Step 7: use a small screwdriver to adjust trimpot VR3 (Intercept Adjust) via the small hole in the top of the RF Measuring Head until the dBV and Vrms values are as close as possible to the level set by your RF signal source. Points to note When setting the level of your RF signal source, make sure that its output cable is terminated in a 50Ω load. Most signal generators are calibrated this way but if you have one that isn’t, you can be fooled into setting the MFM to read high by up to 6dB. Another point to note is that when What To Do If You’re Missing A DLL The Windows installer for the MFM software package includes a required set of DLLs (Dynamic Link Libraries), referred to as the “C Runtime Library” (CRT for short). They contain support routines required by software developed using Microsoft Visual Studio using C or C++. When you run the installer we supply, it checks to see if you already have the required version of the CRT DLLs on your system and if not, installs them. But Microsoft has recently changed the way the CRT DLLs work. To avoid having to install a whole new set of DLLs for software 86  Silicon Chip built using each different version of Visual Studio, they now supply a set of “Universal CRT” DLLs with Windows. Developers then only need to bundle a smaller set of DLLs with their software, which in turn rely on the Universal CRT. Windows 10 comes with these Universal CRT files out of the box but for Vista, Windows 7 and Windows 8, they are instead installed by Windows Update. For some reason, it seems that some PCs still lack these files, even with all the latest updates installed. If you try to launch our software on such a PC, Load resistors The main thing to bear in mind when making dBm or power measurements is that the MFM does not have inbuilt load resistors. This applies to both the Audio Level & Power range and the RF Level & Power range. It would have been very complicated to do this and would not have suited all situations, so the MFM must be used with external load resistors for both these ranges. If you already have AF or RF dummy loads with adequate power ratings for the power levels you want to measure, these can be used without modification. On the other hand, if you do need to acquire one or more dummy loads of a particular impedance and/ or power level, they can be purchased or built for minimum outlay and no modifications to the MFM itself will be required. For example, 4Ω and 8Ω wirewound resistors with ratings of 50W and 100W are relatively cheap and are available from multiple suppliers. you get the following error message: The program can’t start because api-mswin-crt-I1-1-0.dll is missing from your computer. Try reinstalling the program to fix this problem. If this happens, check that your PC has the latest updates installed. If so, you will need to download and install the Universal CRT files yourself. They can be found at the following location: www.microsoft.com/enus/download/details.aspx?id=50410 If you’re still using Windows XP (despite the fact that it’s no longer supported!) this will almost certainly be necessary. siliconchip.com.au RF Measuring Head Modification IN L VR3 2k COM 2 INT OFS SLEEVE 3 100nF CON7 100nF * USING A STANDARD 3.5mm PLUG/3.5mm PLUG STEREO CABLE 33pF 100nF RF IN SC RF HEAD TO MFM 1206 4.7Ω 47nF 20 1 6 R 102 C C 62016 21061140 04116012 TIP 1.5k 1206 5 RING 1206 100nF 100nF IC3 1 OUT 1.5k AD8307 560Ω IC3 AD8307 ARZ 4 1206 1 1206 1206 1206 IN H EN 47nF 560Ω 200k VPS 47nF 8 INTERCEPT ADJ (CAL) 6 7 NP0 1206 200k 47nF S T 1206 33pF 200k TO CON4 ON MFM SHIELD* 1206 CON6 100nF 200k RF INPUT CON7 4.7Ω INT ADJ VR3 2k CON6 ADDED CAPACITOR FOR ARDUINO MFM SHIELD Fig.12: the RF Measuring Head needs to be modified by adding a 33pF NP0 capacitor between pin 8 (INH) of IC3 and ground, as shown here. This capacitor swamps the stray capacitance across the two 200kΩ input resistors and ensures that the RF Measuring Head remains accurate for frequencies above 10MHz. Further testing of the MFM’s RF Measuring Head has revealed the need for a 33pF NP0 capacitor to be added between the pin 8 (INH) of IC3 and ground. Without this capacitor, the very small stray capacitance across the two paralleled 200kΩ input resistors causes the signal level into pin 8 of IC3 to increase as the input frequency rises above about 10MHz. In other words, the input voltage divider’s division ratio gradually falls below its nominal low-frequency level of 158:1. Adding the 33pF capacitor compensates for this stray capacitance by forming an additional, parallel capaci- There is one further small point to remember when making RF level and/ or power measurements via the RF Measuring Head: be sure to connect the RF Head to the MFM (ie, connect CON5 and CON7) before you plug the MFM’s USB cable into your PC. The reason for this is that the +5V line can be briefly shorted to earth when these plugs are fitted into their sockets, so it’s safer to make the connections before power is applied to the MFM. Analysing & plotting data The MFM Windows software saves sample data in a CSV format which can be loaded into Microsoft Excel, LibreOffice/OpenOffice Calc or just about any spreadsheet software. It’s then a simple matter to plot or analyse this data. siliconchip.com.au tive voltage divider, maintaining the input division ratio at its correct level for higher frequencies. It ensures that the calibration of the MFM’s RF Measuring Head is maintained up to the limit of the AD8307 log detector’s measurement capability (around 500MHz). Fig.12 shows the revised circuit and PCB layout diagrams for the RF Measuring Head. If you have the latest PCB version, just follow Fig.12 to build the unit as shown, with the extra capacitor. Alternatively, if you have one of the earlier RF Head PCBs, it’s quite easy to add the 33pF capacitor. First, scrape away a small patch of green resist coating from the top layer earth copper, to For example, after opening a CSV file saved from the MFM App in Calc, simply click OK on the Import dialog and the data should appear, with column “A” containing the time stamp for each sample, column “B” the sample number (starting with 0 for the first sample in each sequence), column “C” the number of milliseconds that each sample was taken relative to the first and the remaining columns (“D” and so on) containing the sample values. If you want to plot a set of samples, first click on column “C” just before the first sample you’re interested in, which should read “Milliseconds”. Then, shift-click on the data you want to plot in the last row, which may be in one of the columns titled “V”, “dbV”, “Vrms” and so on. Then, under the “Insert” menu at the the left of pin 8 of IC3 and just below the 100nF bypass capacitor on pins 6 & 7. Tin this bared copper area (to provide for the capacitor’s earth connection), then scrape away the green resist on the copper above the 47nF input capacitor (the one going to pin 8 of IC3) and tin this small area of bared copper as well. We soldered a 1.5 x 2.5mm rectangle of flattened, thin (eg, 0.15mm) copper foil to the exposed copper to make a pad for the 33pF capacitor. However you could just solder the capacitor directly across the two areas of copper that are now exposed. Its final location should be very close to that shown in the revised PCB overlay diagram, Fig.12. top of the window, go to the “Object” sub-menu and click on “Chart”. There are various types of chart to choose from, we suggest starting with “X-Y (Scatter)”. You can then select either Points, Points and Lines or Lines and click Next twice. Select any data series you don’t want to plot in the box at left and click “Remove” to get rid of them. Then click Finish and the chart should appear. You can make further changes; see the documentation and internet forums for more details. Note that both LibreOffice Calc and OpenOffice Calc are free downloads. They can also analyse this data, doing calculations such as variance analysis, correlations, moving averages and so on (as can Excel). Check the documentation of these software packages for SC more information. May 2016  87