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Arduino Multifunction 24-Bit Measuring Shield Here’s a low-cost PC-linked measuring system project which provides four accurate DC voltage measurement ranges together with an audio frequency level and power meter, plus an optional RF level and power meter which can operate to 500MHz. W ANT TO accurately measure voltages, decibels and power levels using an Arduino? This design combines a 4-range, 24-bit DC voltmeter with both audio frequency and RF level and power meters. The audio meter can measure up to 60V RMS (up to 900W into a 4Ω load) while the RF section will measure up to 1kW into a 50Ω load, with a frequency range extending up to around 500MHz. 62  Silicon Chip Apart from the RF measurement head, everything fits into a small diecast aluminium box (119 x 94 x 57mm) which is hooked up to a PC via USB. No separate power supply is required. The RF Head is tiny at just 51 x 51 x 32mm and connects to the main box via a standard 3.5-to-3.5mm tip/ring/sleeve (TRS) (or stereo phono jack) cable. The unit is designed to be controlled from a PC using the Windows software we’ve written (for Windows 7 or later) but you could also write your own Arduino “sketch” to suit other measurement tasks. In short, this is a seriously useful and accurate test instrument that can be built at moderate cost. Refer to the specification panel for more details. What’s inside the box Fig.1 on the facing page shows a siliconchip.com.au Pt.1: By Jim Rowe CON1 4.742M 1000V +HV INPUT 36k USB CABLE TO PC 250V 25V 12k 450k 0V INPUT 50k AF LEVEL & POWER The MFM shield PCB is housed in a metal diecast case measuring 119 x 94 x 57mm, while the optional RF Head PCB is housed in a small diecast case measuring 51 x 51 x 32mm. RF LEVEL & POWER +LV INPUT CON2 24-BIT ANALOG TO DIGITAL CONVERTER (ADC) (IC1, REF1) S1 2.5V ARDUINO UNO OR ELEVEN +5V CON3 λ FROM RF DETECTOR POWER +5V CON4 CON5 AF INPUT LOGARITHMIC AMPLIFIER & DETECTOR (IC2) +5V Fig.1: a simplified schematic showing the general operating principle of the Multifunction Meter. There are four inputs: low and high-voltage DC, audio level/power and RF level/power. S1 connects these inputs to a high-precision analog-to-digital converter (ADC) which is controlled by the Arduino. This in turn passes the measured values on to the PC for logging or display. IC2 provides AC-to-DC conversion for audio signals while an identical chip mounted off-board (but configured differently) feeds in RF level and power measurements via jack CON4. simplified block diagram of the Multifunction Meter (MFM). At its heart is an Arduino Uno or compatible (eg, Freetronics Eleven or Duinotech Classic). This is shown at upper right and it controls the rest of the circuitry, including the USB link to the PC. The USB cable also provides 5V DC to power all of the meter’s circuitry. Coupled closely to the Arduino is the digital sampling “engine” shown to its left. This comprises a Linear Technology LTC2400 24-bit precision ADC (analog-to-digital converter), together with an LT1019ACS8-2.5V precision voltage reference. This combination forms a high-resolution, high accuracy digital DC voltmeter with a basic range of 0-2.5V, a resolution of 150nV (nanovolts!) and a basic accuracy of around ±0.06% (ie, ±1.5mV). The LTC2400 is a very impressive device. It comes in an SO-8 SMD package, uses delta-sigma conversion technology and connects to the Arduino via a flexible 3-wire interface compatible with SPI and Microwire communication protocols. Other nice features include a built-in pin selectsiliconchip.com.au The MFM is based on an Arduino Uno or compatible, such as a Freetronics Eleven as pictured here. able notch filter providing better than 110dB rejection at either 50Hz or 60Hz (ie, to reject mains hum fields), very low offset and noise and a low supply current of only 200µA. It operates from a single 2.7-5.5V supply. Range switch S1 controls the connections between the ADC and the various inputs and has six positions. Four of these are for the DC voltage ranges, while the remaining two are used for the Audio Level & Power and the RF Level & Power ranges respectively. In the third position of S1, the +LV input connector CON2 is connected directly to the input of the ADC, giving a measurement range of 0-2.5V. In the next position, the ADC is connected April 2016  63 Features & Specifications • • Description: A PC-linked digital measurement system combining the functions of an accurate DC voltmeter, an audio level and power meter and an RF level and power meter. PC link is via USB with an adjustable sampling rate. The application software allows saving data in CSV format for later loading, plotting or analysis. Power supply: All power comes from the host PC. Draws less than 65mA from the USB port (<325mW <at> 5V). DC Voltmeter • • • • Four ranges: 0-2.5V, 0-25V, 0-250V and 0-1000V. Resolution: 24 bits (1 part in 16,777,216) corresponding to 150nV, 1.5µV, 15µV and 60µV for each range. Basic accuracy: approximately ±0.06% on 2.5V range (±1.5mV), ±0.5% on higher ranges (±125mV, ±1.25V & ±5V respectively). Input resistance: 500kΩ on the 2.5V and 25V ranges, 4.79MΩ on the 250V and 1000V ranges. Audio Level & Power Meter • • • • • Input range: 4.2mV RMS (-37.5dBV) to 60V RMS (+35.5dBV) (83dB range) Frequency response: from below 20Hz to above 200kHz. Input resistance: 60kΩ. Power readout: Watts, dBm & dBV for load impedances of 600Ω, 75Ω, 50Ω, 32Ω, 16Ω, 8Ω, 6Ω, 4Ω or 2Ω. Maximum readings: 600Ω: 6W, +37.8dBm 75Ω: 48W, +46.8dBm Artwork for the two labels can be 50Ω: 72W, +48.6dBm downloaded from the SILICON CHIP 32Ω: 112.5W, +50.5dBm website (www.siliconchip.com.au) 16Ω: 225W, +53.5dBm as a PDF file, free to subscribers. 8Ω: 450W, +56.5dBm Print these out (or photocopy Fig.6), 6Ω: 600W, +57.8dBm then laminate them in clear plastic for protection and finally attach them 4Ω: 900W, +59.5dBm using thin double-sided tape. 2Ω: 1800W, +62.5dBm Front Panel Artwork RF Level & Power Meter • • • • • • Input range: 15.8mV RMS (-36dBV) to 223.6V RMS (+47dBV) (83dB range) Frequency response: approximately 10kHz to 500MHz. Input resistance: 101kΩ. Power readout: Watts, dBm & dBV for load impedances of 75Ω or 50Ω (600Ω, 32Ω, 16Ω, 8Ω, 6Ω, 4Ω and 2Ω modes also available). Maximum power readings: 75Ω: 667W, +58.2dBm 50Ω: 1000W, +60.0dBm Minimum readings: 75Ω: 3.33µW, -24.8dBm 50Ω: 4.99µW, -23.0dBm via a 10:1 voltage divider, giving a measurement range of 0-25V. In either case, the load impedance is 500kΩ. In the fifth and sixth positions of S1, the ADC input is connected to the 64  Silicon Chip +HV input at CON1 via a 2-step voltage divider giving a division ratio of 100:1 for the fifth position and 400:1 for the sixth position. This gives two further DC voltage ranges of 0-250V and 0-1000V respectively. The first position of S1 is used for the Audio Level & Power function, with the signal fed into a separate BNC input socket. Signal processing is performed by IC2, an Analog Devices AD8307ARZ logarithmic amplifier/ detector device which uses a progressive compression/successive detection technique to provide a dynamic range of more than 88dB with a linearity of ±0.3dB, at all frequencies between 20Hz and 100MHz. In effect, the AD8307 is a wideband AC-to-DC converter with a logarithmic transfer function. It converts AC signals to a DC output voltage which varies from 0.25V to 2.5V, with a nominal slope of 25mV/dB. This can be adjusted via external components, as we shall see shortly. Finally, the second position of switch S1 connects the ADC input to the tip of CON4, which interfaces with the RF Detector Head. We haven’t included this circuitry in Fig.1 but it will be covered below. It’s based on another AD8307 logarithmic amplifier detector but with a different configuration, to allow it to operate at frequencies up to about 500MHz. Circuit description Fig.2 shows the full circuit diagram for the Multifunction Meter’s main “shield” PCB, ie, everything apart from the Arduino itself and the RF Head. The Arduino interface is via the SIL header pins shown on the right-hand side of the circuit, while the RF level and power measurement head connects via CON4 at centre left. More on this shortly. Fig.2 includes the LTC2400 24-bit ADC (IC1) and LT1019ACS8-2.5 2.5V reference (REF1) which were both mentioned previously. Pin 3 of IC1 is the analog input (VIN), with Schottky diodes D1 & D2 providing input overvoltage protection, in conjunction with a 1.5kΩ series resistor. The 2.5V reference from REF1 is fed to pin 2 of IC1 (VREF), while ZD1 and the series RC circuit between this pin and ground both reduce any noise present in the output of REF1 and limit any voltage rise of the 2.5V reference output due to an accidental input voltage overload. Pin 8 of IC1 is used to set its internal notch filter to either 50Hz or 60Hz, by connecting it to +5V or ground via JP1. Pins 5, 6 & 7 of IC1 form the SPI serial siliconchip.com.au CON1 0.1% 499k DC VOLTS INPUT 0.1% 499k 0.1% 499k 0.1% 499k 0.1% 499k 0.1% +5V 10k 36k JP1 0.1% 0.1% 499k 0.1% 50Hz 2.0k 499k 499k 0.1% (4 x 100k + 50k) 0.1% 250.0V (TOTAL = 450k 0.1%) – RF POWER 50k 0.1% CON3 1.5k D1, D2 1N5711W -7-F K A Vcc SCK IC1 LTC2400 VIN 2 SDO VREF CS 6 7 7 470Ω 8 6 A POWER LED1 5 λ GND A K 4 K 100nF ZD1 3.9V TIP REF1 LT1019ACS8-2.5 6 2 1W SLEEVE IN OUT 5.6Ω 5 TRIM TMP GND A 4 4 5 +5V 6 3 1 16V 10Ω 2 +5V 3 CON5 100 µF 5 6 39k 6 7 22 µF 20k 8 VPS IN H 470pF 1 10k VR1 2k 4 RANGE SELECTOR SWITCH 100nF INTERCEPT ADJUST IN L EN IC2 AD8307 ARZ OUT INT OFS 5 3 22 µF 8 SLOPE ADJUST VR2 50k 1 2 1 µF COM 2 7 S1b 4 1 µF (SS) 3 4 33k (MISO) 5 (SCK) 6 7 LED1 ZD1 K A SC 20 1 6 1N5711W-7-F K K A RST 3.3V 5V GND GND Vin A0 A1 A2 A3 A4 A5 DIGITAL I/O 100nF 10 µF CON4 AF INPUT 1 2 +2.500V IOREF ANALOG INPUTS 3 K RING 5 F0 3 +5V TO RF POWER MODULE 4 RFC1 100 µH 8 AF POWER CON2 2 3 2.500V +LV 10 µF 1 S1a 25.00V 100nF 60Hz 0.1% 0.1% POWER 1 SET NOTCH FREQ 1000V 750k IC1, IC2, REF1 9 8 4 1 A 8 10 IO0 RXD IO1 TXD IO2 IO3 PWM IO4 IO5 PWM IO6 PWM IO7 IO8 IO9 PWM IO10 PWM IO11 SIL HEADER PINS IN THIS AREA MATE WITH HEADERS ON ARDUINO UNO OR COMPATIBLE (8 x 499k + 750k = 4.742M) +HV PWM IO12 IO13 GND AREF SDA SCL ARDUINO MULTIFUNCTION 24-BIT MEASURING SHIELD Fig.2: the full circuit diagram for the MFM shield (the optional RF power head and the components on the Arduino PCB are not shown). The input switching and attenuation circuitry is shown at upper left, while the 24-bit ADC (IC1) and associated components is at upper right. The audio level and power measurement circuitry is based on IC2 at lower left, while the Arduino interface headers are on the righthand side of the circuit. interface connecting it to the Arduino. Pin 5 is the “slave select” (enable) input, while pin 7 is the serial clock (SCK) input. Sample data emerges from pin 6, the serial data output (SDO), which connects to the MISO (master in/slave out) serial input of the Arduino, via digital I/O pin 5 (IO12). Fig.2 also shows a second pole for range switch S1 (S1b). This allows the Arduino to monitor which range the user has selected. Each position goes to a different digital input on the Arduino (pins IO3-IO8), while the rotor is grounded. These inputs have internal pull-up resistors so the firmware can tell the position of S1b (and thus S1a) siliconchip.com.au by sensing which pin has been pulled to ground. The audio level and power meter circuitry is shown at lower left in Fig.2. As noted previously, this is based around IC2, an AD8307 logarithmic amplifier/ detector. Its DC output voltage at pin 4 rises by a maximum of 25mV per decibel increase in the input AC voltage, which is applied between pins 8 & 1. The 25mV/dB slope can be easily reduced by connecting an external load resistance and this is the purpose of trimpot VR2 (50kΩ) and the 33kΩ series resistor. With VR2 adjusted to give a total of 50kΩ, IC2’s output slope drops to 20mV/dB. The audio input from CON5 is fed to pin 8 (INH) via a 60:1 resistive voltage divider and a 22µF capacitor, while pin 1 (INL) is connected to ground via another 22µF capacitor. Note that while the INH and INL pin names imply polarity, they are in fact interchangeable. In determining the resistor values in the divider, we must consider the 1.1kΩ input resistance of the AD8307. So when trimpot VR1 in the lower leg of the nominal divider is set to 1kΩ (ie, to mid-range), the external lower leg resistance is 11kΩ, giving an effective lower leg resistance of 11kΩ // 1.1kΩ = 1.0kΩ. In conjunction with the upper leg resistance of 59kΩ, this April 2016  65 4.7Ω 6 7 47nF IN H 560Ω INTERCEPT ADJ (CAL) 1 IN L VR3 2k EN IC3 AD8307 ARZ RING 1.5k 4 OUT TIP 5 INT SLEEVE 3 OFS 100nF COM 2 CON7 * USING A STANDARD 3.5mm PLUG/3.5mm PLUG STEREO CABLE 100nF 47nF FOR ARDUINO MFM SHIELD gives the required 60:1 ratio. As a result, this section produces an output of 2.5V DC for an audio input of 60V RMS, falling at a rate of 20mV per dB, down to around 840mV DC with an input of 4.26mV RMS. That results in a measurement range from +35.5dBV to -47.5dBV, for signal frequencies from below 20Hz to above 200kHz. You might wonder why VR1 is labelled “Intercept Adjust”. This is because, by adjusting the input divider ratio, it controls the input level that corresponds with 0V output from IC2. The 470pF capacitor connected between input pins 8 & 1 of IC2 is there to attenuate input frequencies above about 300kHz. This is necessary because the AD8307 can respond to frequencies up to above 500MHz, which would result in the circuit being affected by RF interference. The 1µF capacitor between IC2’s output pin 4 and ground acts as a noise filter A1 36k 0.1% TXD 50k NEG IO2 VR2 RF IN 1206 AD8307 50k 0.1% 1.5k 1206 IO4 IO3 SLOPE ADJUST 2k VR1 66  Silicon Chip 4x 100k IO5 0.1% 1206 R T 2 33k CON5 AF IN 1206 100nF 1206 1206 1 1206 1206 47nF 560Ω INT ADJ VR3 2k CON6 IO6 3 A4 1206 10k 0.1% 8x 499k 0.1% A3 39k 2.0k 0.1% 1206 100nF 100nF IO7 A2 20k 1206 IO10 IO9 1 5 A5 RF IN IO11 IO8 4 1206 1.5k (CON3) 750k 0.1% 1 µF 1 µF 470pF 1206 10k 5 IO 12 6 1206 1206 1 GND IO13 S1a R 102 C C 62016 21061140 04116012 4.7Ω AREF 6 A0 22 µF 1206 CON4 S1 1 SDA 4 S1b 1206 100nF 100 µF 1206 1206 IC2 8307 GND 1206 S SCL GND 1206 1206 22 µF 100nF +5V 3 2 RST +3.3V 60Hz S T 200k IOREF 04116011 C 2016 RevE 1206 1206 10 µF 1206 1N5711W-7-F D1 D2 104116011 1061140 C6 12016 02 C 1 1 1210 100nF 50Hz 1206 5.6Ω 1 100 µH IC1 2400 1206 SMD INDUCTOR 1206 REF1 1019 100nF 10 µF 4800S JP1 IC3 MURATA 10Ω LED1 A POWER ZD1 3.9V 50k 0.1% RFC1 1206 SET NOTCH FREQ 470Ω TO MFM for Arduino Uno/Eleven 24-bit Multifunction Meter 1206 RF HEAD Fig.3: the optional RF Head circuit. Note the similarity to the audio level and power measurement circuitry in Fig.2, the main difference being that various component values and placements having been changed to expand the bandwidth out to 500MHz. 5V power comes from the main board via the ring connection of CON7, while the measurement output goes to the tip. The RF input section is housed in a second smaller metal box and connects to CON4 on the main PCB via a 3.5mm “stereo” cable. Fig.3 shows the circuit of this section and again, it’s based on an AD8307 logarithmic amplifier/ detector (IC3), with a changed configuration to make it suitable for measuring RF signals up to about 500MHz. We put it in a separate metal box, to prevent any RF radiation from affecting the operation of the rest of the circuit. In this case, the input divider values provide a nominal division ratio of 158:1, while the output from pin 4 has no external resistance to ground, giving a nominal slope of 25mV/dB. As a result, the RF head provides a 2.5V DC output for an RF input of 223V RMS, corresponding to 1000W into 50Ω or 666.6W into 75Ω. The minimum input voltage level is lower than 7.07mV 47nF SC 20 1 6 RF input head 1206 200k VPS 8 1206 200k CON7 CON6 TO CON4 ON MFM SHIELD* 100nF 200k RF INPUT while the 1µF capacitor bypassing pin 3 (OFS) has a similar function. The 5V DC rail which powers all these ICs comes from the PC’s USB port, via the Arduino. Inductor RFC1 provides some RF filtering in case any unwanted signals have been picked up by the wiring while LED1, in conjunction with a 470Ω current limiting resistor, provides power indication. RXD +LV (CON2) +HV (CON1) Fig.4: follow these two parts layout diagrams to build the MFM shield and RF Head PCBs. All parts are mounted on the top side of each board while CON1-CON3 are chassis-mounted and connect to the MFM shield PCB via short lengths of tinned copper wire. Be sure to fit the SMD components first before moving on to the through-hole types. siliconchip.com.au RMS, corresponding to 1µW into 50Ω or 0.66µW into 75Ω. The RF Head receives its +5V DC power from the main MFM shield, via the same cable used to carry the output from IC3 back to CON4. Construction Most of the parts of the MFM are mounted on a single 96 x 83mm “shield” PCB (code 04116011) which plugs directly into the Arduino PCB via SIL pin headers. Range switch S1 is at the centre of the shield PCB, while the RF and AF input connectors are at lower left. The complete assembly fits into a large diecast box, along with three panel-mount DC input sockets (CON1-CON3) which are mounted just above the shield PCB. All parts for the RF Head are fitted on a second PCB (code 04116012) which measures 42 x 41mm. This slips into a smaller diecast box (see photos). Use Fig.4 as a guide to assemble both boards. All components mount on the top sides of the boards. Begin by fitting the SMD resistors, taking care not to overheat the 0.1% types. Follow with all of the SMD capacitors, which are not polarised. Next mount the diodes (D1, D2 and ZD1) to the main PCB, then IC3 to the RF Head and IC2, REF1 and IC1 to the main PCB, preferably in that order. Finally, fit inductor RFC1. After that, only the through-hole parts are left. Install the trimpots, taking care not to get VR1 and VR2 mixed up. BNC socket CON5 can then be fitted, followed by 3.5mm stereo socket CON4. Then fit the four SIL headers used to make the interconnections between the MFM shield and the Arduino. As you can see from the photos, these mount on the top of the MFM shield PCB, with their pins soldered to the pads underneath. Take care to use the minimum solder necessary to make a reliable joint, as the main length of each pin needs to be free from solder, flux and dirt in order to make good contact with the matching clips in each Arduino SIL socket. With the headers all in place, fit the 3-pin SIL header for JP1 at upper right. Then pass 20mm lengths of 0.75mm tinned copper through the three holes at lower-right on the main PCB, soldering them to the pad underneath. These will later be soldered to DC input connectors CON1-CON3. Alternatively, you can use cut-down IC socket pins as shown in the photo below but soldered connections may be more reliable. Rotary switch S1 can now go in, after first having its spindle cut to around 17mm long (remove any swarf with a small file or hobby knife). Make sure The RF Head PCB fits inside a metal diecast case measuring just 51 x 51 x 32mm. It’s mounted on 12mm spacers, as shown in Fig.5. it’s orientated correctly, with the plastic post on the right side as shown in Fig.4, so the knob will go on the correct way around. The shield PCB can now be completed by fitting LED1. This is mounted at top/rear centre with its leads at their full length so that the underside of the LED’s body is 24mm above the top of the PCB (use a cardboard spacer). To finish the RF Head PCB, fit SMA input socket CON6 to the bottom edge of the PCB and 3.5mm socket CON7 at to the top. The SMA socket is “edge mounted”, with the PCB passing through its side slots and its centre pin resting on the central rectangular pad on the top of the PCB. Solder this pin to that pad, then solder the earthy “side bars” to the matching copper pads on both the top and bottom of the PCB. Both of your MFM boards should now be complete and ready to be mounted in the two boxes. We will describe how to do that next but we recommend you go through the test procedure (to be described in Pt.2) first, before mounting it in the box. Preparing the boxes This view shows the fully-assembled PCB. We used cut-down IC socket pins to accept the tinned copper wire leads from CON1-CON3. siliconchip.com.au The drilling and cutting details for both boxes can be downloaded from the SILICON CHIP website. There are 11 holes to drill in the main case plus one rectangular cut-out, and seven holes in the RF Head case. Note that the rectangular cut-out is sized to suit a USB type B plug as required by the Arduino Uno or Duinotech Classic; you could get away with making a smaller cut-out for the microUSB input on a Freetronics Eleven. Once all the holes and cut-outs have been made, remove any burrs from the April 2016  67 Parts List 1 diecast aluminium box, 119 x 94 x 57mm (Jaycar HB5064 or similar) 1 lid panel label, 103 x 84.5mm 1 Arduino Uno, Freetronics Eleven or Duinotech Uno module 1 USB cable to suit Arduino module 1 double-sided PCB, 96 x 83mm, code 04116011 1 2-pole 6-position rotary switch, PCB mounting (S1) 1 instrument knob, 24mm diameter (Jaycar HK7764 or similar) 1 100µH SMD RF inductor (Jaycar LF1402 or similar) 1 3-pin SIL header with jumper shunt (JP1) 2 red panel-mount banana sockets, fully insulated (CON1-CON2) 1 black panel-mount banana socket, fully insulated (CON3) 1 3.5mm stereo switched jack socket (CON4) 1 PCB-mount BNC socket (CON5) 1 2kΩ multi-turn horizontal trimpot (VR1) 1 50kΩ multi-turn horizontal trimpot (VR2) 1 set Arduino male/female headers 4 M3 x 25mm tapped spacers 8 M3 x 6mm machine screws 4 4.5mm OD, 12mm-long untapped spacers 4 M3 x 20mm machine screws 4 M3 hex nuts 4 stick-on rubber feet 1 14-pin DIL socket (for CON1-3) Semiconductors 1 LTC2400CS8#PBF 24-bit ADC, SOIC-8 (IC1) 1 AD8307ARZ logarithmic amplifier/detector, SOIC-8 (IC2) 1 LT1019ACS8-2.5#PBF precision 2.5V reference, SOIC-8 (REF1) 1 3.9V 1W zener diode, SC-109B (ZD1) 1 3mm green LED (LED1) 2 1N5711W-7-F Schottky diodes, SOD-123 (D1,D2) inner and outer edges with a large drill or needle file. Box assembly Fig.5 shows how the two PCBs are mounted. The procedure is as follows: (1) Plug the MFM Shield PCB into the Arduino module, making sure they 68  Silicon Chip Capacitors (all 1206 SMD) 1 100µF 6.3V X5R 2 22µF 10V X5R 2 10µF 16V X5R 2 1µF 16V X7R 4 100nF 16V X7R 1 470pF 100V C0G/NP0 Resistors (0.25W 1206 SMD) 1 750kΩ 0.1% 1 10kΩ 1% 8 499kΩ 0.1% 1 10kΩ 0.1% 4 100kΩ 0.1% 1 2.0kΩ 0.1% 2 50kΩ 0.1% 1 1.5kΩ 1% 1 39kΩ 1% 1 470Ω 1% 1 36kΩ 0.1% 1 10Ω 1% 1 33kΩ 1% 1 5.6Ω 1% 1 20kΩ 1% RF Head (optional) 1 AD8307ARZ logarithmic amplifier/detector, SOIC-8 package (IC3) 1 diecast aluminium box, 51 x 51 x 32mm (Jaycar HB-5060 or similar) 1 front panel label, 45.5 x 45.5mm 1 double-sided PCB, code 04116012, 42 x 41mm, 1 PCB edge-mount SMA socket (element14 2340518) (CON6) 1 3.5mm stereo switched jack socket (CON7) 1 2kΩ multi-turn vertical trimpot (VR3) 2 12mm x 4.5mm OD untapped spacers 2 M3 x 20mm machine screws 2 M3 hex nuts 1 3.5mm stereo jack to 3.5mm stereo jack cable, length to suit user requirements 4 stick-on rubber feet Capacitors (all 1206 SMD) 3 100nF 16V X7R 2 47nF 50V X7R Resistors (0.25W, 1% 1206 SMD) 2 200kΩ 1 560Ω 1 1.5kΩ 1 4.7Ω are properly aligned. Don’t push both boards together as far as they’ll go. (2) Slip four 12mm long untapped spacers into the locations for the Arduino mounting screws between the two boards. (3) Push M3 x 20mm machine screws up through the Arduino PCB and each The header pins on the back of the MFM shield PCB are plugged into matching headers on the Arduino PCB. The assembly is then secured using 12mm-long untapped spacers and M3 x 20mm machine screws and nuts (see Fig.5). spacer and fit M3 hex nuts on the top. (4) Gradually tighten each screw and nut until the two PCBs are held together. Note that you will need to file a small amount of metal off one “flat” of the nut used on the mounting screw that is very close to the rear end of the 10-way SIL header (the one at upper right in Fig.4) so it doesn’t interfere. (5) Fit four M3 x 25mm tapped spacers into the bottom of the case using M3 x 6mm screws. Don’t tighten these screws up fully yet. (6) Remove the nut and lockwasher from the front of BNC socket CON5. (7) Insert BNC socket CON5 through siliconchip.com.au (LID OF CASE) SIDE-ON CUTAWAY VIEW OF 119 x 93 x 56mm DIECAST ALUMINIUM CASE MULTIFUNCTION MEASURING SHIELD S1 CON5 CON4 12mm-LONG UNTAPPED SPACERS ATTACHING ARDUINO TO UNDERSIDE OF SHIELD PCB VIA M3 x 20mm MACHINE SCREWS & NUTS (AT TOP) ARDUINO UNO OR ELEVEN OR COMPATIBLE M3 x 25mm TAPPED SPACERS SUPPORTING BOTH MODULES IN CASE M3 x 6mm MACHINE SCREWS M3 x 6mm MACHINE SCREWS (LID OF CASE) RF INPUT STEREO AUDIO CABLE TO CON4 ON MFM SHIELD VR3 CON6 INT ADJ 12mm LONG UNTAPPED SPACERS 51 x 51 x 32mm DIECAST ALUMINIUM CASE CON7 PCB M3 NUTS 3.5mm STEREO PLUG WITH METAL CASE M3 x 20mm MACHINE SCREWS Fig.5: here’s how the PCB assemblies are mounted inside the diecast cases. The Arduino board is mounted on the MFM shield PCB using four 12mm untapped spacers and secured with machine screws and nuts. The entire assembly is then mounted on the bottom of the case on M3 x 25mm tapped spacers. The RF Head PCB is secured inside its case on two 12mm-long untapped spacers. POWER INTERCEPT ADJUST 2.50V DC RF INPUT USB LINK TO PC 25.0V DC RF LEVEL & POWER 250V DC SILICON CHIP 1000V DC AUDIO LEVEL & POWER RF MEASURING HEAD FOR MFM OUTPUT TO MFM SILICON CHIP RF HEAD siliconchip.com.au AUDIO INPUT USB LINKED MULTIFUNCTION 24-BIT MEASURING SYSTEM DC VOLTAGE INPUTS – +2.50V/25.0V Fig.6: full-size panel artwork for the main MFM unit lid and the RF Head lid. These can also be downloaded as a PDF from the SILICON CHIP website, printed out and laminated. +250V/1000V April 2016  69 The rear panel has a cut-out to access the USB socket on the Arduino mod­ ule. Note that this cut-out can be made smaller than shown if the module is fitted with a micro-USB socket. Above: this view shows the Arduino PCB (in this case, a Freetronics Eleven) mounted on the rear of the MFM shield PCB. on. Make sure the black socket (CON3) is closest to BNC socket CON5. (11) Rotate the banana sockets so that their tabs are horizontal and do the nuts up tight. (12) Bend the previously soldered lengths of tinned copper wire so they pass through the corresponding banana socket holes, then solder them in place (or push tinned copper wire leads into cut-down IC sockets and then solder these to CON1-CON3). (13) Attach the label to the lid. (14) Drop the lid in place, ensuring that LED1 passes through its 3mm hole, then screw the lid down and attach the knob. RF Head assembly The PCB assembly is installed in the case by first angling the BNC socket down through its hole, then forcing the rear of the assembly down into the case. The three banana sockets are then fitted and wired to the shield PCB. Use the following steps to assemble the RF Head: (1) Push M3 x 20mm machine screws up through the two holes in the bottom of the box, then slip a 12mm long untapped spacer over each. (2) Insert CON6 through the larger hole in the side of the box, then lower the PCB into place and slide it back so that CON7’s ferrule lines up with its smaller hole opposite. (3) Attach the board using two M3 hex nuts. Do these up nice and tight. (4) Attach the label to the lid. (5) Secure the lid in place, making sure that the hole to allow trimpot VR3 to be adjusted is positioned above the trimpot’s screw. Next month its corresponding hole in the front of the box, then force the PCB assembly down into the case until it is resting on top of the four 25mm spacers. (8) Attach the MFM shield board to these spacers using M3 x 6mm screws, then tighten the corresponding screws 70  Silicon Chip in the bottom of the case. (9) Screw the lockwasher and nut back onto the ferrule of BNC socket CON5. (10) Remove the nuts from the three banana sockets for CON1-CON3, push them through the 12mm holes in the front of the box and slip the nuts back Your Multifunction Meter hardware is now complete and you’re ready to tackle the remaining steps such as installing the firmware and software, calibration and finally putting the instrument to use. These topics will be SC covered in Pt.2 next month. siliconchip.com.au