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Virtually all the parts for the Inductance & Q-Factor Meter are mounted on a single PC board, so building it is easy. Here’s how to assemble it, check it out and use it to make inductance and Q-factor measurements. Inductance & Q-Factor Meter Pt.2: By LEONID LERNER B UILDING THE INDUCTANCE & Q-Factor Meter is quite straightforward. Fig.9 shows the parts layout on the single PC board. Begin by installing the wire links. All of the links except one can be formed from 0.7mm tinned copper wire or similar. The link situated to the right of Q1 must be fashioned from light-gauge insulated wire instead. Follow up with the resistors and then continue with all remaining components in order of height. Note that the high-profile components must be mounted as far down on the PC board surface as possible, so as not to inter64  Silicon Chip fere with the keypad and LCD when they are installed in the case later. In fact, the tip of REG3’s metal tab had to be removed on the prototype, to clear the rear of the keypad. This can be done using a hacksaw (with the device held in plastic vice jaws) and then carefully filing the job to a smooth finish . Alternatively, you may be able to bend the regulator to achieve sufficient clearance. When installing the IC sockets, be sure to align the notched (pin 1) ends as indicated in the overlay diagram. Also, check that you have the banded (cathode) ends of the diodes (D1-D8) and the positive leads of the four polarised capacitors around the right way. The keypad and LCD are connected via lengths of 7-way and a 10-way ribbon cable respectively. On the prototype, an 8-way header is used at the keypad end, while two 6-way headers are soldered to the first six and last six contacts of the LCD module. We’ve not included these headers in the parts list because there are several ways the keypad and LCD ends of the cable can be terminated, as well as several types of LCD modules with varying pinouts. You could even solder the ribbon cables directly to the PC siliconchip.com.au Fig.9: follow this diagram when assembling your meter. In particular, make sure that all the high-profile components are seated as close to the board as possible. boards, leaving out the connectors. Trial fit the assembly in the enclosure first to gauge the required ribbon cable lengths. Wire up in accordance with the circuit diagram (Fig.5), noting that the pins of the LCD and keypad will not be in the same order as the wires on the ribbon cable. Finally, in preparation for testing, temporarily fit the three binding posts directly to the PC board in the large holes marked “A”, “B” and “E” on the overlay. The posts must be removed after testing and installed in the top of the case as described later. Basic checks & programming The unit requires a 7.5-9V DC, 200mA power supply. Care needs to be taken here as some 9V DC plugpacks supply much higher voltages when lightly loaded. This extra voltage translates to power dissipation in the 7805 regulator, which may cause it to overheat and shut down. If you’re using an unregulated plugpack with selectable output voltages, you may find that the “6V” or “7.5V” setting is sufficient. As the first step, apply power and check that the +5V and -5V supplies are present on pins 7 and 4 of the AD8055 (IC4) respectively. Also, check for -1.8V on pin 2. Next, adjust siliconchip.com.au This view shows the fully assembled prototype PC board. Note that the final version shown in Fig.9 differs slightly from this unit. March 2005  65 the data entry screen should appear on the LCD. To make measurements using an external tank capacitor, first remove the link between the “A” and “B” terminals if installed earlier. Next, connect a capacitor of a few nF in parallel with an inductor of a few mH between the “A” and “E” terminals. You can now enter the capacitance value. For example, if you’ve chosen an 8.2nF capacitor, press “8” and then “2”. The display will show “8.2” followed by “100pF”, which is the default multiplier. Pressing any key except “*” has the effect of rotating the choice between the 100pF, 1nF, 10nF, and 100nF multipliers. As we’re using an 8.2nF capacitor in this example, press any key once to select the 1nF multiplier. Press “*” once you’re satisfied with the two parameters. If you make a mistake, pressing “*” at any stage lets you enter a choice or abort a measurement. The display should now show the inductance, Q factor and test frequency. To make measurements using the internal capacitor bank, insert a link between the “A” and “B” terminals. Repeat the procedure above but note that only choices of 1.0 x (1nF or 10nF or 100nF) make sense here since only these values are present internally. Q readings with the internal capacitor bank suffer due to the 0.5W (approx.) dynamic series resistance of the transistors, which do the bank switching. This mode is to be used if you’re not interested in the Q and just want to make inductance measurements. After the range is chosen, the micro starts sampling at the maximum rate. The first few acquisitions are used to optimise the sampling rate and set a sampling delay if required. The latter occurs when the ringing saturates ABOVE: the LCD is glued in place behind the display window and is connected to the PC board via ribbon cable – see text. Note that the diode shown connected to the DC socket here is on the PC board in the final version (D9 in Fig.9). the potentiometer (VR1) for optimum contrast on the LCD screen. All you’ll probably see at this point are faint grey blocks of pixels, as the micro is yet to be programmed; simply adjust the pot to get the darkest possible pixels. An ISP programmer can now be connected and the program files LQMeter128.HEX and LQMeter128.EEP Table 1: Capacitor Codes Value 100nF 82nF 8.2nF 4.7nF 1nF 820pF 680pF μF Code 0.1µF .082µF .0082µF .0047µF .001µF   NA   NA EIA Code   104    823   822   472   102   820   680 IEC Code   100n       82n   8n2   4n7   1n    820p    680p loaded into the Flash and EEPROM memories, respectively. These files are available from the SILICON CHIP web site in a file named “LQMeter. ZIP”. If you don’t already have a suitable programmer, then check out the “AVR ISP Serial Programmer” project described in October 2002. Kits for the programmer are available from Jaycar Electronics (Cat. KC-5340). The microcontroller program occupies most of the available memory space. It is quite complicated but should you have the inclination, you can follow its operation in detail in the documented source code included in the download. You can get a top-level understanding of program operation from the flow chart in Fig.10. Operation Power up and assuming all is well, Table 2: Resistor Colour Codes o o o o o o o o o o No. 1 10 2 3 1 9 1 1 2 66  Silicon Chip Value 10kW 4.7kW 1.2kW 1kW 120W 100W 82W 56W 47W 4-Band Code (1%) brown black orange brown yellow violet red brown brown red red brown brown black red brown brown red brown brown brown black brown brown grey red black brown green blue black brown yellow violet black brown 5-Band Code (1%) brown black black red brown yellow violet black brown brown brown red black brown brown brown black black brown brown brown red black black brown brown black black black brown grey red black gold brown green blue black gold brown yellow violet black gold brown siliconchip.com.au This close-up view shows how the three 10mm tapped spacers are fitted to the binding posts (after first snipping off the non-threaded tips). The PC board is secured to these spacers using M3 x 6mm screws and star washers (see text). the input amplifier in the immediate aftermath of the pulse. After a valid sample is acquired the micro performs an FFT and the centre frequency is estimated. During a period of eight cycles, centre frequency and bandwidth measurements are performed and the average taken. Finally, the inductance, Q factor and centre frequency are calculated and displayed on the LCD. A complete acquisition, averaging and display period takes about 0.1s for the 10MHz micro. You may find that the large binding posts are useless when testing physically small inductors. This is easily addressed by making up two short test leads to plug into the binding posts. Each lead consists of an uninsulated crocodile clip soldered to a 4mm banana plug via a very short length of large diameter single-strand copper wire or similar. Housing The completed meter will fit neatly into a console-style instrument case. This should be done after it has been tested and found to be operating corsiliconchip.com.au Fig.10: this diagram gives a very basic idea of how the microcontroller program works. For specific details, check out the fully documented source code, which is available for download from the SILICON CHIP website. rectly, as the keypad and LCD are glued in place and will be difficult to remove later. An opening for the keypad must be cut out and holes for the DC socket and binding posts should be drilled as shown in the various photographs. Note that the binding post holes must be positioned accurately otherwise it will be impossible to assemble the unit later. This can be achieved by using the PC board as a template when marking out the holes. To give the finished unit a professional appearance, a faceplate can be cut from thin aluminium sheeting and fitted around the keypad. The easiest way to achieve this is to lightly mark out the contour of the keypad on the aluminium sheet using a scribe or needle and then drill four 8.5mm holes at the corners. Next, use a straight edge and Stanley knife to score the sheet front and back, joining the four holes at their perimeters. Cut away the centre of the piece to be removed, then place the sheet in a vice and bend along the scored marks. Work the metal back and forth at March 2005  67 Another view inside the completed prototype. The keypad and LCD are secured to the case using epoxy adhesive. Fig.11: check your PC board against this full-size etching pattern before installing any of the parts. 68  Silicon Chip siliconchip.com.au the bend and it will break off, leaving a clean edge. The LCD, keypad and faceplate can then be glued to the case using two-part epoxy adhesive. To achieve a good bond to this plastic, the mating surfaces should first be sanded to a rough finish. Make sure that the LCD is centred left to right when you glue it, otherwise some of the characters will not be visible. Note that even when centred, the entire display width is not visible through the case cutout. This is not a problem, as the program uses only 12 of the available 16 character positions. Binding posts The binding posts are first attached to the case using the supplied nuts and spring washers. Next, snip off the very tip of the binding posts, leaving just the threaded portion. The non-threaded part is not needed and would otherwise obstruct the PC board mounting screws. Now remove the two small M3 nuts and replace them with M3 x 10mm tapped metal spacers, tightening firmly. The PC board can then be held in place by attaching it to the three spacers using M3 x 6mm screws and star washers. Note that the screws and washers must also make good electrical contact with the copper on the PC board. If there is a problem attaching the PC board to the spacers on the binding posts, check for interference between the components and the rear of the keypad. Some minor component repositioning may be necessary to fix this problem. Final word In addition to L and Q measurements, some interesting physical phenomena can be investigated with this meter. First, wind a small choke with a few turns of enamelled copper wire around a Philips screwdriver and check its inductance and Q factor with the meter. Next, insert a small HF ferrite bead into the coil and observe that the inductance and Q factor increases, as expected. Now replace the bead with the blade of the screwdriver and observe that the inductance hardly changes and may even decrease, while the Q drops markedly. Ordinary iron is not a useful core siliconchip.com.au Par t s Lis t – Inductance & Q-Factor Meter 1 PC board, code 04102051, 139.7mm x 86.4mm 1 150mH miniature ferrite choke (L1) (Farnell 432-090) 1 black 4mm binding post (Jaycar PT 0454) 1 blue 4mm binding post (Jaycar PT 0450) 1 green 4mm binding post (Jaycar PT 0455) 1 6-way 2.54mm pitch header (for ISP connection) (Jaycar HM-3416) 1 2.1mm or 2.5mm panel-mount DC socket 1 20-pin IC socket 1 16-pin IC socket 2 14-pin IC sockets 2 8-pin IC sockets 1 16-character x 2-line LCD module (DSE Z 4170 or equivalent) 1 numeric keypad (DSE P 7810) 1 console style plastic case, 150 x 95 x 28.5/49.5mm (Jaycar HB-6090) 1 ribbon cable (see text) 1 miniature 10kW trimpot (VR1) 3 M3 x 10mm tapped metal spacers 3 M3 x 6mm screws & star washers Semiconductors 1 AT90S2313-10 microcontroller (IC5), programmed with LQMeter128.HEX & LQ-Meter128. EEP 1 74HC00 quad NAND gate (IC1) 1 74HC390 dual decade counter (IC2) 1 74HC4066 quad analog switch (IC3) 1 AD8055AN high-speed op amp (IC4) (Farnell 283-976) 1 MAX635ACPA switching regulator (IC6) (Futurlec or RS 655-442) 2 2N4250 or PN4250 PNP transistors (Q1, Q2) (Wiltronics) 6 2N2222A or PN2222A NPN transistors (Q3-Q8) 1 40MHz crystal oscillator module (OSC1) (Farnell 571-830) 1 LM334Z adjustable current source (REG1) 1 7805 +5V regulator (REG2) 1 LM337 adjustable negative voltage regulator (REG3) 1 1N60 germanium diode (D1) (DSE Z 3040) 7 1N4148 diodes (D2-D7) 1 1N4004 diode (D8) Capacitors 2 470mF 16V PC electrolytic 1 220mF 10V PC electrolytic 1 47mF 6.3V tag tantalum 11 100nF 50V monolithic 2 82nF 50V MKT polyester 1 4.7nF 50V MKT polyester 1 1nF 50V MKT polyester 1 820pF 50V ceramic disc 1 680pF 50V ceramic disc Resistors (0.25W 1%) 1 10kW 8 100W 10 4.7kW 1 82W 2 1.2kW 1 56W 3 1kW 2 47W 1 120W 1 130W Note 1: parts shown with catalog numbers can be obtained from the indicated distributor(s). Contact details for all distributors mentioned are as follows: (1) Dick Smith Electronics (DSE): www.dse.com.au (2) Farnell InOne (1300 361 005): www.farnellinone.com.au (3) Futurlec: www.futurlec.com (4) Jaycar Electronics: www.jaycar.com.au (5) RS Components (RS) (1300 656 636): www.rsaustralia.com (6) Wiltronics Research (1800 067 674): www.wiltronics.com.au Note 2: the 40MHz crystal oscillator module could also be obtained from an old 386/486 PC motherboard. material at RF. This is because its magnetic domains cannot keep pace with the fast changing RF field. Rather they vibrate ineffectively and generate heat, introducing nothing but losses SC into the tuned circuit. March 2005  69