Fullscreen HTML5Med Res (??MB)HTML4

Digital Sine/Square Wave Generator; Pl.2 This month we present the constructional details on our new Sine/Square Wave Generator. The parts are all installed on two PC boards which are soldered together at right angles and mounted in a plastic instrument case. By JOHN CLARKE Very little point to point wiring is used in this project as it is virtually all on the two printed circuit boards. What little wiring there is, is mostly associated with the mains transformer. The two printed boards occupy just about all the floor space inside the standard instrument case which measures 263 x 190 x 84mm. The main board measures 225 x 162mm and it has the power transformer mounted directly on it, which does away with the need for a separate transformer baseplate. The front panel is made from a sheet of red Perspex measuring 250 x 75mm, the lower half of which is covered with a Dynamark label. We expect that kitsets will be supplied with screen printed red Perspex front panels. Behind the Perspex front panel is a black plastic film which masks all but the four digit The main regulator (REG2) is mounted on a U-shaped heatsink which is bolted lo the PC board. Notice how the plastic insulating sleeving fitted over the power switch leads is extended back past the heatsink. 52 SILICON CHIP displays so that components on the display board are not visible. The display PCB measures 227 x 75mm and is mounted at rightangles to the main board. The complete 2-board assembly is secured into the case by four selftapping screws which thread into integral pilla rs in the base. The vertical display board is held rigid by slots in the front of the case, along with the red Perspex panel. Board assembly Before you start work on the boards, check them carefully for any open circuit or short circuited tracks by comparing them with the PC artworks included in this article. Also check for any undrilled component holes - it is a pest to have to drill holes in a partly assembled board. Note that three of the tracks on the display board around S1 and S2 are open circuit at one end. This is not a mistake - these tracks are there to provide shielding We suggest you start work on the main board first. There are a large number of links on the board and these should be installed first. Many assembled kit boards we see don't look good because the links are all untidy and crooked. The way to make them look good is to take a length of the tinned copper wire and straighten it by putting one end in a vyce and pulling the other end with a pair of pliers to stretch it slightly. You can then take this length of nice straight wire and cut it to length for each link. The ends of each link should be neatly bent with pliers so that they fit into position nicely on the board. A little care in this aspect can help make your boards look a lot better. Incidentally, some people may question why we have produced a board with a relatively large number of links. Why not go to a REAR PANEL CORO GRIP - - ~/G~~IT \ ACTIVE (BROWN) HEAT SHRINK TUBING ""',1 I I I I • 1% double sided board and eliminate them entirely? That could be dohe but it would be a good deal more expensive and make no difference at all to the final result. Double sided boards are a good idea for mass produced gear where labour costs must be kept low but for this project they would be a waste of money. The four PC stakes are optional but should be installed now if you intend to use them. There are three in the power supply section of the board and one near IC7. The resistors can now be installed. Note that all of the resistors shown with a star in the circuit presented last month and the wiring diagram of Fig.5 are 1 % types. The 5-band colour coding for these can be difficult to decipher. To help \ FRONT PANEL t NPO SEE TEXT Fig.5: here's how to install the parts on the main PCB. Be sure to use 1% resistors where indicated and take care with component polarity. All wiring to the mains switch & to the power transformer must be run using 240V AC cable. Note the wire link under VR3. in this regard, we have included a table of the all resistor codes but if you are not sure about any resistor value, check it with your multimeter before installing it. Now the ICs can be installed and when they are all in, the board will look more than half complete. Don't hurry the job though, as you could make mistakes. Make sure that all the ICs are placed in the correct position and oriented correctly they all face the same way except for IC2, the 74HC42. Note that the parts list and circuit diagram presented last month give a choice of 74LS (low power Schottky TTL) or 74HC (high speed CMOS) for a number of the ICs. The only other permissible IC substitution is to use a TL071 instead of an LF351 for IC5 . Now install the diodes and transistors, making sure that each is correctly oriented. The capacitors and trimpots can then be added, again making sure that the electros are correctly polarised. Cut the shafts of the two dualgang potentiometers (VR2 & VR3) to a length of 29mm. They are soldered directly into the board although note that you might have to make some fine adjustments to AUGUST 1990 53 Cut the shafts of the switches and the single gang pot to 13mm before mounting them on the display PCB. The pot is mounted on three PC stakes. them later when the time comes to mate the two boards together. Regulators REGl and REG3 (at the back of the board) are installed upright, while the main regulator, REG2, is mounted horizontally on a U-shaped heatsink. Apply a smear of heatsink compound between the regulator base and the heatsink to improve the heat transfer. Both the regulator and heatsink are secured to the PCB using a machine screw and nut. Display board Work can now begin on the display PCB. First, install the PC stakes, noting that the PC stake marked A should be installed from the copper side of the PCB. Note that there are three stakes for the connections to the single gang pot, VR5. The links and all the resistors can now be installed. The displays are mounted onto Molex pins so that they are raised from the PCB by about 4mm, to be closer to the Perspex panel. The transistors, trimpots and capacitors go in next, followed by the rotary switches and the single gang pot, VR5. The rotary switches and the single gang pot should have their shafts cut to 13mm before they are soldered to the board. RESISTOR CODES D D D D D D D D D D D D D D D D D D D 54 No 1 1 1 1 2 1 2 3 8 3 1 1 9 2 SILICON CHIP Value 330k0 1% 270k0 1% 120k0 1% 82k0 1 % . 47k0 1% 39k0 1% 33k0 1% 22k0 1% 20k0 1% 18k0 1% 15k0 1% 10k0 1% 10k0 4 .7k0 2.2k0 2700 470 330 4-Band Code (5%) not applicable not applicable not applicable not applicable not applicable not applicable not applicable not applicable not applicable not applicable not applicable not applicable brown black orange gold yellow violet red gold red red red gold red violet brown gold yellow violet black gold orange orange black gold 5-Band Code (1%) orange orange black orange brown red violet black orange brown brown red black orange brown grey red black red brown yellow violet black red brown orange white black red brown orange orange black red brown red red black red brown red black black red brown brown grey black red brown brown green black red brown brown black black red brown brown black black red brown yellow violet black brown brown red red black brown brown red violet black black brown yellow violet black gold brown orange orange black gold brown Finally, the DPDT switch (S3) can be installed. Mating the boards The display PCB is joined to the main PCB by soldering the underside of the main PCB bus to the display bus. The first step in this process is to fit the display board over the shafts of the two ganged pots, VR2 & VR3, and secure it with lock washers and nuts. Now ensure that the PCBs are square and that the bus pattern on the main PCB lines up with the bus pattern on the display board. Tack solder the boards together at several points along the mating bus patterns and then test the boards for fit in the case. Do the same check with the front panel fitted (we're assuming here that the front panel is finished and ready to fit) to make sure that everything lines up in the case. Finally, solder the two PCBs together. The wiring can now be completed. A short wire runs from the Fig.6: install the wire links and PC stakes on the display PCB first, before mounting the other components. The PC stake at point A is installed from the copper side of the board. Point B on the earth pattern is wired to the bodies of the rotary pots (see photo). "A" PC stake on the main PCB to the corresponding "A" stake on the display PCB. Solder wires from the VR2 & VR3 pot bodies to the B track on the display PCB. This earths the bodies of these pots. Transformer wiring The transformer is mounted on the main PCB as shown in Fig.5 . You will need to run hookup wire from the OV, 7.5V and 15V AC secondary connections on the transformer to the three PC stakes on the main board. If you are working from scratch and not from a kit you will need to drill and file a hole in the rear panel of the case to accept a cordgrip grommet. This is to anchor the mains cord. We suggest you use a 3-core flex which comes with a moulded 3-pin plug. Fit the cord and check that it is held securely by the cordgrip grom- Right: here's how to mount the single gang pot. Bend the pot terminals so that they fit over the PC stakes and make sure that the pot body sits flush against the PCB. CAPACITOR CODES □ □ □ □ □ □ □ □ Value AH Value IEC Code Ell Code 0.1uF .022uF 220pF 100pF 22pF 12pF 10pF 100nF 22nF 100n 22n 220p 100p 22p 12p 10p 104 223 221 101 22 12 10 met. The Earth lead of the mains connects to a solder lug which is secured by one of the transformer mounting screws. The Neutral wire goes directly to one of the transformer primary connections while the Active wire goes to the power switch on the front panel. Don't connect it to the switch at this stage though because you need to go through a test procedure before final assembly. Instead, connect the mains Active wire to the other primary terminal of the transformer. Fit heatshrink sleeving over the two primary terminals or tape them up so that there is no chance of accidental contact. If not supplied ready made in the kit, the Perspex front panel can be cut to size and drilled for the switches, pots and BNC output socket. To do this, first attach the Dynamark label, then punch out or cut out the holes in the front panel. GO ~ r,~ -=-=~--==i ~~ 4..'o":::::~ ~ ~ ~~ °°" I .Rd m'IIII 56 SILICO N CHIP 7 I- • ::) 11. I::) 0 UJ er: I N • < ::::) N J: J: .ll: ~ 0 0 (/) oO ,... • eo J: ' N O.ll: ,-o UJ z 0 u, en N J: .";° 0 N ~ J: Oo • g C a, .0 . . 00 0 ,...' N ,- :5 6 ,... 0 0 ,- ,... "-I" • 0 (.) Cl') UJ z u.. I(/) ::::) -, C < > (.) zUJ ::::) UJ • (/) er: < 0 UJ er: u.. 0 (.) a: Figs.7-10: here are actual size artworks for the two PC boards, the front panel and the light mask. • UJ 3: ~ VOLTAGE TABLE The four 7-segment LED displays are mounted using Molex pins so that they sit close to the front panel. Be sure to install each display with its decimal point at lower right. IC1 IC2 IC3 IC3 IC4 IC4 IC5 IC5 IC6 IC? ICB IC9 IC10 IC11 IC12 Q1 Q2 pins 11, 16 pin 16 pin 7 pin 4 pins 7,8, 12 pins 6, 1 3, 1 4 pin 7 pin 4 pins 6,10,14 pin 14 pin 16 pins 2, 1 o, 1 6 pins 4, 11, 1 6 pin 14 pin 18 collector collector +5V +5V +10V -10V +5V -5V +10V -10V +5V +5V +5V +5V +5V +5V +5V +10V -10V do not read waveforms much beyond lkHz. If there are problems, check the component overlay against your circuit for incorrectly placed components and for short circuits beneath the PCB. In our experience, most problems in kits of this sort are due to soldering and little else. Setting up Solder tack the two boards together in a couple of places and test the assembly in the case before soldering all the edge connector pads. Make sure that you don't create shorts between adjacent pads. The masking film is installed directly behind the Perspex panel and will also require matching holes for the switches and pots. Test the front panel for fit in the case [see above) but do not fit it to the board assembly yet since it will interfere with the trimpot adjustments required for the setting up procedure. Power up Before switching on, check the mains wiring carefully. When you are satisfied that everything is correct, switch on and check that the 58 SILICON CI-IIP power supply voltages on each IC are correct. This should be + 5V for all ICs except for IC3 & IC5 which have a + 10V supply. You should also check the - 5V supply to IC4. Most of the relevant voltages are shown in the accompanying table. The display will also be lit if all is well. If the circuit checks out so far, check that there is an output on both sine wave and square wave using either an oscilloscope or multimeter set to read AC volts. Note that many digital multimeters There are three setting up procedures required for the generator. You will need a multimeter and frequency meter. As an alternative, an oscilloscope can be used although its accuracy will be limited if used for the frequency adjustments. The first operation is to set the offset voltage at the output of the generator when it is in the sinewave mode. This is done by measuring the DC level of the sinewave when set to about lkHz. At this stage, the output frequency shown on the display will not be accurate but this is not important for this procedure. Set your multimeter to a low DC voltage scale and measure the voltage at the wiper (2700 resistor) of VR5 (the output level control). Now adjust VRl for a minimum DC reading on the multimeter. You should be able to set VR 1 for a reading of less than 50mV DC the closer to OV, the better. The se_cond procedure is to accurately set the frequency display FOR FAST SERVICE-CALL_TECH-FASTFIRST! Fluke 45 I l I I► l .~, , ,. nocioo _, ' ' f ,,, ' " ' ' , The dual display bench multimeter with simultaneous display of two parameters: 16 measurement functions and two 5-digit displays for maximum versatility in a single instrument ■ Multi-function vacuum fluorescent dual display ■ True RMS voltage and current, including A.C. + D.C. ■ RS-232 interface standard , IEEE-488.2 option ■ Frequency measurement to 1 MHz ■ dB measurements with selectable reference impedances of zn to 8,ooon and audio power from 2n to 16D ■ Compare (Hi/Lo/Pass) function for quick in-tolerance tests ■ 0.05% D. C. current accuracy for 4-20 mA current loop service ■ Touch Hold~ Relative and Min Max ■ Audible continuity and diode test ■ Optional rechargeable battery, carrying case, rack mount kit, and PC Software Package ■ Closed-case calibration ■ D.C. Voltage 0.02% - A.C. Voltage 0.2% [true RMS) $1216 ex tax - $1417 incl tax Fluke 70 Series Fluke 21 and 23 Analog/Digital Multimeters. Low Cost, High-Accuracy Multimeters that are High-Energy Protected ■ Analog and digital display - tests changing or stable signals ■ Easy to use - single rotary switch and automatic ranging ■ Volts, ohms, diode test, milliamps, 10 amps [Fluke 23) ■ Continuity beeper ■ High visibility yellow, high impact ABS case ■ Touch Hold® [Fluke 23) ■ 2000 hour battery life on a common 9V battery ■ Survives 6 kV transients on 660 V major A.G. feeders ■ UL 1244 listed ■ Made in U.S.A. 21 - D.C. Voltage 0.5%, A.G. Voltage 2% Autoranging w ith range hold , Audible continuity/ diode test $243 ex tax $283 incl tax 23 - D.C. Voltage 0.3%, A.G. Voltage 2% $328 ex tax - $382 incl tax Analog/Digital Multimeters. Unbeatable performance and value ■ Three year warranty ■ 3½ -digit, 3200-count display ■ 31-segment analog bar graph ■ Single rotary dial to select all functions ■ Autoranging and automatic polarity selection ■ Batterysaving "Sleep Mode" powers-down display if you foqiet ■ Tough, textured case resists drops and rough handling ■ Touch Hold® (Fluke 77) ■ UL 1244 listed ■ Made in U.S.A. 73 - D.C. Voltage D. 7%, A.C. Voltage 3% Auforanging $150 ex tax - $175 incl tax 75 - D.C. Voltage 0.5%, A.G. Voltage 2% Autoranging with range hold, audible continuity/diode test $240 ex tax - $280 incl 77 - D.C. Voltage 0.3%, A.C. Voltage 2% Touch Hold® $325 ex tax - $379 incl tax Fluke 80 Series Analog/Digital.Multimeters. Top of the ......,,.,, -~............................... 3a21 !_I ® ~ ~ ~'"""la RANGE<at> iii - -- m m, mAIA mAfA µA A {(h•* & C OFF ~ !1000VMAX, e)320~MAX Fluke 25 & 27 range - the world's most wanted handheld multimeters with 11 functions and 40 ranges. ■ Three year warranty ■ Min Max Average recording mode with Min Max Alert™ ■ Frequency, duty cycle and capacitance measurements ■ 1000V rms input protection ; Input Alert™ detects wrong input jack connections ■ 3¾ digit, 4000 count di splay ■ ■ Fast analog display ■ Touch Hold® and Relative modes ■ Splash proof and dust proof case; EM! shielded ■ Protective holster with Flex-Stand" ' ■ Safety-designed test lead set ■ Made in U.S.A. 83 -O.C. Voltage 0.3 %, A.G. Voltage 1%, Frequency 5kHz $399 ex tax - $466 incl tax 85 - D.C. Voltage 0.1 %, A.C. Voltage 0.5% , Frequen cy 20kHz $480 ex tax - $560 incl tax 87 - True rms 4½ di git, 19 ,999 count hi-resolution mode . Back lit display with auto-off $580 ex tax - $676 incl. tax Analog/Digital Multimeters. Precision Multimeters built for Harsh Environments ■ 3200-count digital display, combined with fast 31-segment analog bar graph ■ "Min-Max" and "Relative" modes (27 onl?'l ■ Fast autoranging and Touch Hold functions ■ Rugged , O-ring sealed case for harsh working conditions ■ Separate, sealed battery/fuse door ■ Superior shie)ding against electromagnetic interference (EM!) ■ Meets military shock, vibration and water resistance requirements ■ Operates from -15°C to 55°C, and 95% relative humidity ■ 0.1 % basic D.C. accuracy, overload protection to 1000V rms, fus ed 10A range ■ Survives 6 kV transients on 660V major A.C. feeders ■ UL 1244 listed ■ Made in U.S.A. 25 -O.C. Voltage 0.1 %, A.C. Voltage 0.5% $428 ex tax - $499 incl tax 27 - + Min-Max and Relative Modes $570 ex tax - $665 ex tax Also call us for Oscilloscopes, Signal Generators, Power Supplies, Counters and any other T & M requirement. Full range of Fluke Accessories. Fluke made in U.S.A. t1:1c:r'l-:Fzsl' T & M PTY. LTD. VISA Call Tim Wortman the specialist Fluke Distributor TECH-FAST T & M PTY. LTD. 14B MAXWELL STREET. TURRAMURRA N .S.W. 2074 TELEPHONE: 988 3865 FAX: 988 3861 This view shows how the metal bodies of the dual gang pots are connected to earth (point B) on the display PCB. Also shown in the lead that connects from point A on the display board to point A on the main board. why not go for the maximum frequency possible? The reason that 500kHz should be the maximum obtainable frequency is because this gives the best possible hash reduction with the tracking filter, which operates in tandem with the coarse and fine frequency controls. If you have to change the value of the lOpF capacitor, increase its value to reduce the maximum possible frequency and reduce its value to increase the frequency. Note that there is space on the PCB to add a parallel capacitor if necessary. Now select the 1-lOOkHz range with switch S2 and the O.lHzlOOkHz range with Sl. Check the maximum frequency obtainable with the coarse and fine frequency controls set fully anticlockwise. Adjust VR4 until the output frequency approaches lOOkHz. If VR4 needs to be adjusted almost fully anticlockwise, the 22pF capacitor will have to be increased in value. If the frequency cannot be set close to lOOkHz, the 2 2pF capacitor have to be reduced. In the final result, you should be able to adjust VR4 so that the frequency range is slightly beyond lOOkHz. This will allow some overlap between ranges. Note that there will be some interaction between all these adjustments and so you may have to repeat them to get the final result. Final assembly The two transformer mounting nuts should be soldered to the copper pattern of the main board to ensure a good earth connection. meter. VR6 adjusts the O. lHzlOOkHz range readings, while VR7 adjusts the 100-500kHz range. Connect a frequency meter or oscilloscope of known accuracy to the output of the generator and select the 0. lHz-lOOkHz range. Set the output to a high frequency, say 90kHz, and adjust trimpot VR6 so that the frequency display reads the same as the external meter. Similarly, select the 100-500kHz range and set the output to a high frequency, say 450kHz. Adjust trimpot VR7 for the same reading on the frequency display as on the external meter. 60 SILICO N CHIP Finally, the overall frequency range of the generator needs to be set. Select the 100-500kHz range and determine the maximum frequency you can obtain from the generator with VR4 set to about half way. This frequency should be about 500kHz. If this frequency is reached well before the coarse and fine frequency adjust pots have reached their full travel or does not reach anywhere near this figure , then the lOpF capacitor at pin 3 of IC7b will require changing. If you are obtaining a frequency well in excess of 500kHz, you may think you have a bonus. After all, Now the front panel can be installed. Before you do that though, fit the BNC socket which is retained with a single nut. Don't forget the earth lug for the BNC socket - it fits under the retaining nut. Make the connections to the BNC socket with two short pieces of the hookup wire, then fit the front panel and the light mask to the display PCB. Secure the panel by fitting the nuts and lockwashers to the switches. Now you need to wire the mains switch. The Active wire from the mains cord goes to one side of the switch while the other side goes the transformer primary. Fit heatshrink tubing over the switch to make the mains connections safe from accidental contact. Finally, fit the lid onto the case and you are finished. ~