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Low distortion audio signal generator; Pt.2 This wide range audio signal generator has low distor­tion, very good envelope stability and a digital display. Last month we presented the circuit details and in this article we present the construction procedure. By JOHN CLARKE There is very little wiring inside this project because just about everything is mounted on the two PC boards. This includes most of the front panel hardware. Most of the assembly work just involves putting the two PC boards together. The two PC boards used are the main board coded 01402991 and measuring 212 x 141mm and the front panel PC board coded 01402992 and measuring 210 x 73mm. These two PC boards are sol­dered together at right angles and 62  Silicon Chip they mount in a plastic in­strument case measuring 256 x 190 x 84mm. The front panel has a red Perspex panel inserted directly in front of the LED displays. There is a label measuring 249 x 76mm which is fitted to the front panel. You can begin construction by checking the PC boards for any shorted or broken tracks and that the holes are drilled to accept the various components. You will need 1.5mm (1/16") holes for the terminals of the rotary switches on the front panel PC board and also there should be 3mm (1/8") holes for the corner mounting positions on the main PC board. Also 1.5mm holes are required for the potentiometers VR1 & VR2. Holes for the PC stakes should be such that they are a tight fit into the PC board before soldering. Two 10mm holes are required for the potentiome­ter shafts to protrude through the front panel PC board. Start assembly of the PC boards by inserting all the links and resistors. You will need to follow the component overlay diagrams of Fig.1 & Fig.2. Table 1 shows the resistor colour codes, to help you in choosing the correct value. Alternatively, you can use a digital multimeter to measure each resistor before it is inserted. The 27Ω 5W resistor is mounted so that its body is about 1-2mm above the PC board to allow cooling. Fig.1: this is the component overlay for the main PC board. Note that the LDR and LEDs1 & 2 are mounted in a light-tight tube (see text and photographs). Take care to ensure that all polarised parts are correctly oriented and note that regulator REG3 is bolted to the PC board and a U-shaped heatsink. MARCH 1999  63 Fig.2a (left): this is the component overlay for the display PC board. Note that the decimal points of the 7-segment displays should be adjacent to the associated driver transistors. Fig.2b at right shows the full-size PC artwork. Next, mount the PC stakes which are located at the wiring posi­tions on the main PC board. On the front panel board, PC stakes should be inserted for the BNC outputs, for switches S3, 64  Silicon Chip S4 & 6, and for the earth connections near S3 and the sync output. Mount the PC stakes for the switch and earth connections from the rear of the PC board to facilitate wiring and so that there is less to cut off when mounting the switches. Now insert the ICs, making sure that you place them in their correct positions with the orientation as shown. The display board carries the two rotary switches, the three toggle switches and the 7-segment LED displays. Note that the displays are mounted off the PC board using 5-way pin headers. The two BNC sockets on the front panel connect to the display board via PC stakes. Diodes D1-D12 can then be mounted, paying attention to their orientation. Make sure that the power diodes are placed in the D9-D12 posi­tions. The regulators can also be mounted at this stage. Note that the 7805 (REG3) is mounted horizontally and onto a heatsink. Next, the capacitors can be mount­ ed. Table 2 shows the IEC and EIA marked with EIA codes rather than the resistance value. VR3 is 100kΩ and may be coded 104. In the same vein, trimpots VR4-VR6 may be marked 10k or 103. When mounting the transistors, insert them so that their leads are about 6mm long above the board. The two LEDs and the LDR are at first inserted into the PC board and oriented as shown. Both the LDR and the LEDs are bent over at 90° so that the LEDs can shine directly onto the face of the LDR. Keep the front lens of the LEDs about 3mm away from codes which may be found on the MKT and ceramic types. Use the table to sort out the values and insert them in the positions as shown. The electrolytic types must be oriented with the polarity shown. Be sure to use 35V rated capacitors where indicated. You can mount the trimpots next. Make sure you insert each one in its correct position. Often trimpots are Table 1: Resistor Colour Codes                        No. 1 1 1 1 1 5 1 1 8 1 7 1 2 3 2 1 2 2 9 1 1 1 Value 560kΩ 470kΩ 360kΩ 330kΩ 120kΩ 100kΩ 47kΩ 20kΩ 10kΩ 5.6kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1kΩ 510Ω 470Ω 160Ω 51Ω 39Ω 24Ω 16Ω 7.5Ω 4-Band Code (1%) green blue yellow brown yellow violet yellow brown orange blue yellow brown orange orange yellow brown brown red yellow brown brown black yellow brown yellow violet orange brown red black orange brown brown black orange brown green blue red brown yellow violet red brown orange orange red brown red red red brown brown black red brown green brown brown brown yellow violet brown brown brown blue brown brown green brown black brown orange white black brown red yellow black brown brown blue black brown violet green gold brown 5-Band Code (1%) green blue black orange brown yellow violet black orange brown orange blue black orange brown orange orange black orange brown brown red black orange brown brown black black orange brown yellow violet black red brown red black black red brown brown black black red brown green blue black brown brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black brown brown green brown black black brown yellow violet black black brown brown blue black black brown green brown black gold brown orange white black gold brown red yellow black gold brown brown blue black gold brown violet green black silver brown MARCH 1999  65 The display board is attached at right angles to the main board by soldering two sets of matching copper pads together. Note that the two potentiometers are mounted on the main board and their shafts pass through holes drilled in the display board and the front panel. the LDR surface; this will allow the maximum amount of light coverage. The whole assembly is encapsulated in black heatshrink tubing with the ends blocked with some light proof sealant. You could use some automotive windscreen sealant or the commonly available “Blu Tak” or similar sticky adhesive for temporarily mounting lightweight items to walls. Setting the rotary switches Cut the shafts for the rotary switches to a length of 10mm and cut the potentiometer shaft 30mm long. Remove the nuts for each rotary switch and take out the locking pin washer. Rotate each switch fully anticlockwise. Now insert the locking pin washer for S2 (3-pole) in the “4” position and replace the nut. Check that this switch only rotates to four positions. Switch S5 (1-pole) has its locking tab washer inserted in the “9” position so that it can be rotated to nine positions. Having been adjusted, the rotary switches can be installed onto the PC 66  Silicon Chip board. Be sure that you do not stress the pins of the switches when inserting them into position. If the switch is difficult to insert, check that the holes are large enough and that the switch body is rotated so that the wiper pins are aligned correctly with the holes on the PC board. Table 2: Capacitor Codes  Value IEC Code EIA Code  0.56µF   560n   564  0.47µF   470n   474  0.18µF   180n   184  0.1µF   100n   104  .039µF   39n  393  .018µF   18n  183  .01µF   10n  103  .0047µF   4n7  472  .0018µF   1n8  182  .0015µF   1n5  152  180pF   180p   181  10pF   10p   10  3.3pF   3p3   3.3 The two potentiometers (VR1 & VR2) are mounted directly onto the main PC board. Switches S3, S4 & S6 mount by soldering the terminals onto the PC stakes allocated. Cut these down almost flush with the PC board so that the switch will sit as low as possible. Solder the terminals to the PC stakes. The four 7-segment LED displays are mounted off the PC board using pin headers. Install the 5-way pin headers in posi­tion for the displays and solder each display’s 10 pins to two 5-pin headers. They should be soldered so that the front face of the display is 20mm above the PC board. Make sure that each display is oriented with the decimal point located near the transistors. Connecting the PC boards As mentioned previously, the front panel PC board is at­tached to the main PC board by being soldered to it at right angles. To do this, first place the main PC board in position in the base of the case and check that none of the integral standoff pil­lars prevent the board from sitting on the four corner pillars. Any unused pillars can be cut down with a large drill to prevent them fouling Fig.3: this chassis wiring diagram shows the connections to the two PC boards and the power supply wiring. MARCH 1999  67 Fig.4: this is the full-size etching pattern for the main PC board. Check your board carefully against this pattern before installing any parts. the PC board. Now place the front panel PC board at right angles to the main PC board, with its lower edge on the base of the 68  Silicon Chip case and check that the edge is not siting on a raised rib section; some cases have these ribs and others don’t. If one of the ribs is in the way, remove it using a sharp chisel. Mark each end of the front panel PC board where it meets the main PC board. Then remove both boards and ELECTRONIC COMPONENTS & ACCESSORIES •  RESELLER FOR MAJOR KIT RETAILERS •  PROTOTYPING EQUIPMENT •  CB RADIO SALES AND ACCESSORIES •  FULL ON-SITE SERVICE AND REPAIR FACILITIES The aluminium rear panel carries the fused IEC mains socket, plus the power transformer and the earth terminal lugs on the inside of the case. Fit star washers and locknuts to all mounting screws, so that they cannot work loose. •  LARGE RANGE OF turn the main board upside down. Align the two PC boards so that the copper patterns for each match and the markings are in the correct position. Temporarily tack solder the two boards together at right angles in a couple of positions on the large copper areas and check that the positioning is correct when placed back in the case. If all is correct, you can now solder the remaining con­nections. Make sure all connections are soldered to ensure cir­cuit continuity. Croydon Ph (03) 9723 3860 Fax (03) 9725 9443 Front and rear panels The front panel can now be drilled out for the switches, poten­tiometers, LED display and input sockets, plus the Earth screw. Use the front panel artwork as a guide to drill the holes. Once the panel is drilled and the rectangular cutout made for the displays, you can attach the front panel label. The LED display cutout will require a red Perspex window which should be made to fit tightly in the hole. Wiring Place the front panel over the front panel PC board and wire the output and sync socket to the PC pins on the board using short lengths of tinned copper wire. You can now use the chassis diaThe LDR and the two LEDs are bent over at 90°, so that the LEDs shine directly onto the face of the LDR. These parts are then encapsulated in black heatshrink tubing and the ends blocked with light proof sealant. M W OR A EL D IL C ER O M E The input sockets must be insulated from the panel using an insulating kit. This requires two fibre washers and a short length of tubing. Secure these in place and do not forget to place a solder lug beneath a retaining screw for each socket. The rear panel requires mounting holes for the transformer, the earth terminal and the cutout for the fused IEC mains socket. This can be cut out by drilling a series of holes around the cutout border and removing the inside piece. The hole can then be filed to shape. Two holes are required for the mounting screws for this socket. Install these components with screws, nuts and lockwashers. ELECTRONIC DISPOSALS (COME IN AND BROWSE) Truscott’s ELECTRONIC WORLD Pty Ltd ACN 069 935 397 30 Lacey St, Croydon, Vic 3136 gram of Fig.3 to complete the remaining wiring. The mains wires must be 250VAC-rated and must be insulated at the switch terminals with heat­shrink sleeving. An insulating boot should be fitted over the IEC socket to prevent accidental contact with the terminals. The Earth wires must be run in the standard green/yellow striped wire and are terminated to solder or crimp lugs. These lugs are secured to the panels with a screw and nut and star washers, plus a further locknut to ensure that the earth lugs cannot possibly come loose. Tie the mains wires together with cable ties at the switch and the IEC socket. An Earth lead runs from the front panel solder lug to the GND PC stake on the front panel PC board. A separate wire is then soldered from this pin to the potentiometer bodies of VR1 and VR2. You will need to scrape the plating off the pot where it is to be soldered, to allow a clean joint. Testing When you have completed the assembly and wiring, check all your work carefully for mistakes. In particuMARCH 1999  69 POWER SILICON CHIP Hz kHz audio signal generator SYNC OUT DISPLAY MAX FLOAT MIN EARTH RANGE FREQUENCY SINE OUTPUT MIN SQUARE FINE LEVEL OFF 10k-100k 1k-10k 100-1000 10-100 70  Silicon Chip Fig.5: this full-size artwork can be used as a drilling template for the front panel. MAX OFF 1V 1mV 3.16V 316mV 3.16mV 100mV ON Setting up 31.6mV 10mV lar, be sure that the ICs are oriented correctly. Also check that each regulator is in its correct position and that it is oriented correctly. Now apply power and check that the Neon glows in the power switch (S1) and that the displays are alight. Check the voltages on the circuit using your multimeter. Clip the negative lead of your multimeter to the metal tab of REG1 and measure the supply pins for each IC. IC1, IC2, IC4 and IC5 should each have +15V at pin 8 and -15V at pin 4. IC3 should have +5V at pin 11 and -5V at pin 6. IC6 should have +5V at pin 14 and -5V at pin 7. IC7 should have +5V at pin 18. IC8, IC10 and IC12 should have +5V at pin 16. IC9 should have +5V at pin 14 and IC11 should have +5V at pins 4 & 8. Now check that the display is operating correctly. Firstly, make sure the display on/off switch is in the ON position. Now check that the display indicates a reading and that the decimal points light for the upper two frequency ranges. Note that you may not obtain a correct reading of frequency yet since the signal generator needs to be set up first. There are several adjustments required on the trimpots and trimmer capacitor before the Audio Signal Generator can work properly. First, the output level must be adjusted so that the generator produces a maximum of 3.16V RMS. This can be done by measuring the output with a multimeter which is set to read AC volts. The multimeter should have a useable AC response to at least 1kHz. Set VR3 to its mid setting and set the range switch to 1001000Hz, with the frequency adjust pot set midway. Now set the attenuator to the 3.16V setting and the output level pot to maximum (fully clockwise). Select the sinewave output. Measure this output level with your multimeter and adjust VR5 so that the level is 3.16V. Next, set the output to square wave. If your multimeter reads in RMS then set the square wave level using VR6 for a reading of 3.16V. If your multimeter does not read true-RMS values, it will be average-indicating and it will be calibrated to read the correct RMS value for a sinewave. To do this, it scales (or multiplies) the average value of a sinewave by 1.11. 1.11 is the “form factor” of a sinewave and is the ratio between the RMS value and the average value of a sinewave. When an “average indicating” multimeter reads the average value of other waveforms, it also multiplies them by the same scaling factor of 1.11 and this leads to an error when measuring the RMS value of square wave signals. Now the average value of a square wave when it is fullwave rectified is equal to its peak value and this is also equal to the RMS value. In other words, when rectified, a square wave signal of 1V RMS will have an average value of 1V and a peak value of 1V. So instead of setting the Audio Signal Generator to produce a reading of 3.16V on the top scale, we set it to 3.51V (ie, 3.16V x 1.11). The multimeter will read 3.51V but the generator will actually be delivering 3.16V RMS. Frequency setting Next, you can adjust VR4 so that the frequency readout reads correctly. On the 100-1000Hz range the meter should display from about 90Hz to 1100Hz. The last two adjustments set the operation of the oscilla­tor at the lowest and highest frequencies. VR3 sets the operation of the feedback control so that it maintains the amplitude This is the view inside the completed prototype. Keep the mains wiring neat and tidy (use cable ties) and be sure to earth the front and rear panels and the pot bodies as shown in the wiring diagram of Fig.3. level at the output of IC1b over the frequency range. You will not be able to use a multimeter to measure the output signal below about 45Hz and above about 2kHz since most multimeters are extremely inaccurate beyond these frequencies. However, you will be able to gauge the output quite simply by using the frequency display itself. Any sudden change in the frequency readout back to 0000 will indicate that the signal level has changed from its correct 3.16V maximum output, either to a value lower than or higher than this. The digital frequency readout thus becomes a signal indica­tor which stops working if the signal level is too high or too low. Adjust the frequency control to its lowest frequency and check that the display reads about 9Hz. If it is showing 9Hz and then suddenly drops back to 0000, then adjust VR3 slightly more anticlockwise and set the frequency control to maximum to regain amplitude control. The display should now read correctly. Now return to the lowest frequency and check that the readout stays at about 9Hz. If it drops back to 0000 again, readjust VR3. Note that you will need to wind the frequency control to a higher frequency again each time to regain a frequency readout. When you can obtain a constant 9Hz readout, observe this for a few seconds to be sure that the reading remains. At this low frequency, the amplitude can slowly drift higher and higher unless VR3 is set correctly. Now set the range switch to the 10-100kHz position and wind the frequency control up to its maximum. The frequency readout will probably drop back to 000.0, either because the signal has dropped to zero or because it has begun to oscillate of its own accord. Either way, trimmer capacitor VC1 will need to be adjusted to regain control. This is simply a trial and error adjustment until the frequency display reads cor­rectly on this range. Finally, you may wish to calibrate the frequency meter. This will usually not be necessary because it will be accurate enough for most purposes. You can check the frequency accuracy using a frequency meter or by checking the period on an oscillo­ scope. Most oscilloscopes have a calibration output which produc­es a 1kHz signal. The 1kHz output from the signal generator should match this calibration output. Calibration involves changing the value of the .01µF capacitor on pin 2 of IC11. Make the value larger if the reading is too low or smaller if the SC reading is too high. MARCH 1999  71