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In this final article, we describe construction of the VGA Oscilloscope, plus testing and operation. This is a relatively straightforward process, with most components mounted on printed circuit boards. By JOHN CLARKE Part 3: Constru ction Build a VGA digital oscilloscope The VGA Oscilloscope is mounted in a plastic instrument case measuring 262 x 189 x 84mm. A Dynamark label measuring 252 x 76mm is fitted to the metal front panel. Most of the components are mounted on five PC boards and these are: the front panel PC board coded 04307961 and measuring 252 x 75mm; the main PC board coded 04307962, measuring 213 x 142mm; the rear timebase board coded 04307963 measuring 252 x 75mm and finally, two memory surface mount PC boards coded 04307964 and measuring 20 x 32mm. Begin by checking the all the PC board patterns against the published artworks. Check for undrilled holes, broken tracks or shorts and fix these before proceeding. Also check that the front and rear boards fit neatly into the slots of the case and file the board 16  Silicon Chip edges to size if they are too large. Memory boards Work can start on the two small memory boards. These, for IC4 and IC10, are intended to be used with the copper side up, suitable for surface mount devices. The board overlay diagrams for both of these ICs are shown in Fig.1. For best results we recommend that pads for the ICs are pretinned using a fine tipped iron. Once the pads are tinned, locate the IC in position, making sure it is oriented correctly and solder the four end pins in place using a minimal amount of solder. Now solder the remaining pins, taking care not to solder any two pins together. Once done, you should check with your multimeter that each pin of the IC does in fact connect to the track, as shown on the published PC artwork. Also check that adjacent pins are not shorted except where the tracks on the overlay show that they are intended to connect. Solder blobs between adjacent pins can be removed with solder wick and a soldering iron. After any repairs have been done, we recommend a thorough final check of the connections. Do not forget to install the 0.1uF capacitor from the copper side. In final assembly, the memory boards are attached to the main PC board using short lengths of tinned copper wire, soldered to the top side of the memory board and to the underside of the main board. Main PC board Now move on to the main PC board. Its component overlay is shown in This photo shows the location of the various front panel controls. The vertical PC board behind the front panel supports all of these controls and associated components. Trimpot VR6 and the PC stakes are installed next. We did not use stakes in the two 4-way locations above IC24 and IC28. Fig.2. Insert all the links, using tinned copper wire, and solder them in place. All the ICs, with the exception of IC4 and IC10, can be inserted. Take care to install the correct type in each place and with the correct orientation. Now insert and solder the diodes and resistors in place. The accompanying resistor table gives the colour codes for each resistor value. It is also good practice to use a digital multimeter to verify each resistor value. The voltage regulators (REG1 & REG2) are mounted horizontally and held in place with a screw and nut. Bend the regulator leads to insert them into the holes provided before installation. Make sure you place the 12V regulator (REG1) in the position closest to the PC board edge. Capacitors can be mounted next. The 1000uF capacitor is placed on its side with the orientation shown. The remaining electrolytic capacitors also must be oriented with the correct polarity. 8-way header pins are installed in the positions adjacent to IC15, and near IC11 and IC12. Finally, insert the trimpots and PC stakes. assembly can proceed in the same order as the main board. Take care to orient the ICs and diodes with the polarity as shown. When installing the transistors, take care to place the BC338s in positions marked Q3, Q6 and Q9. The BC548’s go in positions marked Q4 and Q7 while the BF199 devices are installed at Q5 and Q8. Timebase PC board Two plug-in memory boards are used in the VGA Oscilloscope. The ICs on the boards are surface mount devices and require care when soldering. Fig.1 (above left) shows the PC board layouts and patterns, with a close-up photo of one assembled board at right. The component overlay for the timebase board is shown in Fig.3. Its Front panel board The front panel PC board is shown in Fig.4. Carefully check out the board pattern as before and then install the links, resistors, diodes and capacitors, with the exception of the 0.22uF types. Note that LED1-LED4 are September 1996  17 Fig.2: the parts layout for the main (horizontally mounted) printed circuit board. Solder in all of the wire links first, then proceed with the passive components and finally the diodes, transistors, regulators and finally the ICs. Opposite is the fullsize main printed circuit board pattern. mounted flat to the PC board with the cathode lead (the shortest one) bent sideways to fit into its hole. Do not shorten the leads for LED5. They need to be the full length so LED5 can reach the front panel. VC1-VC3 are mounted on the rear of the PC board for ease of adjustment later on. Before installing any of the pots or rotary switches, cut their shafts to about 12mm long, so that the knobs will fit neatly in place. Switches S2, S5, S4, S6 - S10 and S12 are all soldered directly into the PC board. PC stakes are required to mount the slide switches S1, S3 and S11 and pots VR2, VR4 and VR5. The leads on the pots are bent over to solder to the top of the PC stakes. Soldering in the slide switches S1, S3 and S11 requires a little more patience. The connecting lugs of each switch are inserted between the rows of PC stakes and carefully soldered in place. Check that the pins are connected by testing with a multimeter. Then solder in the 0.22uF capacitors and the PC stakes required for off-board connections. The vertical attenuator switches S2 18  Silicon Chip and S4 will be supplied as single-pole 12-position types and will need to be set to provide eight positions. This is done by rotating the switch fully clockwise and then lifting out the locking washer and repositioning it so that its tab sits in position 4. After this is done for S2 and S4, check that each switch will provide eight positions. Switch S5 needs to be set to 11 positions. In this case the switch is rotated fully anticlockwise and the locking tab placed in position 11. Check that the switch rotates through 11 positions. Before installing any of the boards in the case, it is best to drill the rear panel holes for the VGA lead and for the DC socket. These holes must line up with those on the rear panel PC board. The PC board hole for the socket is made large enough to accom- modate the DC socket pins which will protrude through it when assembly is complete. Fit a grommet into the rear panel for the VGA cord. Pull the VGA cord through the hole and secure it to the PC board using a cord clamp. Then fit the DC socket to the rear panel. The front panel label can be affixed to the metal panel and drilled to accommodate the switches, pots and BNC sockets used for the input connections. The rectangular holes for the three slider switches are filed to shape after they have been drilled out. Then secure the BNC sockets to the front panel, using a star washer, nut and solder lug on each. The socket is connected to the front PC by soldering the centre pin to the PC stake and the earth connection via a short length of tinned copper wire to its GND PC stake. Attach the front panel to the front PC board by securing it with the switch nuts. The pot nuts are not required. Fit all the knobs to the shafts of the pots and switches. Before installing the main board in the case, it is necessary to shorten all the integral standoffs on the base. They should all be drilled off except for those at the outermost four corners. Also cut off the small upright spikes with side cutters. Then attach the main PC board in place, using self tappers into the four remaining integral standoffs. Slide the front and rear panel PC board assemblies into the case slots and the remaining wiring can be done. Wiring Fig.5 shows the wiring between the PC boards. Most of this is done with hookup wire. We used ribbon cable split into strips of four for connecting September 1996  19 20  Silicon Chip Fig.3 (top): the component layout for the timebase PC board, which mounts vertically at the rear of the case. Above is its associated PC board pattern, reproduced full size. September 1996  21 Fig.4 (top) : the component layout for the front panel (vertical) PC board, with its PC board pattern, reproduced full size. The three photographs above are effectively an exploded view of the VGA Digital Oscilloscope, with the front and rear vertically mounted PC boards "folded out" from the main PC board similar to the component overlay diagram on the facing page. Both vertical boards mount in slots in the case with their components towards the front. 22  Silicon Chip Fig.5: the wiring diagram showing how the various boards are interconnected. Use this in conjunction with the photographs on the opposite page along with the circuit diagram in last month's issue. the 8-way pin headers on the main PC board to the rear panel board. Shielded cable is used to connect from the front panel to op amps IC1 and IC7. The VGA cable is terminated onto the rear panel board at the positions indicated. Fig.6 shows the pin-out arrangement for a VGA socket. We September 1996  23 CAPACITOR MARKING CODES ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ Value    IEC Code 0.22µF 220n 0.1µF 100n .047µF 47n .0039µF 3n9 .0015µF 1n5 .001µF 1n0 680pF 680p 560pF 560p 470pF 470p 390pF 390p 150pF 150p 47pF 47p 22pF 22p EIA Code 224 104 473 392 152 102 681 561 471 391 151 47 22 purchased a VGA cable from Dick Smith Electronics and it used white for the line sync, dark brown for frame sync, orange for the blue trace, red for the green trace, light brown for the red trace and purple, light blue, light green, dark green and un-insulated wire for the ground. This may not be the same for your VGA cable so check this carefully with your multimeter. When complete, tidy up all wiring with cable ties. Testing Before applying power, check your wiring carefully for errors. In particular, check that the positive and GND wires from the main PC board connect         RESISTOR COLOUR CODES ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ ❐ No. 1 1 1 1 2 1 2 1 1 2 3 1 2 2 2 3 2 1 2 8 3 1 1 1 5 10 2 1 8 2 1 1 3 Value 10MΩ 3.9MΩ 2.2MΩ 820kΩ 510kΩ 390kΩ 240kΩ 220kΩ 150kΩ 130kΩ 100kΩ 82kΩ 75kΩ 51kΩ 47kΩ 39kΩ 27kΩ 20kΩ 12kΩ 10kΩ 7.5kΩ 6.8kΩ 3.9kΩ 3.3kΩ 2.7kΩ 2.2kΩ 1.8kΩ 1.5kΩ 1kΩ 330Ω 220Ω 120Ω 75Ω 24  Silicon Chip 4-Band Code (1%) brown black blue brown orange white green brown red red green brown grey red yellow brown green brown yellow brown orange white yellow brown red yellow yellow brown red red yellow brown brown green yellow brown brown orange yellow brown brown black yellow brown grey red orange brown violet green orange brown green brown orange brown yellow violet orange brown orange white orange brown red violet orange brown red black orange brown brown red orange brown brown black orange brown violet green red brown blue grey red brown orange white red brown orange orange red brown red violet red brown red red red brown brown grey red brown brown green red brown brown black red brown orange orange brown brown red red brown brown brown red brown brown violet green black brown 5-Band Code (1%) brown black black green brown orange white black yellow brown red red black yellow brown grey red black orange brown green brown black orange brown orange white black orange brown red yellow black orange brown red red black orange brown brown green black orange brown brown orange black orange brown brown black black orange brown grey red black red brown violet green black red brown green brown black red brown yellow violet black red brown orange white black red brown red violet black red brown red black black red brown brown red black red brown brown black black red brown violet green black brown brown blue grey black brown brown orange white black brown brown orange orange black brown brown red violet black brown brown red red black brown brown brown grey black brown brown brown green black brown brown brown black black brown brown orange orange black black brown red red black black brown brown red black black brown violet green black gold brown to the correct points on the front and rear PC board. Reverse polarity on a PC board may cause IC damage! Apply power, check that the LED lights and that the regulators provide an output voltage of +12V from REG1 and +5V from REG2. Now you can check supply on all the ICs. Checking the front panel ICs can be done from the rear of this PC board. IC1, IC2, IC7 & IC8 should have 12V between pins 7 and 4. IC3 & IC9 should have 5V between pins 20 and 8. IC4 & IC10 should have 5V between pins 14 and 28. IC5, IC6, IC11, IC12, IC16, IC17, IC18, IC24 & IC25 should have 5V between pins 8 and 16. IC13, IC20, IC22 & IC28 should have 5V between pins 8 and 1. IC14, IC15, IC19 IC23, IC26, IC27 & IC29 should have 5V between pins 7 and 14. IC21 should have 12V between pins 11 and 8. If all voltages are correct you can test the oscilloscope using a VGA monitor. Turn all power off and connect the VGA lead to your monitor. Apply power to the oscilloscope first, then switch on the monitor. You should obtain at least steady blue vertical graticule lines on the screen. The horizontal graticule lines may not be present. If the graticule is broken up with rolling or with S-shaped patterns, then you have lost vertical or horizontal sync or the ground connections are disconnected. Check wiring to the timebase and main boards for shorts, dry solder joints or discontinuities in tracks. Also recheck the VGA socket connections. Select a timebase other than 50us Fig.6 (above): the standard pin-outs for a VGA socket. . \ \ CH1 SLOPE . . POSITION \ \ - STORE + . TRIGGER LEVEL TIME/DIV . SOURCE . CH2 . . POWER UPDATE . TRIGGERED REALTIME . NORM FAST SLOW VOLTS/DIV AC GND DC . . MAG x1 x2 x4 FREE RUN POSITION VOLTS/DIV AC GND DC . CH2 .5. . .2 (RED) . .1 . . .05 1. 2. 5. . 10 VGA OSCILLOSCOPE 2ms . .1ms . .5ms . . .2ms . . .1ms 50µs 5ms . 10ms . 20ms . 50ms . . .1s .5. . .2 . .1 . . .05 will be seen as many dots in a disjointed arrangement on the screen. When the frequency is adjusted so that the A-D converter operates correctly, the trace will appear normal with all dots following each other. If correct adjustment is not possible, increase the 47pF value at pin 2 and 6 of IC13 to 56pF. The VGA oscilloscope is now ready for use. Note that if GND input is selected, you will also need to switch to Free run triggering to obtain the update straight line on the screen. Any deviation from the straight line is due to noise and least significant digit error in the A-D conversion process. This is normal in a digital oscilloscope. If the timebase selected is too slow for the signal being measured, a phenomenon called “aliasing” will occur. This happens since the sampling rate is not fast enough to obtain half a cycle of the waveform and a trace will be displayed which is of a much lower frequency than the incoming signal. The problem is instantly recognised on the VGA oscilloscope since the waveform cannot be triggered correctly so that it remains steady. In most cases the waveform also shows as an envelope where two traces are evident with one being 180 degrees out of phase to the other. If the oscilloscope is to be used to measure mains voltages take note of these precautions. Set the volts per division switch to 10V. Use only a x10 probe and do not use the earth connection since you may incorrectly attach it to Active. The oscilloscope is earthed via the VGA monitor. If the mains voltage is above 250VAC, the trace will over­ range. To prevent this, the VR1 & VR3 calibration trimpots can be adjusted so that the trace level is reduced. This will uncalibrate SC the volts/division setting. 1. 2. 5. 10 . Fig.7: the front panel artwork for the VGA Digital Oscilloscope, reproduced full size. CH1 (GREEN) and check that the red and green traces can be moved up and down the screen using the position controls. Note that if the traces are moved above the top of the screen they will produce a slanted two line trace on the lower screen portion. This is a sign of overrange. Signals brought to the bottom of the screen will flatten out to a straight line. Several adjustments are required before the VGA oscilloscope is ready for use. The first is to adjust VR6 to obtain the horizontal graticule lines. You will find that there are several settings for VR6 which will give the horizontal lines. Use the setting which centrally locates the graticule in the screen. Check operation of the VGA oscilloscope by applying a square wave signal to the inputs and adjust the timebase and sensitivity for the best display. Note that you will need to select the Free run and Real time switch positions. To trigger the trace, select the source (CH1 or CH2) the polarity and the Triggered position. Now adjust the trigger level so that the trace is triggered and is updated (as indicated by a momentary loss of display periodically). Use the update selection which best suits your purpose. Check that the MAGnification switch provides an expanded timebase. Adjust the trimmer capacitors VC1 and VC2 for best square wave response. This means that the waveform should be square without overshoot or rolloff at the rising and falling edges. Adjust trimpots VR1 and VR3 for correct vertical calibration. If the peak-to-peak voltage of your signal is not known, measure the voltage of a battery using a multimeter. Then measure it on the VGA oscilloscope with the DC input selected. Now adjust the trimpot for a correct volts per division reading. If the frequency of the oscillator is accurately known, check that the timebase calibration is correct. Now select the 50us timebase and adjust VC3 until the traces stop breaking up. In other words, adjust VC3 to set the maximum frequency before the A-D converters stop operating correctly. Incorrect A-D operation September 1996  25