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A high quality audio oscillator: Pt.2 Our new high quality oscillator has a number of interesting circuit features, some of which were described last month. This month, we describe the remaining circuitry and give the construction details. By LEO SIMPSON Last month, we concluded with the description of the oscillator circuitry. Now let us move to the square wave circuitry. This needs to provide square waves which are as square as possible (ie, with 50% duty cycle) and with minimum overshoot or droop. The rise and fall times also need to be very fast which is necessary if the oscillator is to be faster than any audio equipment under test. The output signal from the low distortion oscillator section is fed via a 2.2µF bipolar capacitor and 8.2k0 resistor to a 74C14 CMOS Schmitt trigger. Two Schmitt trigger sections, ICta and ICb, are cascaded to give a very good square wave which is precisely ad32 SILICON CHIP justable for duty cycle by tweaking the DC voltage fed to IC1a with tkO trimpot VR2. While the square wave from the second Schmitt trigger is pretty good, it is only about 12 volts peakto-peak as set by the + 12V rail from the associated 78112 3-terminal regulator. We need a square wave with an amplitude of 20 volts peak-to-peak to give the same RMS amplitude as from the oscillator and it must also have a low output impedance. The first step in achieving this is to connect the signal from IC1 b to the remaining four sections of IC1. These sections are connected in parallel to form a fast buffer circuit and drive the gate of VMOS Fet Qt 1, which is a Siliconix VNtoKM. This is an extremely fast device with rise and fall times of potentially only a few nanoseconds. Qt 1 acts as a very fast switch to boost the square wave signal to just over 20 volts peak to peak. From the drain of Qtt, the square wave signal is coupled via a 100µF capacitor to a rather unusual buffer circuit consisting of four transistors: Q12, Q13, Qt4 and Q15. It , is based on the circuit configuration used in the National Semiconductor LH0022. The four transistors effectively act as complementary compound emitter followers and they are arranged so that there is virtually no DC shift from input to output. This is accomplished in the following way. As the signal passes from the base to the emitter of Qt2, it is shifted up in potential by the baseemitter voltage. Then, as it passes from the base to the emitter of Q14, it is shifted down by almost exactly the same amount. The same process occurs in the complementary transistcirs Q13 and 015. Note that Q12 to Q15 are spec- SEE SEPARATE DIAGRAM FOR S1, S2, S3 AND S4 CDNNECl'lbNS Fig.4: here's how to install the parts on the PCBs and wire up the chassis. The idea is to build & test the power supply board first, before moving on to the oscillator board. Refer to Fig.7 for the connections to S1, S2, S3 & S4. ified as complementary types and this is important for stabilising the quiescent current through Q14 and Q15, which is set at around 1.2 milliamps. Any tendency for the current through the output transistors to increase will result in a slight increase in the voltage across the 6.80 emitter resistors and this will throttle them back. At the same time, any tendency for the Vbe of the output transistors to drift will be compensated for by an equivalent drift in the input transistors, Q12 and Q13. Since the whole buffer is effec- tively an emitter follower using small signal transistors, it has a very wide bandwidth we estimate that it is at least 30MHz. The buffer runs from a + 22V supply and so can deliver a square wave of 10 volts RMS. The 2000 trimpot VR3 in the drain circuit is provided FEBRUARY1990 33 The two lamps in the negative feedback path of the oscillator are mounted directly on the PCB. Be sure to use the types specified in the parts list (Farnell Electronics Cat. CM7374). as simple as you will ever see in an instrument of this kind but has the virtue of good linearity and excellent frequency response - more than adequate for the frequency range to 1 lOkHz. In fact, the metering circuitry is the same as used for the AC ranges of most cheap analog multimeters. D3 and D4 are simply halfwave rectifiers connected in a bridge arrangement with two 10k0 resistors. The circuit has advantages over a conventional bridge rectifier in that (a) only two diodes are used instead of four; and (b) the resulting linearity is better since only one (germanium) diode is operative at any instant. The linearity of the circuit is very good because the signal being measured is large, up to 14 volts peak or more, in relation to the germanium diode forward voltage drop of 250mV or less. Separate trimpots, VR4 and VR5, are required for metering the sine and square wave outputs since the peak to peak voltages of the waveforms are different. As mentioned above, a 10V RMS square wave has a peak-to-peak voltage of 20 volts whereas a 10V RMS sinewave has a peak-to-peak swing of 28.28 volts. Output attenuator The output attenuator is a simple resistive divider with 10 lOdB steps from 1mV to 10 volts RMS. The 4700 resistor in series and the resistance of the divider plus VR6 combine to give a nominal output impedance of 6000. Check the output voltages from the power supply before wiring it to the oscillator board. All connections to the mains switch must be run using 240V AC cable and any exposed connections sleeved with heatshrink tubing. to adjust the square wave output to exactly 20 volts peak to peak which is equivalent to 10 volts RMS. Sine/square switch S5 is the sine/square selector. It has three sections: S5a, S5b and S5c. In the sine mode, S5a switches the wiper of trimpot VR2 to the + 12V rail. This pulls the input of ICta high and effectively disables the square wave circuitry. It also turns Mosfet Qt 1 off so that no current is consumed. S5b connects either the sinewave 34 SILICON CHIP output from the oscillator or the square wave output from the aforementioned buffer stage to the tkO variable output control, VR6, and thence to the output attenuator, S6. S5c connects the signal from the wiper of VR6 to the metering circuitry. This involves germanium diodes D3 and D4, the associated 10k0 resistors, and trimpots VR4 and VR5. Metering circuitry The metering circuitry is about Power supply The power supply uses a transformer with two 12V windings connected in series. These feed two half wave rectifiers and lOOOuF filter capacitors to give unregulated supplies of about ± 33V. These are fed to LM317 and LM33 7 3-terminal regulators to give ± 22V. These regulated rails supply the oscillator and square wave buffer circuits while a 78L12 3-terminal regulator feeds the 74C14 (ICt). Construction Our prototype oscillator was built into a standard Horwood This view shows the arrangement of the hardware inside the metal case. The Horwood case will have to be disassembled into its component panels before drilling the necessary mounting holes for the PCBs, switches, meter and other hardware items. Take care with the mains wiring. metal instrument case, measuring 305 x 102 x 203mm. Two printed circuit boards are used, one for the power supply and one for the oscillator, although many of the components are mounted around the rotary switches. The oscillator board measures 207 x 93mm and is coded 04101901, while the power board measures 108 x 64mm and is coded 04101902. Before we start on the details of construction, we'll point out that this is a tricky unit to build. There is a preferred order of assembly and if you don't follow it you will find the job a lot harder. The Horwood case should be disassembled into its component panels, some of which will need drilling to mount PC boards or hardware such as switches and so on. Having disassembled the Horwood instrument case, the next task is to assemble the power supply board and get it going. This will then give you a power supply to get the oscillator board going. Check the power supply board carefully before assembling any components onto it. Look for shorts between tracks, open circuit tracks, and holes that have not been drilled. The board can then be used as a template to drill four 3mm mounting holes in the righthand side panel of the Horwood box. Now fit the board with the five PC stakes for the outputs. Next, mount and solder all the components with the exception of the power transformer. Take particular note of component polarities: diodes. electrolytic capacitors and 3-terminal regulator. Follow the component layout shown in the main wiring diagram (Fig.4). Take particular care with the 3-terminal regulators and note that the terminals for the LM337 are different from those of the LM317 (see the circuit diagram, Fig.3, in last month's issue). Before mounting the transformer, fit short lengths of sleeving over the three primary termination lugs which protrude from the side of the bobbin. These are a safety hazard and otherwise could cause an electric shock later, when you are working on the project. Now mount and solder the power transformer to the board. Temporarily terminate a 3-core mains flex (with moulded 3-pin plug) to the board and then mount the whole board assembly on the case side panel. The side panel can be temporarily earthed via the earth lead of the mains cord to make the whole assembly electrically safe. Now apply power to the supply board and measure the output supply rails. They should be close to ± 22V. It is a good idea at this stage to mark the three supply pins on the FEBRUARY1990 35 Keep all wiring leads to the front panel hardware as short as possible. The LED is mounted using a plastic bezel. board with " + ", " - " and "OV" in pencil so that you can easily identify them. It is also a good idea to temporarily wire the LED across its terminals so it can function as a power indicator. 2 x 1 POLE, 12 POSITION WAFERS ~ SHIELD PLATE \ + S1 ,S2,S3 Oscillator board The procedure with the oscillator board is to assemble it with all components except for the 74C14 (IC1) and the Mosfet (Q11). After checking the copper pattern carefully for any defects, use the oscillator board as a template to drill four 3mm holes in the base plate of the case. The only other hole which needs to be drilled in the base plate is for the earth point solder lug which is situated just behind and between the two front panel BNC sockets. With the holes in the base plate drilled, you can proceed to assemble the board. First insert the PC stakes (25 required) and then the resistors and diodes. When this has been done, install the transistors and capacitors. Note that the resistors associated with the attenuator are all 1 % types (with five colour bands) while the rest of the resistors can be 5 % types. Use a digital multimeter to check each resistor value before you insert and solder it into circuit. The two miniature incandescent lamps are 28V 40mA types (not 24V types as shown on the circuit diagram, Fig.3, in last month's 36 SILICON CHIP ALL WAFERS MAKE BEFORE BREAK Fig.5: you can either buy the wafer switches fully assembled or you can save money by assembling your own as shown here. The parts are all available from Farnell Electronic Components (see text). Don't leave out the shield plates used on switches S1-S3. issue). They have a plastic base which allows them to sit squarely on the board. For the trimpots, we suggest you use good quality Cermet types. Note that VR3 should have a rating of 0.5 watt. Oscillator checking With all components except IC1 and Ql 1 installed, you are ready to power up the board and check its operation. First, we need to check the DC conditions on the board. To do this, the circuit is set up as a DC feedback amplifier by connecting a 22k0 resistor in parallel with a .068µF capacitor between points J to G. Now connect the ± 22V rails from the power supply board and check the voltage at the junction of the 150 emitter resistors for Q9 and Q10. It should be be close to OV. Now check the quiescent current through Q9 and Q10 by measuring the voltage across the two 150 emitter resistors. For a current of 15mA, the voltage should be 0.45 volts. Provided the measurement is between 0.3V and 0.45V, no adjustment is needed. If a measurement between 0.3V and 0.45V is not obtained it will be necessary to change the 1k0 resistor between collector and base of QB. To increase the voltage, try changing the resistor to 1.2k0. To reduce it, try shunting the 1k0 resistor with a value of 10k0. To make the circuit oscillate, connect another 22k0 resistor in series with a .068µF capacitor between points H and J. Now apply power and the circuit should oscillate at close to 100Hz factually around 106Hz). With your digital multimeter set to AC voltage, use VR1 to set the output amplitude to 10 volts RMS. Also, while the board is powered up, check that + 12V appears at point N on the oscillator board. This checks that the 78112 is OK. Incidentally, if you can't obtain a 78112, you can use a 7812 instead. Square wave section With the sinewave circuit operational, you can now install IC1 and Ql 1, set VR2 to mid-travel and then power up once more. If you have an oscilloscope, you can check that a 100Hz square wave appears at pins 2 to 6 and pins 8 to 13 of ICl. It should have an amplitude of close to 12 volts peak. The square wave amplitude at the drain of Q11 should be larger, depending on the setting of VR3, and it should be almost identical at the output of the high speed buffer stage, at point 1. You can now adjust the square wave amplitude and duty cycle. Several methods are possible, depending on the test gear you have available. To set the square wave duty cycle, set your multimeter to the lowest DC range, connect it to point 1, and adjust VR2 for a zero reading. To set the amplitude to 10 volts RMS, set your multimeter to measure AC voltage and adjust VR3 to obtain a reading of 11 volts. (Yes, you read it right: 11 volts). After the oscillator is fully assembled you will want to go through and do a final tweak on VRl, VR2 and VR3. You can now put both the power and oscillator boards to one side and then work on the front panel. Front panel assembly Quite a lot of work is involved in the front panel. First, it needs to have all holes drilled and the meter cut-out made. You can use the front panel artwork as a template for this task. After that, the multiwafer switches must be assembled, according to the diagrams in Fig.5. You can purchase the switches fully assembled from C&K Electronics (Aust.) Pty Ltd, 15 Cowper Street, Parramatta, NSW 2124. Phone (02) 635 0799. By the way, all the wafer switches should be make-before-break types. This was not specified in the parts list. o-0 re re re 0--0 • .... 0 .... o--ol!"4 0--0--0 • V 0 u Ol ~ Fig.6: this is the full size artwork for the main oscillator board. Alternatively, you can buy the individual parts for the wafer switches from Farnell Electronic Components Pty Ltd, 72 Ferndell Street, Chester Hill, NSW 2162. Phone (02) 645 8888. By assembling the switches yourself you save some money and you also get more rugged switFEBRUARY 1990 37 WAFER NEAREST CLICKER PLATE S4d WAFER NEAREST CLICKER PLATE CAPACITORS ON S4a TERM IN ATE ON S4b CAPACITORS ON S4c TERMINATE ON S4d OpF 70pF WAFER REMOTE FROM CLICKER PLATE ·\ WAFER REMOTE FROM CLICKER PLATE G are 2 % types or better, as specified in the parts list in last month's article. Fit switches S4 and S2 with the three leads which run down to the oscillator board, at points G, Hand J. They should be left about 10cm long for easy termination to the board. Now, before fitting switches S1 to S4 to the front panel, you should fit the meter movement and the remaining hardware such as the two insulated BNC sockets, the sine/ square switch S5, the float/GND switch S7 and the output control VR6. WAFER NEAREST CLICKER PLATE WAFER REMOTE FROM CLICKER PLATE WAFER NEAREST CLICKER PLATE WAFER REMOTE FROM CLICKER PLATE All WAFERS VIEWED FROM REAR Fig.7: wire up the wafer switches as shown here before installing them on the front panel of the case. It is a good idea to check each resistor value with a digital multimter before mounting it in place. ches (in our opinion). However, we would have preferred to have the switches somewhat easier to rotate - their springs are just too stiff. If you do decide to assemble the switches, you will need the following Farnell Electronic Components parts: 4 mechanisms (146033) 6 wafers 1-pole 12-position (146038) 2 wafers 2-pole 6-position (146039) 3 screens (146042) 14 6mm spacers(146044). They should be assembled as shown in the two diagrams of Fig.5. Note that S6, the attenuator, is a standard 1-pole 12-position switch which is set to give 10 operating positions. It can be purchased as an over-the-counter item from most parts stockists. The shafts of all the rotary switches should be cut to a length of about 12mm before any components are wired to them. That is the easy part. You now have to wire the switch wafers as 38 SILICON CHIP shown in the diagrams of Fig.7. Note that all the resistors shown on the switch wafers are 1 % types and, as before, you should check each value with a digital multimeter before installing it. All the capacitors on switch S4 With all hardware and switches mounted on the front panel, you can complete the wiring to the oscillator board. Attach the base panel to the front panel with the two selftapping screws and then proceed to do the wiring in a way so that you won't have to push the soldering iron through existing leads to make additional connections. For example, wire the two BNC sockets first, then the float/GND switch S7, then the sine/square switch S5, then VR6 and so on. Leave the attenuator switch, S6 till last. When all the wiring between the front panel and the oscillator board is complete, you can finish the assembly of the case and do the mains wiring to the power supply. Note that there is no connection between the circuitry of the oscillator and the mains earth, although the case is earthed back to the mains. This is to avoid earthing SC04101 901 00 Fig.8: full-size artwork for the power supply board. D.DAUNER ELECTRONIC COMPONENTS WE STOCK A WIDE RANGE OF ELECTRONIC PARTS for • Development • Repair • Radio Amateur • Industrial Electronic • Analog and Digital WHILE STOCKS LAST Quartz in filter 10.9MHz 6kHz BW. $12.50 US Filter capacitor 4µF 3kV . . ... $15.00 Electromagnetic Airpump for Aquarium. . ...... $9.00 Timer Motor 240VAC 6RPH ...... $6.00 Come and see. This view shows the mounting details for the four wafer switches which are used to set the output frequency. Note that the lead length of the components mounted on the switches are kept as short as possible. problems when you are making measurements on sensitive audio equipment. Final adjustments Now power the oscillator up and repeat the adjustments to VRl, VR2 and VR3. This done, set the oscillator to the sine mode and, with the output adjusted to give 10 volts RMS, adjust VR5 to give full scale deflection. Finally, set the unit to the square wave mode and adjust VR4 for full scale deflection of the meter movement. After that, all that remains is to check the operation of the attenuator and then the frequency setting switches. You will need an oscilloscope or a frequency meter for this latter task. Showroom: 51 Georges Crescent, Georges Hall, NSW 2198 (Behind Caltax Sanlca Station In Blrdwoad Road) Phone 724 6982 TRADING HOURS: Monday to Friday 9.00 a.m. to 4.00 p.m. Saturday from 9.00 a.m. to 12.00 noon. Lamp option As an interesting exercise, we have tried quite a number of different incandescent lamp combinations to see if we could get better performance. Basically, the lamp(s) required need a high resistance filament. The best lamp we found, apart from the Farnell type specified, was a 240V 15W pilot lamp with a standard bayonet fitting, as available from most hardware and lighting stores. This lamp gave lower distortion at low frequencies but longer amplitude settling times. ~ ACTIVE SHORT WAVE ANTENNA TECHNIKIT AT4SW j: (SEE SC JAN '90) COMPLETE KIT $59 BUILT and TESTED $119 (BATTERIES INC) CASE $10 LOOP ANTENNA Q TECHNIKIT PX1 COMPLETE KIT $44 BUILT and TESTED $69 (SEE SC JUNE 89) Improved signal strength & signal quality in a portable tunable antenna. .I.V....;. r.:■i;it:■i;~ PACKING & POSTAGE IN AUSTRALIA INCLUDED IN PRICES QUOTED. WRITE OR RING FOR BROCHURES ORDERS ACCEPTED ANYTIME PAYMENTS BY BANKCARD, VISA, MASTERCARD, CHEQUE or MONEYORDER TRADE ENQUIRIES WELCOME RCS Radio Pty Ltd is the only company which manufactures and sells every PCB £, front panel published in SILICON CHIP, ETI and EA. JILOA PTY LTD 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. P.O. BOX 73, GLENHUNTL Y, VIC 3163 Phone (03) 571 6303 (TECHNIKIT DIVISION) FEBRUARY1990 39