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Jiu.ii.a·this versatile test instrument Techni]ab 301 function generator This versatile test instrument packs a 10Hz-110kHz function generator, a power supply, and an audio amplifier and loudspeaker into one compact package. By DAVID WHITBY If your funds don't extend to a workshop full of exotic test gear, this multi-function test instrument is for you. It features a function generator, power supply and audio amplifier all in one package, and is ideal for testing prototype circuits and for service work. We think that it will more than earn its keep in many small workshops and labs. The idea behind the Technila b 301 was to provide a versatile test instrument at an affordable price. This has been achieved with a very clever circuit that uses just three low-cost ICs and a couple of 3-terminal regulators. As well as keeping the cost low, this also makes the unit extremely easy to build. There are many potential applications for the instrument. Here are just a few: (1) Audio servicing: you can use the unit as a signal tracer for servicing audio circuits. A signal injected from the function generator can be traced by the built-in amplifier and loudspeaker. (2) Loudspeaker testing: by connecting the generator output to the amplifier, you can ·use the unit for frequency response testing of loudspeakers or amplifiers. (3) Variable frequency code practice oscillator: a Morse key connected between the generator output and the amplifier input is all The Technilah 301 is housed in a grey plastic case with red front-panel lettering. It can generate sine, triangle and square waves from 10Hz to 110kHz. that's required to make a Morse code practice oscillator. (4) Power supply: the unit provides both ± 6V regulated and ± 15V filtered supply rails for powering prototype circuits. Other voltages can be derived from these rails by means of external voltage regulation circuits (eg, zener diodes or 3-terminal regulators). The instrument can provide up to 200mA which is sufficient for most small projects using op amps or logic ICs. Perhaps the most important feature of the Technilab 301 is the built-in function generator. A function generator is useful for checking out audio and logic circuits. Despite the very simple circuit employed, the Technilab is capable of providing sine, triangle and square waveforms with frequency continuously variable from lOHz to 1 lOkHz over four ranges. MARCH 1988 43 FREQUENCY VR1 1M LIN. 100k +6V 82pF VR5 10k H~ .~. .OOl X100 ~ .01 33k +6V x10 Sla 0.1 K Slb 1k RANGE l ':' 1-4) AXED OUTPllT .I1IL 4069 IC1a 1 LE01 15pF 39k 0 2 .It. 100k \I\ 10 S2a S2b 220k 40 400 mV/DIV S3b 1M +6V D1 1N4002 OFF SINE • SYMMETRY VR7 10k +15V HOM ON 0 I -~ +6V 02 1N4002 _;~ ~;_, OV 03 1N4002 S3a 12VAC INPUT OUTPUT OUTPUT VR8 1k UN REG AMPLIFIER IN iov ~1 .1 7806 ':' ":' +15V • LUME 100pFI -6V REG ,. i:k LOG . . GND 7906 -15V NOM 06 1N4002 IN TECHNILAB 301 Fig.1: the function generator circuit is based on CMOS hex inverters IC1 and IC2, while IC3 is the audio amplifier stage. D1, D2 and the two 3-terminal regulators provide the ± 15V and the ± 6V power supply rails. The output level is also continuously variable (from 0-4V over three ranges). And, as a bonus, there is a separate 6V p-p square wave output which is completely independent of the level set. This feature allows reliable external oscilloscope triggering and/or frequency measurements, regardless of the level being fed into the test circuit. The output from the generator is made available on small binding post terminals on the front panel. It can then be fed directly to the circuit under test or to the in-built 44 SILICON CHIP. audio amplifier. The amplifier can deliver 1W into an 80 load and connecting an external lead via the 3.5mm OUT socket automatically disconnects the internal loudspeaker. How it works Take a look now at the circuit details in Fig.1. This can be split into three sections: a function generator based on CMOS hex inverters ICl and IC2; a power supply stage built around two 3-terminal regulators; and an audio amplifier stage based on IC3. We'll consider the function generator circuitry first. ICla is connected as an integrator with four switched capacitors from input to output. These capacitors ar e selected by Sla and provide the four decades of frequency range. IClb and IClc together form a Schmitt trigger. This is fed from the output of ICla via one of the trimpots VR2-VR5 , as selected by Slb. The output of IClc is then fed back to the input of ICla via a lkD resistor and the main frequency control (VRl) to form a surprisingly *MOUNTED ON UNDERSIDE OF BOARD 12VAC INPUT Fig.2: install the parts on the PCB as shown in this diagram. Note that the 3-terminal regulators, the 4700µF filter capacitors and the loudspeaker are . mounted on the back of the board. Take care with component polarity. simple but stable 4-decade oscillator with both triangle (pin 1) and square wave (pin 6) outputs. The four trimpots (VR2-VR5) 11llow adjustment of each frequency decade to match the dial calibration. Inverter stage ICld buffers the output of IClc to provide the fixed 6V p-p square wave output. Additionally, the square wave output of IClc is fed direct to switch S2a and to S2a via a 39k0 attenuator. The triangle wave is derived from pin 2 of ICla and applied to buffer/amplifier stage ICle which is wired in linear mode. After that, the signal is fed to a shaping network (33pF // 18kn) and then fed to S2a. Similarly, the sinewave output is produced by driving the triangle wave into soft limiting stage IClf which is also wired in linear mode. VR6 and VR7 provide adjustment for level and symmetry to produce a rough approximation of a sinewave. Switch S2a selects the appropriate waveform and feeds it to an output stage consisting of six 4069 inverters (IC2a-IC2f) wired in parallel. This stage is used in linear mode in the first three switch positions for sine, triangle and square waves and provides a 4V p-p signal to the output attenuator. In the fourth switch position, the 27kn feedback resistor is switched out and IC2 inverts the output of IClc to provide a 6V p-p square wave to the attenuator network. The 6V p-p variable output is useful for driving digital circuits operating from 6V supply rails and Specifications Waveform functions Frequency range Output level Output impedance Amplifier power output Power supply rails Maximum supply current Sine, triangle and square wave 1 OHz-11 OkHz 0-4V p-p continuously variable on sine, triangle and square wave, 0-6V p-p continuously variable on square wave, 6V p-p fixed square wave output 600 ohms 1W into 8 ohms ± 15V unregulated, ± 6V regulated 200mA also has faster switching times, especially on the highest frequency range. Note: this output is independent of the 6V p-p fixed square wave output from ICld. The signal from the 4069 output stage is AC-coupled via a 470µF capacitor to the attenuator network. This network consists of a lkn pot, fixed 9.lkO and lOOkO resistors, and switch S3b. Depending on the setting of the pot, the output impedance will be no more than about 6000. The audio amplifier circuitry is about as simple as you can get and is based on an LM380 audio IC. This has a power output of 1W into 80, a gain of about 10 and a frequency response from 30Hz to 30kHz (-3dB). Starting at the input, a O. lµF ceramic capacitor couples the incoming signal to 500kn pot VR9 which functions as a volume control. From there, the signal is coupled via another O. lµF capacitor to the pin 2 input and also to the pin 6 input via a 220k0 resistor and parallel 33pF capacitor. The 220k0 limits the gain, while the 33pF capacitor determines the upper frequency rolloff. The amplified output signal appears at pin 8 and is fed to the loudspeaker via a 470µF capacitor and series 4. 70 resistor which provides short-circuit protection. The series 4.70 resistor and O. lµF capacitor across the output form a Zobel network which ensures stability of the amplifier. Power for the circuit is derived from a 12V AC plugpack transformer. Dl and Cl half-wave rectify the incoming AC to provide a nominal + 15V rail, while DZ and C2 provide a nominal - 15V rail. Note: these rails will be closer to + 18V and - 18V under no-load conditions. Finally, regulated ± 6V rails are derived using 7806 and 7906 3-terminal regulators. Diodes D3-D6 protect the supply against reverse polarity connection to external voltages (eg, charged capacitors). Construction A complete kit of parts for this project is available from Technikit Electronics (see panel). To make MARCH 1988 45 PARTS LIST 1 plastic case with silkscreened front panel (predrilled) 1 carrying handle 3 knobs 1 PCB, code Technilab 301, 146 x 86mm 1 8 n loudspeaker with attached pedastal 1 0 threaded brass spacers 3 2-pole 4-position slide switches 1 3.5mm DC power socket 1 3 .5mm switched line socket Semiconductors Above shows the completed PCB, ready for installation in the case. Note the threaded spacers and screws which form the 10 binding post terminals. 2 4069 hex inverter ICs 1 LM380N audio amplifier IC 1 7806 +6V 3-terminal regulator 1 7906 -6V 3-terminal regulator 6 1 N4002 or 1 N4004 diodes Capacitors A small pedastal is used to support the loudspeaker on the back of the board. Note that the PC pattern has been modified to eliminate the wire link. construction really easy, the case comes pre-drilled with the speaker grille already fitted to the rear panel. The front panel features red screen printing on a dark grey background for a professional finish. All the components, except for the power input jack, are mounted on a printed circuit board [PCB) measuring 146 x 86mm. The three pots, along with the 3-terminal regulators, 4700µ,F filter capacitors and the loudspeaker, are mounted on the back of the board, with all other parts mounted on the front. Begin assembly by installing all the parts on the front of the PCB as 46 SILICON CHIP shown in Fig.2. You can install the parts in any order you wish but make sure that the ICs, diodes and electrolytic capacitors are correctly oriented. The three electrolytics used (2 x 470µ,F and 1 x 10µ,F) are all RB types and should be installed with their bodies flat against the PCB [see Fig.2). To do this, bend the leads of each capacitor at right angles before mounting it on the PCB. The power indicator J;..ED should be stood off the board by about 8mm [the long lead is the anode). Ten terminals must also be mounted on the board for the 2 4700µ,F 25VW axial electrolytics 2 4 70µ,F 16VW PC electrolytic 1 1 Oµ,F 16VW PC electrolytic 5 0 .1µ,F ceramic 1 0.1 µ,F greencap 1 .01 µ,F green cap 1 .001 µ,F greencap 1 1OOpF ceramic 1 82pF NPO ceramic 2 33pF ceramic 1 1 5pF ceramic Resistors (0.25W, 5%) 1 x 1 MO, 3 x 220k0, 3 x 1 OOkO, 1 X 39k0, 1 X 33k0, 3 X 27k0, 1 x 22k0, 1 x 18k0, 1 X 9.1 kO, 2 x 1k0, 1 x470, 2 x4.70, 1 x 1MO trimpot, 1 x 500k0 log potentiometer, 6 x 1 OkO trimpots, 1 x 1 kO linear potentiometer various inputs and outputs. These consist initially of 25mm nickelplated screws which are fastened to the PCB by means of 12mm tapped brass spacers. The ends of the screws are later passed through the front panel and fitted with washers, nuts and plated knurled knobs to finish the terminals. Check that the three 4-position slide switches are pushed down firmly onto the PCB before soldering. The loudspeaker socket is installed with its earth terminal towards the bottom of the PCB. You can now install the parts on the frequency control on the x1, x10 and xlO0 ranges. The x1k range begins at l0kHz, so how much of this range you hear will depend on your hearing. Finally, use your multimeter to check the supply voltages. The ± 6V rails should be very close to their nominal values for loads up to 200mA (within 5%). The ±15V rails should vary from around ± 18V at no load down to a minimum of ± 14V with a l00mA load. Calibration The loudspeaker, 3-terminal regulators, and 4700µF capacitors are mounted on the back of the PCB. The regulators are kept cool by finned heatsinks. the rear of the PCB. The pots go in first. Bend their leads at right angles so that they mate with their respective pads on the board. Secure the pots from the front of the board with the washers and nuts provided before soldering the terminals. Note that the pots are all different values so be sure to use the correct pot at each location. Next, solder 35mm lengths of hookup wire to each of the speaker output pads on the back of the PCB. The two 4700µF capacitors can now be mounted, followed by the 6V regulators. Bolt small TO-220 style heatsinks to the 6V regulators as shown in the photograph. Make sure that these don't short with the leads from the 4700µF electrolytics. The loudspeaker supplied with the kit comes with a "pedestal" attached to its magnet (see photo). This pedestal consists of a fibre disc, a 12mm threaded spacer and a screw. The whole assembly is simply mounted on the back of the PCB and secured from the front using a nut. You can now complete the wiring by attaching the speaker leads and by connecting leads from the PCB AC-input pads to the 3.5mm socket on the rear of the case. Testing A final check of component orientation and placement is advisable before switching on. When you are satisfied that everything is correct, apply power and check that the LED comes on. Now turn the gain full on and touch the amplifier input terminal - you should hear a healthy "blurt" from the speaker. If everything is OK so far, set the output level to 40 x 5, select the square waveform, and connect a wire link between the generator output and amplifier input terminals. You should now hear tones from the loudspeaker as you vary Where to buy the kit A kit of parts for this project is available from Technikit Electronics. The kit includes all parts and comes with a pre-drilled case and a silkscreened front panel. Price: $69 .50 plus $6 .50 p&p ($8 .50 to NZ) . Add $1 0. 00 for the 1 2V AC plugpack transformer. Payment may be made by cheque or Bankcard/Mastercard number with mail order, or by Bankcard/Mastercard number for telephone order. Send your order to: Technikit Electronics, 654 Calder Hwy, Keilor, Vic. 3036. Phone (03) 336 7840. The Technilab 301 is also available in fully built-up form. Contact Technikit Electronics for further details . Calibration involves adjusting the six preset pots (VR2-VR7) along one edge of the PCB. For non-critical applications, these can all be set to mid-travel. The dial calibrations will then be accurate to ± 10% and you will get quite a reasonable sinewave. To accurately calibrate the instrument, you will need a digital frequency meter (eg, the 1GHz DFM described in SILICON CHIP from Nov.87 to Jan.88). The procedure is as follows: (1). connect the DFM to the generator output terminals; (2). set the output to maximum on square wave and set the main frequency dial to 110; (3). set the frequency range to xl and adjust trimpot VR2 so that the DFM reads 110Hz; (4). adjust VR3 on the xlO range for a reading of 1 lO0Hz, VR4 on the xlO0 range for a reading of 11kHz, and VR5 on the xlk range for a reading of 1 lOkHz. An oscilloscope is required to accurately set the sinewave shape. Set the output frequency to lkHz, then adjust VR7 (symmetry) so that the positive and negative peaks are as close as possible to the same shape. After that, it's simply a matter of adjusting VR6 (sine level) for smooth rounding of the sinewave peaks. Don't go too far or you will flatten the peaks too much. Once calibration has been completed, the PCB can be fitted to the front panel and secured by fitting nuts and washers to the terminals. Complete the terminals by fitting the round knurled nuts, then screw the handle to the rear panel. Finally, fit the front panel assembly to the case and secure it using the four corner screws. ~ MARCH 1988 47