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Digital Audio Oscillator Design By DARIAN LOVETT Words by MAURO GRASSI Do you need to test audio equipment, including amplifiers and speakers, in the field and in the workshop? If so, you could use this compact and inexpensive digital audio oscillator. It can produce sine, square, triangle and sawtooth waveforms in the frequency range from 10Hz-30kHz and features three output ranges: 20mV, 200mV & 1V. T HIS COMPACT HAND-HELD digital audio oscillator will allow you to quickly test wiring and to diagnose faults in audio systems. It is ideal for testing amplifier and speaker set-ups and is portable and easy to use. To use it, you simply select one of four waveforms – sine, square, triangle or sawtooth – and set it to a frequency between 10Hz and 30kHz. The digitally synthesised waveform is then available at the two RCA outputs. These two outputs are in parallel and are doubled-up simply for your convenience. It means you can test a stereo amplifier and speaker set simultaneously. Turning to the front panel, there is a 4-position slide switch that selects one of three levels for the output signal: 68  Silicon Chip 20mV, 200mV and 1V. Each selected level can be continuously varied down to zero with the “Level” control. There are also three pushbuttons on the front panel. The two on the right increase or decrease the frequency of the output waveform. The output frequency and the waveform type are shown on a blue backlit LCD screen. Pressing the “Wave” button on the left while at the same time pressing the “Down” button on the right lets you scroll through the four different waveform types: sine, square, triangle and sawtooth. It’s that easy! Circuit details Fig.1 shows the circuit details. It uses an Atmel microcontroller (IC1) to implement most of the features. The unit is powered from a single 9V battery. As shown, the +9V rail is fed via reverse polarity protection diode D1 to one pole of a 2P4T (2-pole 4-throw) switch, S4a. In three of the four positions, the switch feeds the resulting +8.4V rail on D1’s cathode to voltage regulator REG1. REG1, in turn, outputs a +5V rail which is used to power the microcontroller, while the +8.4V rail from diode D1 is used to power op amps IC2a & IC2b. In operation, IC1 monitors pushbuttons switches S1-S3. These switches are respectively connected to digital inputs PD2-PD4 which have weak internal pull-ups. When a switch is pressed, the relevant input is pulled low and this is detected by IC1 and siliconchip.com.au siliconchip.com.au June 2009  69 PD0 PD1 PD5 PD6 PD7 PD4 PD3 PD2 GND 22 PB0 PB1 PB2 PB3 PB4 PB5 PB6 PB7 PC5 PC4 PC3 PC2 PC1 PC0 14 15 16 17 18 19 9 10 28 27 26 25 24 23 30k 30k 30k 30k 30k 30k 30k 30k DIGITAL AUDIO OSCILLATOR GND 8 IC1 ATmega8-16PI PC6/RST 21 Aref 2 3 11 12 13 6 5 4 1 20 AVcc 7 Vcc 100nF 30k 15k 15k 15k 15k 15k 15k 15k 6 4 100k 1 F NP 2 3 100k 11 12 13 14 10 3 CONTRAST 8 120k IC2a 2.2k 1 5 1 180 20mV 200mV 1V 10 F RANGE S4b 10k IC2: TL072 R/W GND (Z 7006) 16x2 LCD MODULE D4 D5 D6 D7 D3 D2 D1 D0 9 8 7 EN RS 2 Vdd VR1 10k LCD CONTRAST 100k 4.7 F 100k KBL ABL 100nF 16 15 GND OUT 6 5 IN REG1 78L05 4 IC2b 1k A K 1N4002 7 +8.4V 100 F 220 F OUTPUT LEVEL VR2 1k 100 POWER S4a IN OUT CON2 CON1 9V BATTERY A 78L05 GND K D1 1N4004 Fig.1: the circuit diagram of the Digital Audio Oscillator. The design is based around a microcontroller (IC1), which drives an LCD module and a DAC made up of a R-2R ladder network and an op amp buffer stage. The blue backlit LCD screen shows the waveform shape and the frequency. SC 2009 WAVE S3 DOWN S2 UP S1 10nF 47k +5V 100nF 1 F NP 9V 100 F -– REG1 78L05 CON1 + +D1 CON2 4004 10k 10 F 120k 2.2k S4 (SX2040) 4.7 F 180 15k 15k 15k 30k 15k 30k 30k 30k IC2 TL072 Z 7006 100k 100k 100k 100k (LCD MODULE) 30k 100 15k IC1 ATmega8-16PI 220 F 30k 30k 30k S1 10k 16 1 WAVE VR2 2a3452.K 1k UP 1k LEVEL OUTPUT 10nF 15k S3 VR1 CON3 (TO LCD) 15k 30k 100nF 47k S2 processed by the internal software. This sets the waveshape ( sine, square, triangle or sawtooth) and the frequency and displays the result on a 16x2 LCD module. As shown, the microcontroller drives the 16x2 LCD module using its PC0-PC5 digital output lines. Trimpot VR1 sets the display contrast, while power for the LCD is derived directly from the +5V rail. The 47kΩ resistor and 10nF capacitor on pin 1 of IC1 reset the microcontroller at switch on. D-to-A converter As well as the LCD module, the microcontroller also drives a digitalto-analog (D/A) converter via its PB0PB7 digital output lines. This D/A converter is made up of a R-2R ladder network and has 8-bit resolution. In DOWN Fig.2: follow this parts layout to build the digital audio oscillator. Be sure to install the electrolytic capacitors and semiconductors with the correct polarity and note the orientation of switches S1-S3. this case, R = 15kΩ and there are seven 15kΩ resistors and nine 30kΩ resistors in the ladder network. The output of any N-stage R-2R network is given by: VOUT = DN x V/2N where VOUT is the output voltage, DN is the digital value as an N-bit number and V is the supply rail. In our case, N = 8 (so 2N = 256), V = 5 and DN is given by bits PB7 (MSb) to PB0 (LSb) which are digital outputs of IC1. The accuracy of such a DAC is constrained by the accuracy of the resistors. Note that, in this case, 1% resistors are used throughout. The output of the DAC is AC-coupled to op amp stage IC2a (TL072) via a 1µF non-polarised (NP) capacitor. This op amp stage has its non-inverting input biased to half-supply by two 100kΩ resistors and is wired as a unity-gain buffer stage. Attenuator IC2a’s output appears at pin 1 and is AC-coupled to an attenuator stage (switch S4b and associated parts) via a 10µF capacitor and 10kΩ resistor. As shown, the signal is fed to the wiper of switch S4b which selects the output level range. In operation, S4b selects a divider consisting of the 10kΩ resistor and either a 120kΩ, 2.2kΩ or 180Ω resistor to GND, in parallel with the two 100kΩ bias resistors for IC2b. These selections correspond to the 1V, 200mV and 20mV amplitude ranges, respectively. Selecting the 120kΩ resistor provides the 1V range, while selecting the 2.2kΩ resistor provides the 200mV range. The 180Ω resistor gives the 20mV range. Output buffer This side-on view shows how the LCD module is secured to the PC board using Nylon spacers and screws. 70  Silicon Chip The output of the divider is ACcoupled to op amp output stage IC2b, this time via a 4.7µF capacitor. This op amp is also biased to half-supply using two 100kΩ resistors and also operates as a unity-gain buffer stage. Its output signal at pin 7 is fed via a 220µF capacitor and series 100Ω resistor to potentiometer VR2 which sets the output level. The resulting signal at VR2’s wiper is then fed to RCA output sockets siliconchip.com.au CON1 & CON2 which are connected in parallel. Construction The Digital Audio Oscillator is built on a double-sided PC board measuring 76 x 62mm. Fig.2 shows the parts layout. Begin by carefully inspecting the PC board for hairline cracks and for shorts between adjacent tracks. It will be rare to find any problems but such checks are easier done at this stage than later on, when all the parts are in place. Once you have inspected the board, start the assembly by installing the resistors. Table 1 shows the resistor colour codes but it’s also a good idea to check them using a DMM as some colours can be difficult to distinguish. The diode can then be installed, taking care to orientate it exactly as shown on the parts layout diagram. Voltage regulator REG1 in the TO-92 package can be soldered in next. It can only go in one way! Don’t force the body down too close to the PC board or you may damage its connecting leads. It should ideally sit about 7mm off the board. The non-polarised MKT capacitors are installed next, followed by the polarised electrolytic capacitors. Make sure the latter are orientated correctly. Note also that the electrolytic capacitors must all be installed so that they sit flush with the PC board, to ensure they don’t later foul the lid of the case. A 28-pin IC socket is used for the microcontroller and this can be installed now. Be sure to orientate it with its notched end to the right, as indicated on Fig.2. Leave IC1 out for the time being – its plugged in later on, after some basic checks of the supply rail have been performed. IC2 (TL072) is next on the list. It’s The PC board is secured inside the case using metal screws that go into integral mounting posts. Note that the battery leads are run under the PC board and into the battery compartment via a slot in the back wall. directly soldered to the PC board and goes in with its notched end towards switch S4. Be sure not to apply too much heat at any one time to its pins, as this could damage it. The LCD connector (CON3) can now be soldered in, followed by potentiometer VR1, the two RCA sockets (CON1 & CON2), the 2P4T switch (S4) and trimpot VR1. Follow these with the three pushbutton switches (S1-S3) making sure that they are orientated correctly. Note that each has a straight edge and this must go to the right as shown on the component overlay. The last thing to do is to solder in the battery clip lead. The red lead goes to the +9V PC pad, while the black lead goes to the negative (-) pad. These pads are located at the top of the PC board, immediately to the left of the two RCA sockets. That completes the PC board assembly, apart from plugging in IC1. As mentioned earlier, that’s done only after making a few basic checks. Table 1: Resistor Colour Codes o No. o   1 o   4 o   1 o   9 o   7 o   1 o   1 o   1 o   1 o   1 siliconchip.com.au Value 120kΩ 100kΩ 47kΩ 30kΩ 15kΩ 10kΩ 2.2kΩ 1kΩ 180Ω 100Ω 4-Band Code (1%) brown red yellow brown brown black yellow brown yellow violet orange brown orange black orange brown brown green orange brown brown black orange brown red red red brown brown black red brown brown grey brown brown brown black brown brown 5-Band Code (1%) brown red black orange brown brown black black orange brown yellow violet black red brown orange black black red brown brown green black red brown brown black black red brown red red black brown brown brown black black brown brown brown grey black black brown brown black black black brown June 2009  71 Fig.3: this oscilloscope screen grab shows a 1kHz sinewave (yellow trace), as captured at the output. The distortion waveform for THD+N (blue trace) can also be seen, as well as the FFT (Fast Fourier Transform) of the distortion. Note that the highest distortion peak is at the lower harmonics. Fig.4: an oscilloscope screen grab of a triangular wave at around 1kHz. This shows that the waveform is very close to linear on the rising and falling slopes although there is some very slight drooping discernible at the waveform troughs. Fig.5: this shot shows the square wave output from the unit at around 1kHz. There is a 2% overshoot on the rising edge of the waveform but little droop. Droop will only be apparent at low frequencies in the tens of Hertz. Fig.6: a sawtooth waveform at a nominal 10kHz. As shown, the actual frequency is 10.4kHz and the RMS value is also indicated. Note: this screen grab was obtained with the unit at full level on the 1V range. Specifications Note also that the LCD module is not attached at this stage. Frequency Range: 10-200Hz in 10Hz steps, 200Hz-1kHz in 100Hz steps & 1-30kHz in 500Hz steps Initial tests Amplitude Ranges: 0-20mV, 0-200mV & 0-1V RMS (output amplitude adjustable within the selected range) Waveforms: sine, square, triangle & sawtooth Frequency Accuracy: ±4% Total Harmonic Distortion + Noise: approximately 3% Output connectors: 2 x RCA parallel mono outputs Power supply: 9V alkaline battery Current drain: 25mA 72  Silicon Chip To test the assembly, first connect a 9V alkaline battery to the battery clip, then switch on and use a DMM to check the voltage between the OUT terminal of REG1 and the body of either RCA socket. You should measure close to 5V and this voltage should also be present on pin 7 of IC1’s socket. If this voltage is correct, you can jump to the final installation section. If not, you should disconnect power immediately and perform a few checks: (1) Are you using a fresh 9V battery? siliconchip.com.au Parts List Performance W E CHECKED the Digital Audio Oscillator on our Audio Precision Test set and the results are shown in Fig.7. Keep in mind, though, that it is not intended as a high-precision instrument. For the sinewave output, we measured the THD+N (Total Harmonic Distortion + Noise) over the frequency range with four different bandpass filters – see Fig.7. The typical THD+N figure was around 3% which is higher than most amplifier and speaker sets. While this is not enough to worry about, it means you cannot use this oscillator in precision applications, where low distortion is paramount. It’s quite good enough, however, for most troubleshooting tasks. Figs.3-6 on the facing page show screen grabs of the four different waveforms that can be selected. The frequency accuracy is within ±4% across the whole range from 10Hz to 30kHz. 1 plastic case, 79 x 117 x 24mm, Altronics H-8971 (supplied drilled & screen-printed) 1 16x2 LCD with blue backlight, Altronics Z-7006 2 PC-mount RCA sockets 1 28-pin 0.3-inch machined IC socket 1 16-way PC-mount FFC/FPC connector, Altronics P-4516 1 10kΩ horizontal 5mm trimpot (VR1) 1 1kΩ log pot (9mm PC-mount), Altronics R-2480B 3 PC-mount pushbutton switches, Altronics S-1094 1 2-pole 4-position PC-mount slide switch, Altronics SX-2040 4 M3 x 9mm Nylon spacers 8 M3 x 6mm Nylon screws 1 9V battery snap connector Semiconductors 1 programmed Atmel ATmega816PI microcontroller (IC1) 1 TL072 dual op amp (IC2) 1 78L05 regulator (REG1) 1 1N4004 silicon diode (D1) Capacitors 1 220µF 16V 1 100µF 16V 1 10µF 16V 1 4.7µF 16V 1 1µF 16V NP 2 100nF MKT polyester 1 10nF MKT polyester Fig.7: the sinewave THD+N vs Frequency for four different filter combinations. The filters range from <10Hz – >500kHz), <10Hz – 22kHz , <10Hz – 30kHz and <10Hz – 80kHz. The distortion is less with a more restrictive filter. (2) Is there between 7-8.4V at the cathode of diode D1? If there isn’t, then you may have D1 in the wrong way around. (3) If the voltage is still incorrect, double-check the PC board assembly. In particular, check for incorrect component orientation and for incorrectly placed parts. Check also for dry solder joints on the underside of the board. Assuming that REG1’s output is correct, switch off and plug IC1 into its socket (notched end to the right). The LCD module can then be installed. To do this, mount the LCD module in position on the PC board using four M3 x 9mm Nylon spacers and eight M3 siliconchip.com.au x 6mm Nylon screws. The module’s flexi connector is then plugged into CON3 on the PC board. The PC board can now be mounted inside the case and secured using the four Phillips-head 10mm screws supplied with the kit. When doing this, make sure that the two battery-clip wires pass underneath the PC board and into the battery compartment – see photo. The top of the case can then be fitted into position and secured using the two Phillips-head 18mm screws. Your Digital Audio Oscillator is now complete and ready for use. You can check that it is working properly by Resistors (1%, 0.25W) 1 120kΩ 1 10kΩ 4 100kΩ 1 2.2kΩ 1 47kΩ 1 1kΩ 9 30kΩ 1 180Ω 7 15kΩ 1 100Ω Where To Buy a Kit This Digital Audio Oscillator was designed by Altronics who own the design copyright. A complete kit of parts is available from Altronics for $89.00 (Cat. K-2543). The kit includes the PC board, the machined case and all specified components (including a preprogrammed microcontroller) but does not include a battery. monitoring its output with a scope or failing that, feeding its output into an SC audio amplifier system. June 2009  73