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Low distortion audio signal generator; Pt.1 A low distortion wide frequency range audio oscillator is always a useful test instrument for your work bench. This Audio Signal Generator produces high quality sine and square waves and incorporates a 4-digit frequency readout and switched output attenuator. By JOHN CLARKE If you’re an enthusiast who likes to dabble with audio equipment, you won’t get too far unless you have a high-quality audio signal generator and preferably, an AC Millivoltmeter to go with it. We published an AC Millivoltmeter in the October & November 1998 issues and now we present a matching Audio Signal Generator. This completely new audio signal generator effectively supersedes both the Digital Sine-Square Generator from the July 1990 issue of SILICON CHIP and the High Quality Audio 24  Silicon Chip Oscillator from January 1990 issue. While the new generator does not have the ultra-low distortion of the January 1990 circuit, it is much simpler in its range and frequency switching and it actually has better distortion below 100Hz. As well, the new design is consid­erably simpler in construction. Operating features As you can see from the photos, the new Audio Signal Gen­erator comes in a standard plastic instrument case and Features • • • • • • • Sine or square wave output 10Hz-100kHz range Fast settling time Digital frequency readout Stepped attenuator with fine adjustment Sync output for oscilloscope Display off switch has four knobs and a 4-digit display on the front panel. On the lefthand side are the frequency controls: a 4-position range switch and a variable frequency knob. Then there are the amplitude controls which comprise the 8-position attenuator and the vernier control knob. There are three toggle switches, one to select sine or square wave output, one to ground or “float” the instrument and one to turn off the frequency display. This last-mentioned switch is included so that when you are doing critical measurements with the oscillator, you can switch off the display and thereby elim­inate any multiplex hash from the sinewave signal. Finally, there are two BNC sockets, one for the main sine/square output and one for the sync output to an oscillo­scope. Settling time Where this new design is notably superior to our previous high quality design is in settling time. Many very low distortion audio oscillators suffer from a long settling time whereby the output amplitude bounces badly after each change in frequency. Our new signal generator has a negligible settling time and the frequency control knob can be swept rapidly from one extreme to the other on the three lowest ranges without any level change occurring. On the highest frequency range, there is a short duration dip in output level at around 60kHz if the control knob is swept too quickly. The new Audio Signal Generator is also far superior in its output level flatness versus frequency compared to both previous oscillators. Output level flatness is of particular importance in an audio signal generator. If you wish to make measurements of an amplifier’s frequency response from 20Hz to beyond 20kHz, any variation in level from the generator will also be measured at the amplifier output. This will lead to an incorrect amplifier response measurement. Similarly when checking a filter, any generator level variation will be reflected in the filter’s response. Sine & square output This latest Audio Signal Generator can produce either a sine or square wave output with the latter being particularly useful for measuring the slew rate of amplifiers. The 33ns rise and fall times of the square wave output correspond to a 300V/µs slew rate for a 1V signal. This is more than adequate to check any audio amplifier’s response to square waves. Also included is a sync output which can be used to lock an oscilloscope to the output waveform. This output is constant in level (280mV Fig.1: the “state variable oscillator” comprises three op amps, two of which are configured as integrators and the third as an inverter. RMS), regardless of the output level set on the attenuator. The output attenuator provides eight steps, ranging from 3.16V down to 1mV, in 10dB steps. There is also a variable con­trol (vernier) which can reduce the output level to zero. The output frequency is displayed on a 4-digit LED readout. It has a relatively fast update time so that the output can be varied quickly using the frequency adjust control without having to wait for the display to catch up. State variable oscillator Our new Audio Signal Generator is based on a “state vari­able oscillator”. As shown in Fig.1, it comprises three op amps, two of which are configured as integrators and the third as an inverter. Each integrator has a frequency response which reduces with increas- ing frequency at 6dB/octave (10dB/ decade) and they each introduce a 90-degree lagging phase shift. We have shown the output of op amp 1 as being the reference waveform with 0° phase shift. Its output is coupled to the inverting input of op amp 2 via resistor R2. Op amp 2’s gain is -1, as set by the input and feedback resistors which have the same value (R2). The negative gain figure comes about because op amp 2 is an inverter. The output of op amp 2 is 180° out of phase to its input. Op amp 3 is an integrator producing a 90° phase shift and this is followed by op amp 1 producing another 90° phase shift. The phase changes through three op amps add up to 360° and so we have the perfect recipe for an oscillator. The oscilloscope waveforms of Fig.2 show how the circuit oscillates. Fig.2: these waveforms demonstrate the operation of the state variable oscillator. The top trace shows the output of op amp 1 while the lower trace is op amp 2. Note that the lower trace is 180° out of phase to the top trace. The centre trace, op amp 3, lags behind the lower trace by 90°. February 1999  25 Fig.3: the block diagram shows that the state variable oscillator of Fig.1 needs a lot more circuitry for a practical instrument. The frequency of the state variable oscillator is multiplied by four to drive the digital counter circuitry. The top trace shows the output of op amp 1 while the lower trace is op amp 2. Note that the lower trace is 180° out of phase compared to the top trace. The centre trace, from op amp 3, lags behind the lower trace by 90°. The frequency of oscillation is equal to 1/(2πR1.C1), provided that op amp 2 has a gain of -1. An oscillator of this type will produce an output level which is only limited by the amount of peak-to-peak swing from the amplifiers. In other words, the output will rise until the circuit clips, which is hardly what we want for a low distortion design. To prevent this from happening, some form of feedback is required to maintain a constant signal level. VRx introduces amplitude control by applying a small amount of negative feedback from op amp 3’s output to the input of op amp 2. A practical oscillator would require an automatic amplitude control which monitors op amp 1’s output and varies VRx accord­ingly to maintain the output level. VRx could be any device which can vary signal level and could be a FET, transistor or even a light dependent resistor. Unfortunately, these devices all intro­duce some form of distortion into the signal, either by their non-linearity or via the control circuitry which drives them. Interest26  Silicon Chip ingly, some of this distortion is then reduced via the 6dB/octave low pass rolloff from op amp 3 to op amp 1. Block diagram The complete block diagram for the Audio Signal Generator is shown in Fig.3. The oscillator itself comprises op amps IC1a, IC1b and IC2a, with the integrator components VR1a & VR1b and capacitors selected by 2-pole switch S2a & S2b. The sinewave output of IC1b is applied to several sections of the block dia­gram. Firstly, it is applied to the precision rectifier (IC4a, IC4b) which converts it into unfiltered DC. This DC signal is compared in error amplifier IC5a against a reference DC voltage set by trimpot VR5. Buffer transistor Q5 drives LED1 and LED 2 which illuminates light dependent resistor LDR1. The above components form a feedback loop so that the signal applied to the LEDs varies the LDR’s resistance to maintain a con­stant signal level at IC1b’s output. As already noted, the DC output from the precision rectifier is not filtered and this means that the error amplifier (IC5a) will be fed with the same raw DC. However, the filtering of this control loop is achieved by virtue of the slow response of the LDR – it ignores the harmonics in the sign­al. The waveforms of Fig.4 show the action of the control loop. The top trace is the output of IC1b, while the middle trace shows the rectified signal applied to error amplifier IC5a. The third trace shows the drive to the LEDs. These are short pulses which occur at the peak of the sine waveform. As well as driving the precision rectifier, IC1b’s output is applied to the output level control VR2b and the sync output. VR2b is buffered by op amp IC5b which drives the attenuator switch S5. The attenuator provides 10dB steps in signal levels from 3.16V to 1mV. IC1b also drives the Schmitt trigger IC3b which produces a square wave output which is fed to paralleled CMOS inverters in IC6. These give the square wave signal very fast rise and fall times. Fig.5 shows the square wave rise and fall times at 33ns and 30ns, respectively. Frequency multiplier We now come to the frequency display part of the block diagram and there are a few unconventional features in this section. First, there is the frequency multiplier. This uses a diode mixer to add the signal outputs of IC1a, IC1b, IC2a Fig.4: these waveforms show the action of the control loop for the state variable oscillator. The top trace is the output of IC1b while the middle trace shows the precision rectified signal applied to error amplifier IC5a. The third trace shows the drive to LEDs 1 & 2 Fig.6: these four waveforms are added together in a diode mixer to obtain a frequency multiplication of four. and IC2b. These signals are shown in the oscilloscope waveforms of Fig.6. The output of the diode mixer is a waveform with a funda­mental frequency which is four times the sinewave at IC1b’s output. Comparator IC3a squares the multiplier output, as shown in Fig.7. The top trace is the output of IC1b, the middle trace is the mixer output applied to IC3a and the bottom trace is the output of IC3a. This frequency multiplication enables the digital readout to have a relatively fast update time. The signal Fig.5: these are the square wave rise and fall times. Fig.7: these waveforms shows the action of the diode frequency multiplier. The top trace is the output of IC1b, the middle trace is the mixer output applied to IC3a and the bottom trace is the output of IC3a. is then divided by 10 and 10 again, with each of these signals applied to the range selector. The range selector output drives the counter and display driver. Circuit details Fig.8 shows the circuit for the Audio Signal Generator. It uses 12 ICs, four 7-segment LED displays, several transistors, regulators and switch­es, plus various resistors, capacitors and diodes. IC1b, IC2a and IC1a comprise the state variable oscillator. These op amps are LM833 types which have low distortion and low noise, making them ideal for this application. Switches S2a and S2b select the various frequency range capacitors for the inte­grators while the dual-gang potentiometer VR1a and VR1b adjusts the resistance for continuous frequency control. The 8.2kΩ resis­tors at the inputs to IC1a and IC1b limit the maximum frequency for each range. Inverter IC2a is set with a gain of -1 using the 100kΩ resistors from pin 6 February 1999  27 28  Silicon Chip Fig.8: the circuit can be broken down into a number of sections. In the middle is the state variable oscillator and the square wave driver. At the top is the frequency multiplier and at the bottom is the frequency counter circuitry. to pin 7 and the input resistor to pin 6 from the output of IC1b. Trimmer capacitor VC1 is used to compensate for phase shifts in the oscillator at high frequencies. It is adjusted so that the oscillator does not become uncontrollable at the highest frequen­cies. The precision full wave rectifier comprises op amps IC4a and IC4b together with diodes D1 and D2 and associated resistors. When the input signal goes negative, IC4b’s output goes high and the gain, set by the 10kΩ input and feedback resistors, is -1. This signal is seen at the cathode of D1 and is coupled to the inverting input of IC4a via the 10kΩ resistor. Gain is set for IC4a by the 10kΩ input resistor and the 47kΩ feedback resistor at -4.7. Overall gain for the input signal is therefore (-1 x -4.7) = +4.7. Note, however, that there is an extra path for the input signal via the 20kΩ resistor at pin 6 of IC4a. This produces a positive signal at the output of IC4a with a gain of 47kΩ divided by the 20kΩ resistor or -2.35. Adding the two gains gives us +2.35. For positive signals the output of IC4b is clamped due to the conduction of D2. Signal then passes via the 20kΩ resistor connected to pin 6 of IC4a. IC4a inverts the signal and provides gain of -2.35. Since the input signal is positive the signal at pin 7 of IC4a is negative. Thus for positive input signals the output at IC4a is nega­tive, with a gain of -2.35. For negative signals the output of IC4a is also negative, with a gain of 2.35. So a full-wave recti­fier results. Note that the output of IC1b is AC-coupled to the precision rectifier, to prevent any DC offset in the signal from affecting the rectifier operation. Error amplifier Op amp IC5a is the error amplifier. It compares the preci­ sion rectifier output with the reference voltage set at its pin 3 input. This reference voltage sets the sinewave output level February 1999  29 Audio Signal Generator – Parts List 1 PC board, code 01402991, 122 x 141mm 1 PC board code, 01402992, 210 x 73mm 1 front panel label, 249 x 76mm 1 plastic case, 256 x 190 x 84mm 2 aluminium panels, 249 x 76mm 1 red transparent Perspex sheet, 59 x 21 x 2.5mm 1 6672 30V centre-tapped mains transformer (T1) 1 IEC mains panel socket with fuseholder 1 insulating boot for IEC socket 1 250mA 2AG 250VAC fuse (F1) 1 IEC mains cord 1 SPDT mains rocker switch with neon indicator (S1) 1 3-pole 4-position rotary switch (S2) 2 SPDT toggle switches (S3,S6) 1 DPDT toggle switch (S4) 1 single-pole 12-position rotary switch (S5) 1 100kΩ 24mm dual-gang linear pot (VR1) 1 10kΩ 24mm dual-gang linear pot (VR2) 1 100kΩ horizontal trimpot (VR3) 3 10kΩ horizontal trimpots (VR4-VR6) 1 8.5-50pF trimmer capacitor (VC1) 2 BNC panel sockets with insulating kits 1 TO-220 heatsink, 28 x 25 x 35mm 4 19mm knobs 21 PC stakes 1 40-way pin header (broken into groups of five) 1 600mm length of 0.7mm tinned copper wire 1 300mm length of 7.5A green/ yellow 250VAC rated wire 1 400mm length of 7.5A brown mains wire 1 300mm length of 7.5A blue mains wire 1 100mm length of yellow hookup wire 1 100mm length of blue hookup wire 1 100mm length of green hookup wire 4 M4 screws x 9mm 4 M4 nuts 4 M4 star washers 2 M3 screws x 9mm 2 M3 nuts 2 M3 star washers 4 self-tapping screws and is adjusted with VR5. The error amplifier has a gain of about 70, as set by the 330kΩ resistor and 4.7kΩ resistor at pin 2. The 3.3pF capacitor across the 330kΩ resistor provides a high fre­quency rolloff of 146kHz and prevents any tendency to spurious oscillation. IC5a’s output is buffered by transistor Q5, connect­ed as an emitter follower. It drives LED1 and LED2 and these illuminate LDR1 for amplitude control of the state variable oscillator. IC1b’s output is fed via two back-toback 470µF capacitors to the sinewave level control, VR2b. The other half of this dual-ganged potentiometer is the square wave output level control (VR2a). VR2b is connected to pin 5 of op amp IC5b which amplifies the signal by a factor of 2 and drives the output attenuator, switch S5. This switch has eight positions giving 30  Silicon Chip Semiconductors 4 LM833 op amps (IC1,IC2,IC4,IC5) 1 LM319 high-speed dual comparator (IC3) 1 74C14, 40106 hex Schmitt trigger (IC6) 1 74C926 4-digit counter/7segment display driver (IC7) 1 4017 decade counter (IC8) 1 4093 two-input quad Schmitt NAND gate (IC9) 1 4518 dual 4-bit decade counter (IC10) 1 555 timer (IC11) 1 4052 dual 4-channel analog switch (IC12) 1 7815 +15V 1A 3-terminal regulator (REG1) 1 7915 -15V 1A 3-terminal regulator (REG2) 1 7805 +5V 1A 3-terminal regulator (REG3) 1 7905 -5V 1A 3-terminal regulator (REG4) 5 BC337 NPN transistors (Q1-Q5) 8 1N4148, 1N914 switching diodes (D1-D8) 4 1N4004 1A 400V rectifier diodes (D9-D12) 1 LDR (LDR1), Jaycar RD-3485 or equivalent 4 HDSP5303 common cathode 7-segment LED displays 2 high intensity (1000mcd <at> 20mA) red LEDs (LED1,LED2) Capacitors 2 1000µF 25VW PC electrolytic 2 470µF 16VW PC electrolytic 2 330µF 16VW PC electrolytic 2 10µF 35VW PC electrolytic 1 10µF 25VW PC electrolytic 6 10µF 16VW PC electrolytic 1 0.56µF MKT polyester 1 0.47µF MKT polyester 3 0.18µF MKT polyester 2 0.1µF MKT polyester 1 .039µF MKT polyester 2 .018µF MKT polyester 1 .01µF MKT polyester 1 .0047µF MKT polyester 2 .0018µF MKT polyester 1 .0015µF MKT polyester 2 180pF ceramic 2 10pF ceramic 1 3.3pF ceramic Resistors (0.25W, 1%) 1 560kΩ 7 4.7kΩ 1 470kΩ 1 3.3kΩ 1 360kΩ 2 2.2kΩ 1 330kΩ 4 1kΩ 1 120kΩ 2 510Ω 5 100kΩ 1 470Ω 1 47kΩ 2 160Ω 1 20kΩ 2 51Ω 9 10kΩ 9 39Ω 2 8.2kΩ 1 27Ω 5W 1 5.6kΩ 1 16Ω 1 7.5Ω Miscellaneous Heatshrink tubing, solder, black sealant, etc. steps of 10dB each. The ninth position connects the output connector to ground. The output impedance is around 600Ω, depending on the attenuator setting. Switch S3 connects the circuit ground to case (mains Earth) when closed. When the switch is open, the circuit earth is con­nected to mains Earth via a 0.47µF capacitor. This switching arrangement allows the Specifications Frequency range: 10Hz-100kHz in four ranges Total harmonic distortion (THD): 0.02% at 3V out from 20Hz to 2kHz with frequency display off; (.03% with display on); .04% at 10kHz (display off) and 0.1% at 100kHz Output flatness: ±0.1dB from 20Hz to 100kHz; ±0.35dB from 10Hz to 100kHz. Maximum output: 3.16V RMS on sine wave; 3.16V peak on square wave Attenuator: seven steps in -10dB increments plus vernier Attenuator accuracy: within ±0.5dB for all ranges Output impedance: 600Ω (nominal) Sync output: 280mV RMS sine wave Square wave rise and fall times: typically <33ns Frequency readout resolution: 1Hz for 10-1000Hz ranges, 10Hz for 1-10kHz range and 100Hz for 10k-100kHz range Frequency accuracy: typically less than 5% uncalibrated (can be calibrated) Frequency readout update time: 312ms (3.2 per second) signal generator to be earthed when neces­sary or disconnected if a hum loop is evident. Square wave generation To obtain a square wave, IC1b’s output is applied to com­parator IC3b which is connected as a Schmitt trigger with posi­tive hysteresis applied between its pin 7 output and pin 9 via a 100kΩ resistor. Pin 9 is also tied to the midpoint of the ±5V sup­plies via 10kΩ resistors. The positive hysteresis sets the switching thresholds for pin 10 at +0.24V and -0.24V respect­ ive­ly. So when the input goes above +0.24V, pin 7 goes low and when the input goes below -0.24V, pin 7 goes high. Note that IC3b’s output is an open collector stage which requires a pullup resis­tor. This resistor is only connected when switch S4a is selected for square wave output. The output from IC3b is further squared with Schmitt trig­ger inverter IC6a which drives the five paralleled inverters IC6b-IC6f. They drive trim­ pot VR6 and dual-gang pot VR2a. Frequency multiplier As discussed previously, diodes D3, D4, D5 & D6 mix the sinewave outputs from IC2a, IC1a, IC2b and IC1b. The resultant waveform is squared up in Schmitt trigger IC3a. Because the output of IC3a is a single open-collector NPN transistor and its load resistor is connected to the +5V rail, and the control pin (pin 3) connected to ground, the output swing is limited to 0V and +5V which is what is needed for the following divider stages. The output of IC3a connects to the 4518 dual BCD counter, IC10. The two counters produce a total division of 100 at pin 14. The output of IC3a, the Q4 output from IC10a and the Q4 output from IC10b are all connected to IC12, a 4052 analog switch, and this acts as the range switch for the display. Depending on the voltages fed to its inputs at pins 9 & 10 from switch S2c, IC12 selects one of the inputs and feeds it out at pin 13. When range switch S2c is in positions 1 & 2, pins 9 & 10 of IC12 are tied low via 4.7kΩ resistors. This selects the pin 12 input from IC3a. When S2c is in position 3, diode D7 pulls pin 9 of IC12 to the +5V supply and so IC12 selects the signal at pin 15 which is the divide-by-10 signal from IC10a. Position 3 of S2c also applies 5V to the decimal point (DP1) of display DISP2 via a 39Ω resistor. Position 4 of S2c pulls pin 10 of IC12 high and pin 9 high via diode D8. This selects the pin 11 input of IC12 which is the divide-by-100 signal from IC10b. Decimal point DP2 is now select­ed and driven via a 39Ω resistor on DISP3. The signal from pin 13 of IC12 is applied to the pin 6 input of Schmitt P.C.B. Makers ! • • • • • • • • • If you need: P.C.B. High Speed Drill P.C.B. Guillotine P.C.B. Material – Negative or Positive acting Light Box – Single or Double Sided – Large or Small Etch Tank – Bubble or Circulating – Large or Small U.V. Sensitive film for Negatives Electronic Components and Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 • ALL MAJOR CREDIT CARDS ACCEPTED February 1999  31 The construction of the Audio Signal Generator involves two PC boards, with very little else in the way of interconnecting wir­ing. We’ll publish the full constructional details in Pt.2 next month. NAND gate IC9d. The second input at pin 5 is under control from the timebase signal derived from IC11. IC11 is a 555 timer which is connected in the astable (free running) mode. The capacitors at pins 2 & 6 are charged via the series 360kΩ and 120kΩ resistors and discharged via the 120kΩ resistor. The result is a pulse waveform at pin 3 which is high for 0.25 seconds and low for 62ms. This is inverted with IC9a and inverted again with IC9b. IC9b controls the pin 5 input to IC9d and this gates through the signal from pin 13 of IC12 to the clock input (pin 12) of counter IC7. Each time pin 3 of IC11 goes low, pin 15 (Reset) of IC8 is pulled low via IC9b. Also the high output at pin 10 of IC9a allows oscilla­tor IC9c to operate and it clocks IC8. This is a 32  Silicon Chip decade coun­ter and it provides the latch enable (LE) and Reset signals for IC7. When pin 2 of IC8 goes high, it latches the counted signal in IC7 into the display. After that, pin 7 of IC8 resets IC7 for the next count cycle. The latched count signal in IC7 is indicated on the 7-segment LED displays. IC7 drives the display in multiplex fashion via transistors Q1-Q4. This has the advantage of a reduced number of connections between the counter and the 7-segment displays but it does have the drawback of all multiplexed dis­plays and that is increased “hash” on the supply rails. Inevi­tably, some of this hash finds its way into the audio output of the signal and to eliminate that problem we have included toggle switch S6 into the circuit. S6 disconnects the +5V supply to pin 18 of IC7 and this turns off the displays. Note that the clock, LE and R signals are still be applied to IC7 even when the +5V rail is switched off. However, this will not cause damage to the counter IC. Power supply The power supply uses a fairly large power transformer and this is mainly required to satisfy the current drain of the 4-digit 7-segment LED display. The transformer secondary windings are connected as a 30V centre-tapped output to drive a bridge recti­fier and two 1000µF filter capacitors. The resulting ±20V DC rails are applied to a +15V regulator (REG1) and a -15V regulator (REG2) and these supply the op amps. The +20V supply is also fed to a +5V regulator via a 27Ω dropping resistor while the -20V rail feeds a -5V regulator directly. This completes the circuit description. Next month we will give the full SC constructional details.