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Thomas Henry just doesn't give up on an IC. Whether it's an XR, an SN, or even the humble CD, he seems to always find some way to make that IC give up more than the datasheet even implies. Such is especially the case with the often maligned ICL This is hardly a surprise - the has always had backing it a datasheet that puts it in the situation of having to crank out a modest frequency range while providing a certain more-than-warm glow to keep the engineer or technician warm throughout the winter.
In other words, the poor thing can't help but start out on the wrong foot when someone refers to the datasheet for guidance on how to use the thing. So, our has been consigned many a time to sit out the game on the bench. All it needed was a good coach, someone habitual enough to pluck it off the bench and force it to run wind sprints, stair laps and line drills.
And that certain someone was Coach Henry. As it turns out, what the needed was a good change of diet. Instead of junking out on positive voltage rails, Thomas fed it a steady diet of good old fashioned negative rail. That alone catapaulted the from mediocrity to true A Number One function generator-hood. And, all the while, this little champion maintains its sine, triangle and pulse outputs to a "T". Parts of the circuit may look familiar to you - Thomas had an earlier version of it posted on Midwest Analog for some time.
When Midwest Analog closed down, it was For a while, unauthorized PCBs for the earlier version were sold, until Coach Henry spotted the rogue board and put it to an abrupt end. After that, Thomas took that basic circuit and created this much improved and superior design. So, you may ask, what is this audio generator for?
Is it a piece of test equipment or a component for a synthesizer? The answer to that question is "Yes. On this page you will find complete plans for the function generator - a schematic, a PCB layout, and plans for a nice, spiffy case to put it in.
That allows you to put it on your bench top and use it to test your audio circuitry. Though it does use linear control voltage, it's doubtful it would track a linear voltage produced by, say, a keyboard of a Korg MS, which produces a linear control voltage, but it would be great for sweeping and modulation effects.
On the electronics bench, this input would be great for tests that require a swept oscillator. For example using a scope with XY inputs and sweeping the audio generator with another VCO to check out the response of a filter. The audio generator operates in the the range of exactly 20 Hz to 20 kHz. It features sine, triangle and square wave outputs. The frequency control set consists of a coarse and fine tune arrangement - the coarse control obviously will move the frequency in greater increments, and the fine control allows one to set the frequency exactly at the desired point.
An input attenuator is provided to scale this voltage to the desired range of control. As you peruse the schematic, you'll find the typical elegant economy of of a very effective design that is Thomas' virtual trademark. Note that there are only two integrated circuits used in the audio generator! This is not a difficult project to build. If you'd like to etch a PCB for your audio generator, the PCB layout and parts placement document are available for download below.
Here's a pic of Thomas' completed project you can click on the image to enlarge it. This approach turns the audio generator into a handy portable device you can take out to the field or to a gig! The panel decal can be made from the panel document provided below, and Thomas has also supplied a drilling guide for it, which can also be downloaded. We'll start off with the general topology that Thomas has put into place.
As Thomas puts it:. This is the main bulk of the magic Thomas has devised to make the play super-nice. It should be noted that you will probably want to use a good quality pot for R25, the Coarse control.
In fact, if you are going to use a scale for that pot on your panel, this will be essential. Even though the marks on Thomas' panel design are only approximations to help find the ballpark, a better quality coarse pot will put you closer to those marks. Notice there are a number of trimpots - these are in place to ensure that the specified frequency range is met, while maintaining the correct linear waveforms of the triangle and sine outputs throughout that range.
There are five trim pots. Two trim pots are used to set the maximum and minimum frequency range of the audio generator. R10 sets the maximum frequency and R23 sets the minimum frequency.
Three trimpots are used to calibrate the quality of the waveforms - two set high frequency and low frequency symmetry, while another trims the distortion of the sine wave. R1, Symmetry Trim sets the symmetry of the waveforms above Hz or so. R22 is the Low Frequency Symmetry Trim - it is used to ensure the waveforms hold their symmetry below Hz. Once the symmetry is good across the frequency range of the audio generator, R21 is used to trim the distortion of the sine wave to the lowest possible level.
The calibration is more or less an "around the horn" affair - one has to go back and forth a few times between trimpots before everything is set. An oscilloscope is pretty much required for setting symmetry of the waveforms. Distortion may be set by that most precise piece of equipment you possess the set of ears you have placed on each side of your head , or an oscilloscope, or a combination of the two.
Frequency may be set with a counter, though I personally used an oscilloscope when I had it on the bench as my counter has been defunct for some time. Obviously, there are just not a whole lot of parts in this project, and all are fairly easy to get hold of. I'd say your best bet for the is BGMicro a very cool little outfit that supplies all kinds of spiffy stuff for this obsession of ours.
Other places have it as well, though I was disappointed to recently discover I couldn't get them at Jameco, a place I'd always relied on before for my procurement. Abra Electronics, Debco, Futurlec, Unicorn - they all have it. Shop around - prices vary widely! As always, there is the Thomas Henry Designs forum at electro-music. I urge you to go there and browse, and even sign up hey, it's free!
A great group of guys, including TH himself, are known to hang out there. This publication is available for preview and purchase at Lulu. You'll see it is fed the proper nutrients as well. This site copyright c Scott Stites.
All rights reserved. Go back and forth between these two pots until you get good symmetry at the low and high ends. Turn R25 Coarse and R26 Fine to their highest settings. While monitoring the output on a frequency counter, adjust trimmer R10 Max to give 20 kHz. Turn R25 and R26 to their lowest settings. Adjust R23 Min to give 20 Hz. Go back and forth between these to pots to arrive at a solid 20 Hz to 20 KHz range.
While listening to the sine output and watching it on a scope, adjust R21 Sine Distortion Trim for minimal harmonic distortion. This is easy to do with ears alone. Repeat the entire sequence a couple times to lock in all trimmers as desired.
8038ACP EXAR/XR DIP-14 Other Components
Precision Waveform. D Wide Frequency Range 0. D Low Distortion Variation with Temperature. June D Precision Waveform Generation. D Sweep and FM Generation. D Tone Generation.
The 8038 Audio Generator Project
The core of this circuit is IC L — which is basically a function generator chip capable of generating different waveforms. The ICL is a function generator chip, capable of generating triangular, square , sine, pulse and sawtooth waveforms. There is an option to control the parameters like frequency,duty cycle and distortion of these functions. This is the best function generator circuit for a beginner to start with and is of course a must on the work bench of an electronics hobbyist. The circuit here is designed to produce waveforms from 20Hz to 20 kHz. The ICL has to be operated from a dual power supply. Function Generator Circuit Diagram Notes.