VCO testing update

gstopp at gstopp at
Thu Aug 17 02:59:12 CEST 1995

     Hi all,
     After getting the Electronotes tri-square VCO working, I went back to 
     see if I could fiddle with the other VCOs (the Electronotes sawtooth 
     VCO and the National App Note #299 VCO). Also another AHer was kind 
     enough to fax me the schematic for the Chroma VCO so I'll talk about 
     that one too.
     If you remember I had waveshape and frequency range problems with the 
     Electronotes sawtooth VCO when I first built it last week. Well that 
     one was buggin me because I was sure it would work better than it did 
     even with a TL082 instead of the 3130 - the extra current draw of the 
     integrator's (-) input should kill the low end, not the high. Anyways 
     I suspected a construction boo-boo and sure enough, my eyesight gets a 
     whole lot better when I'm not excited. If it oscillated at all before 
     it must have been from incompletely dried flux cleanup residue.
     (These VCOs are all on one vectorboard thingey, with wires connecting 
     all the parts together, scrubbed with isopropyl after soldering.)
     Anyway, added the missing wire and it works fine up to almost 100 khz, 
     and gets rounded waveforms and stops oscillating around 5 hertz, and 
     will probably go lower when I stick the CA3130 in there. Audio tests 
     at home are next.
     This VCO is attractive because it is simple. It consists of one dual 
     op-amp for the exponential and linear input summers, one CA3046 for the 
     exponential converter, one FET op-amp for the integrator, one LM311 
     comparator for resetting the integrator, and one JFET that does the 
     actual cap discharge. A cap in the positive feedback loop of the LM311 
     along with a beefy JFET allow a sawtooth reset time of 400 nsec. 
     Electronotes shows two different exponential converters - this one and 
     one that uses a matched NPN pair (AD818). I chose this one because the 
     CA3046 is easier (and cheaper) to get than the MAT-02 (AD818 
     replacement) from Analog Devices. However the CA3046 version has a 
     high-frequency track trimmer and the matched pair version does not.
     Adding one more comparator to generate a pulse wave, plus a summer for 
     the pulse width modulation inputs, will get you a complete saw/pulse 
     VCO with PWM, linear FM, and sync. The hardest parts to get will be 
     the tempco resistor, which judging by the results of my weekend test 
     may be optional after all.
     I did not get around to doing anything to the National App note #299 
     VCO so no change there.
     Now to talk about the Chroma VCO. I looked up the 4151 in the EXAR 
     databook, yup, sure enough, V-F converter. Then I got the fax of the 
     schematics. Here's my take on it:
     The VCO is of the same basic design as the Electronotes sawtooth VCO 
     that I have been talking about above. The input CV summing op-amp is 
     missing so that higher voltages in mean lower frequencies, at about .5 
     volts per octave. The FET has been replaced with the charge pump 
     thingey. Now here's the kicker - two VCO's are built around a single 
     CA3046, with all other op-amps shared between the two VCOs with dual 
     op-amp packages. This means that one 4558, one TL082, one CA3046, and 
     two 4151s make up two independent VCOs. Since these VCOs are being 
     controlled by the microprocessor in this beast, they simply did not 
     need the input CV summers, but you would want these for stand-alone 
     VCOs (in a modular system for example) so if I were to build this 
     circuit I would add another TL082 to add one CV input summer to each 
     VCO. Oh yeah - there is one tempco resistor per VCO as well.
     I think it was Ric who posted that he has access to some 4151's - can 
     I get some? Email me. Thanks!

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