VCO testing update
gstopp at fibermux.com
gstopp at fibermux.com
Thu Aug 17 02:59:12 CEST 1995
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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