interesting VCO was RE: [sdiy] Imperfect VCO

[Scott Gravenhorst]

I looked at the CD4046 version of this and now the 74HC4046, the difference
is that (in the CD4046 diagram) the current source is detailed as a CMOS
transistor current mirror driven by a CMOS buffer and the flipflip inputs
are actually a chain of inverters (I assume this is to square up the
capacitor charge curve).  It's an interesting design in which the capacitor
is reversed every charge cycle to eliminate reset.  I had tried to fool the
4046 VCO into behaving more linear by using a voltage controlled current
sink from pin 12 to Vss, however, I'm not sure that 1) I did it correctly
and 2) that the CMOS current mirror is a good current mirror or just a
"sortof current mirror".  

I also thought it might be possible to make this VCO with discrete
components, but never got to the breadboard.  I was hoping that the
capacitor charge switches could be implemented using a 4069 or 4007, but it
seemed to me that the with a current source at the top of that, we would
have problems with the static protective diodes.  Am I close with that?  If
so, I suppose the cap switches could be built with discrete MOS transistors
or what about a 4066 to do that?  Wouldn't that solve the diode problem
(assuming there is one)?  If it could be done with 4069, that seems to be
simpler perhaps.

I know that I could get sawtooth from this with a few opamps, and I see no
reason why not also triangle.

"Czech Martin" <Martin.Czech at Micronas.com> wrote:
>Hi Ian,
>
>did you have a look at the VCO schematic of the (Fairchild?)
>74HC4046 in the data sheet? 
>
>This could be done with discrete components also.
>
>Maybe this could be turned into a tri output.
>Maybe also phase reversal is possible.
>
>m.c.
>
>-----Original Message-----
>From: Ian Fritz [mailto:ijfritz at earthlink.net]
>Sent: Freitag, 1. August 2003 18:06
>To: Czech Martin; music.maker at gte.net; synth-diy at dropmix.xs4all.nl
>Subject: RE: [sdiy] Imperfect VCO
>
>
>At 04:36 AM 8/1/2003, Czech Martin wrote:
>
>>I have the impression that simple circuits like diode clippers
>>give only minor effects, and the more interesting effects
>>like "folding" , "resonance" need considerable circuit effort.
>>
>>There is no free lunch.
>>
>>So on the bottom line: wave mangling is interesting, but you should
>>not expect wonders from simple circuits.
>
>
>Agreed.  I looked at simple circuits many years ago and didn't find 
>anything very exciting.  That's why my waveshapers have generally been 
>getting more and more complicated.  The double-pulse generator, the 
>5Pulser, the Wavolver ... these are the interesting ones.  I also have on 
>paper a SNIC-based folder, similar to the Buchla except the folded peaks 
>evolve into sawteeth rather than smooth peaks.  This one will really pump 
>out the harmonics.
>
>   Ian
>

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