[sdiy] square2sine/continuos

[Scott Gravenhorst]

Ragozzino Pasquale <ragozzino at pentium.isu.polimi.it> wrote:
>i want to make a couple of 555 vcos and then use a vco to fm modulate
>another, but a square wave isn't fit, since the frequency of the
>modulated note
>would only shift beetween two frequencies... so i need a continuos
>waveshape. how could i get a tri. by the pin3 output?

Pin 3 is a typical totempole TTL output.  It is designed 
to be in only one of 2 states, near Vcc or near ground.
An integrator could turn the square output into ramps,
but it acts like a lowpass filter.  If the cutoff frequency
of the LPF is fixed, then the output amplitude will change
as the frequency does.  I would suggest not using the 555
unless you want square.  And then you'll have to add circuitry
to make it a VCO, which isn't necessarily easy for a true 50%
duty cycle square output.  The pin 5 control input will give 
you only limited control over frequency.  The FatMan uses
a 555 with a linear voltage to current converter to make
a linear sawtooth VCO.  There are better ICs to do a tri
VCO.

The LM13600 & LM13700 data sheets have several VCO 
circuits, a saw/pulse, and a tri/square that use both 
OTAs of one package, also a  simpler (tri I believe) 
that uses one OTA (you could make a dual from one 
package).  The latter circuit may not be very 
linear; I built one once just for kicks, but never
tried to apply it to music.  All of those circuits, 
however, do not show an adequate current source to 
drive it in a linear (or exponential) fashion, they 
all use a simple resistor for supplying Iabc.

Also, if you look closely at the tri/square and saw/pulse
configurations, they are somewhat similar and you may be
able to cleverly switch from one to the other.  I've
never tried to do that though.  

Using a linear voltage to current converter, I was able to 
get linear performance from the saw/square circuit over
6 octaves.  (I didn't test any more than that since 6 was
adequate for my application.)

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