SV: Re: [sdiy] Expo converters, lin log etc.

[Karl Ekdahl]

Hi all,

> Not really. A "normal" VCO is exponential and
> therefore an exponential function
> is needed. A freq-to-CV converter is logarithmic in
> its frequency to CV
> conversion for a "normal" VCO.

So, i've got two answers, "normal" is linear and
normal is exponential.

Basically, if i input 1V and get 100hZ, i then input
3V and get 500hZ it'd be assumed that inputing 2V
should give 300hZ - that's a linear response if i'm
not mistaken, am i correct in my thinking?

> For some reason the terms is widely missused.

I figured there was something fishy...

>        U
> f = f 2
>      0
> 
> where U is the input frequence. The base 2 is
> natural since you want to double
> the frequency for the rise in voltage of 1 V. f0 is
> the frequency of the
> oscillator with the net sum of 0 V.

Okay awesome, what if i want to do some
reverse-engineering; basically i'm experimenting with
this oscillator which i'm unsure of what kind of
response it has, i've measured a couple of voltages
and the resulting frequencies and i'm unsure wheter
the response is in fact exponential or what it is.

> The transistor has the "right" properties, i.e. the
> exponential function but
> with incorrect values. For most practical matters it
> is:
>          qV
>         (--)
> I = Is e kT
> 
> where Is is a very small current due to material
> aspects
> e is the natural base (about 2.718281828... )
> q is the elementary charge (a constant)
> k is the Boltzman constant
> T is the NP-junction temperature in Kelvins
> V is the NP-junction voltage
> 
> Assuming a stable (or compensated) temperature (this
> is a big issue to us),
> the input voltage will be exponentially converted
> into a current. This current
> we use to feed into a CCO core (Current Controlled
> Oscillator). We scale our
> voltage down to about 1/57 V per octave, compensate
> for the scaling error and
> the frequency offset error.

I gave this a shot a while ago, but i've forgotten
most of the math skills (i once had) necessary to
understand this. 

> It is not too hard to have those strange transistor
> to have some 15-20 octaves
> of nice exponential response. It is still a valid
> method even if it may feel
> strange. Infact, I'm not fully sure that I
> understand all that happends in
> there, but for all practical reasons it does the
> right thing and that is what
> matters in the end.

That's my problem; i have a hard time accepting "black
boxes" until i understand the underlying functions -
at that point i might as well just forget about it
because then it is "valid" in my head.

Karl



	
	
		
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