[sdiy] exponential vs linear vco cores

[cheater cheater]

Scott,

great explanation, thanks. This makes me wonder if there are any
inherently exponential VCO cores. The circuit you've shown is 'linear'
in that the frequency rises linearly with voltage/current.

D.

On Fri, Feb 26, 2010 at 23:04, Scott Nordlund <gsn10 at hotmail.com> wrote:
>
> http://www.art-sci.udel.edu/ghw/phys245/05S/classes/images/opamp-integrator.gif
>
>
>
> This is the integrator circuit that makes up the "heart" of every VCO
> that I know of.  Of course this can also be realized with discrete
> transistors.
>
>
>
> The op amp's inverting input acts as a "virtual ground", so the circuit
> can be fed either a voltage (with the resistor in the diagram) or a
> current (omitting the resistor).  Since the exponential part
> outputs a current (you're basically getting I = exp(V) from a PN junction), an exponential VCO will usually put a current straight into the op amp, while a linear VCO will use a voltage from the mixed CV inputs.  It's really still a current going in, in any case.
>
> The negative feedback of the op amp forces the input current through the capacitor, and from the relation I = C*dv/dt, the output voltage is the (inverted) integral of the input current.
>
> I guess some variant would be possible using inductor's V = L*di/dt relation, but this doesn't strike me as a fantastic idea for a number of reasons... it might need a spark gap or something.
>
>
> ----------------------------------------
>> Date: Fri, 26 Feb 2010 21:24:04 +0100
>> Subject: Re: [sdiy] exponential vs linear vco cores
>> From: cheater00 at gmail.com
>> To: jays at aracnet.com
>> CC: synth-diy at dropmix.xs4all.nl
>>
>> Jays and David,
>> sorry to hijack your SSM2164 thread :P but I have a follow up question
>> to the topic of I/V stage designs.
>>
>> Is the exponentialization always integrated with I/V or does it
>> sometimes happen purely in the voltage domain or alternatively purely
>> in the current domain?
>>
>> Aaron,
>> thanks a lot for the link. I can't have a look right now being on
>> GPRS, but as soon as I have some sort of useful internet connection
>> I'll make sure to dig up the link.
>>
>> Cheers
>> D.
>>
>> On Fri, Feb 26, 2010 at 21:06,  wrote:
>>> David,
>>>
>>> Several quesions if you don't mind. Thanks!
>>>
>>> If I remember right you need to get a magic voltage good to 3 digits (down to mV) for your expo to work right.
>>>
>>> Is this voltage used for the expo converter or the temp comp for the expo converter?
>>>
>>> How hard is it to get that tweaked in for proper operation with cheap test equipment (in this case for me cheap means < $50 DVM)?
>>>
>>> Thanks
>>> Jay S.
>>>
>>> David G. Dixon wrote:
>>>
>>>>
>>>>> Tempco class discussion based on Rene's website, with a side note that if
>>>>> you really want to go down the rabbit hole, you should check out Ian
>>>>> Fritz's website.
>>>>
>>>> Didn't want to go down the hole, or even near the hole.  That's why the
>>>> 2164-based approach was so appealing to me.  No gluing of expensive and hard
>>>> to find parts (tempcos) to other expensive and hard to find parts
>>>> (super-matched pairs)!
>>>>
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