SV: Re: [sdiy] OTA VCO bad or not!

[René Schmitz]

Hi Karl,

karl dalen wrote:
> Yes but was/is that so bad really?

I don't remember, but I bet the answer can be found in the archives...

> To be honest i havent build a OTA osc for years and my memmory is decomposing!

Me neither.

>>You need to set the levels for the OTA to "clip" so that the output 
>>maxes out at +-Iabc, with a much reduced thermal coefficient.
> 
> 
> The lower the worser the thermal drift i suppose.

I'd say the harder you drive the inputs the better. But some designs 
seem to deliberately keep it well below the clipping level. Perhaps to 
kill other temperature dependancies (like that of clipping diodes, these 
will have their own 2mV/K)?!

> Hmm, when doing the electronotes VCO or a  one with fast and accurate
> comparator
> i se almost perfect symetry but the amplitude start to expand at higher
> frequencies,
> but it appears to be a slewrate problem of the comparator, still its pretty
> much 
> very good at 0 to 20Khz! (did this with a Ne5517 and a LM13700).

Use a fast comparator (LM319?), and keep the impedances of the 
hysteresis setting network fairly low. (some kOhms) And you can possibly 
use a small "speed up cap" across one of the two resistors to increase 
the switching speed at high frequencies.

> My problem is that i need a aboslute symetry, and no dent/glitch/slant
> overshot/undershot on the tri wave from 35hz to 14Khz. 

So you want waveform purity. How are your demands on drift and expo 
conformance?

> But how much poorer in *cents' is this inreallity?

Poorer in the sense that you need more Rbe compensation. Some figures 
are at my page. The actual figure depends on your particular 
application. (Reference current, sensitivity of the CCO core and so on.)

>>This all doesn't say its impossible to build an OTA-VCO, but I wouldn't 
>>really recommend using them if you have high performance demands.
> 
> 
> So what is high performance demands actually?

It usually involves demands with words like "absolute" or "perfect" in 
it. ;-)

> LM13700 obsolete?

I don't know their current status. But I wouldn't bet on them being 
still available in the next few years.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159