AGC (was: RE: [sdiy] quadrature sine wave oscillator question

[René Schmitz]

Hi Martin, Jürgen, Jim and List!

The main problem for any AGC'd oscillator is the distortion at
lower frequencies (be it FET, PTC or LDR). Once your frequency 
approaches the timeconstant of the peakdetector, the wave will have 
distortion.

Also the thermal time constants involved in PTC or NTC stabilisations 
will probably be way higher than the delay an LDR introduces. So indeed 
its a good question why nobody does that.

For a quadrature LFO the method with the sine-squares is probably the 
best idea, unless you can tolerate a couple percent of THD. (Often you 
can, see the 3-phase LFO at my site, which is merely stabilized by 
nonlinear action of the amplifiers.)

Cheers,
  René

jhaible at debitel.net wrote:
>>I always wonder why jfets are used for AGC,
>>and not LDRs.
>>Perhaps someone came up with a jfet servo loop
>>and all the others just copied it?
> 
> 
> FETs are faster.
> There are some classic opto compressors which have
> a "special" sound that may partially benefit from
> the LDR's limited speed. But if you want to have
> the full range of useful attack / release times,
> The FET just dosn't has a speed limit like a LDR.
> 
> 
> 
>>The distortion the LDR causes should be way lower
>>than those of the jfet.
> 
> 
> Depends on the voltage level. Lower level, no more distortion
> problems - now your enemy is noise.
> But I was pleasantly surprised how well a simple FET 
> can work for dynamic reduction, if you have a low noise
> (discrete) amplifier to deal with the low signal levels.
> (See my 1176 clone schemos for details.)
> 
> 
> 
>>Also diode forward conduction
>>problems simply do not exist.
> 
> 
> Maybe this is why the control circuit of the 1176 has such
> high resistor values at the FET base? 
> 
> 
> 
>>Perhaps the useable range of the LDR is too small?
> 
> 
> I can't see why it should be smaller than a JFET's.
> Only that the LDR gets even slower on one end of its range ...
> 
> 
> 
>>Applies for quadrature as well as Wien bridges.
> 
> 
> Ooops, your still talking about AGC for oscillators,
> not about general dynamic reduction.
> 
> Well, I guess most sine oscillators have *thermal*
> AGC - an incandescent lamp that serves as PTC. 
> FWR and tru-RMS built-in, and the thermal
> capacity provides the time constant. (;->)
> 
> JH.
> 
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> debitel.net Webmail
> 

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