[sdiy] Those AS33xx chips again...

[Steve Lenham]

Very interesting and useful - thanks Roman! My immediate thoughts:

1. Re the tempco, the 3340 is not the only potential source of 
variation. What dielectric are you using for the timing cap? C0G/NPO is 
fairly blameless, but others have significant tempco (I remember the 
tempco of a polystyrene timing cap being used to cancel an opposite 
tempco in a classic National Semiconductor appnote). Even trimpots have 
thermal drift.

2. Re the different voltages at the bottom of the triangle, this will be 
set by the offset voltage of the internal comparator. One would need to 
test multiple examples of each type to determine whether there really is 
a difference between the CEM and AS parts or whether it is simply 
chip-to-chip variation. The original CEM3340 datasheet specifies that 
the triangle bottom point can vary between +15mV and -15mV.

3. I don't know how much they matter in real applications, but the 
larger glitches on the triangle midpoint and bottom look like the 
biggest differences between the two devices. They look like the sort of 
thing you get when the opamp/comparator has slightly too much bandwdth - 
a case of modern IC technology being a bit too good? For example, there 
are certain vintage sawtooth VCO designs where you have to use a 741 for 
the main opamp, otherwise you get a colossal spike at the sawtooth 
reset; the sluggish 741 simply isn't capable of doing it.

Cheers,

Steve L.
Benden Sound Technology


On 29/11/2017 02:04, David G Dixon wrote:
> Interesting!  I would have expected better performance.  The waveforms 
> look a little dicey and the tempco is definitely not too great.
> 
>     ------------------------------------------------------------------------
>     *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On
>     Behalf Of *Roman
>     *Sent:* Tuesday, November 28, 2017 2:34 PM
>     *To:* synth-diy at synth-diy.org
>     *Subject:* [sdiy] Those AS33xx chips again...
> 
>     During few last days I've spent wonderful time with AS3340 VCO,
>     giving it a closer look, and also a few measurements. Nothing
>     spectacular, just something that any geeky nerd would find interesting.
> 
>     It's about vintage chips, so it seems appropriate to have it written
>     in ancient vintage static html, basicaly the 90's style.
>     http://www.sowa.synth.net/synthchip
> 
>     Roman
> 
> 
> 
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