_[sdiy]_ICL8038 worst ic off all time!

[Karl Dalen]

 --- Scott Gravenhorst <music.maker at gte.net> skrev: > Magnus Danielson
<cfmd at bredband.net> wrote:
> >From: Scott Gravenhorst <music.maker at gte.net>

> >As I recall it, in the 4046 you have basically two inputs and one of
> them is
> >really a current input, so that's your entry into the world. I did
> exchange
> >emails with Björn Juhlin back then, and I don't recall exactly but I
> think I
> >helped him with that particular part (I might have just caused
> confusion, since
> >I was a little hasty to analyse things and made a few errors where he
> was
> >right as I recall it).

I think he did that on his own, but there was a later list discussion
on the matter as i can recall it. However referring to archive and some
off list correnspondance, the usage was of pin 11 and the later discussion
where about using pin 12,(Magnus suggestion) however what he didn't say
(i almost said, "as usual") was "why" he used pin 11 and not pin 12.
If reading the text really close of the design it speaks about the
"non linearity" of the internal current Pfet mirror.

And off list he says:_
:as much as a shot in the dark one idea are to "use" the current sink fet
:of pin 11 to compensate for the slight exponentially behavior of the
:internal current sinks by either statically set an offset voltage on pin
:9 or dynamically by a continuos voltage on pin 9 controlled by the very
:same V/octave voltage sent to the input of the exponential converter
:scaled in one way or the other!? A crude way to use the Nfet of pin 11 to
:compensate for the Pfet in the mirror!!...No satisfaction guarrentee..!!
:And a shot in the dark!......

However, quite interesting idea, could be worth for Scott to try out?
So, i think he did that pretty much on his own, he just didn't say
everything or "why" certain things was done. 

Also looking closer to the design he also suggest de franco compensation
of some sort,And i know he did some pretty complex "sort of de franco"
compensation by splitting up this slightly expo response of the mirror
into several smaller expo stages by using resistor dividers on both side
of the charge cap and some biasing! Quite bizarre as it sound to me but
he really tried to squeeze out the bananas of that IC to se what could
be done.

Great work anyway. And as he says a "neat cheap syncable LFO!!""

KD

> Hmm, I'd still like to get this working.  Yes, you're right, one pin
> (12?) is a 
> current sink pin.  I've designed a standard linear v-i converter that
> sinks to 
> ground, but again, I think I was trying to run the current up too high. 
> The 
> idea was to allow the v-i converter to control the current by setting
> the 4046 
> VCO CV input to the positive rail (thereby opening it up full) and let
> the 
> external sink do the work.  I'll have to try this again, but be more
> mindfull 
> of the control current.
> 
> Something else I've wanted to try is to build the oscillator core from a
> 4007.  
> The one thing that stopped me was that the reversing cap charge scheme
> would 
> cause the cap voltage to go below Vss and that concerned me.  Then I
> re-read 
> the data sheet stuff on the VCO and found that, by design, the cap is
> actually 
> discharged through the protection diodes.  I'm thinking that I can do
> this with 
> a 4007 as long as the diodes can withstand the discharge current (that
> means 
> the cap should be as small as possible).  Using a 4007 this way, I could
> avoid 
> the possible nonlinearity of the MOS current mirror that supplies cap
> charge 
> current in the 4007.  It's a curiosity, I know I can make a saw VCO
> other ways, 
> I just think the design is interesting.
> 
> >I haven't heard from Björn for several years now.
> 
> Yes, he did disappear.  Too bad.
> 
> >Cheers,
> >Magnus
> >
> 
> =========================================================  
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> 
> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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>  

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