Re: [sdiy] VCO and expo converter question

[Roman]

	Hi, 

	Rene's idea of 4069 (I think) was to make it small and cheap. If you modify it by adding precision opamp and even more precision transistor pair (in human language precision=expensive), then why bother using 4069 as integrator and schmit trigger. CMOS can't give you the stability that your new expo converter provides. I'd change it into standard opamp integrator and comparator. But anyway it's only my sick opinion. 
	The 100p is there for stability, and that's why everybody uses it. In such circuit you need to create another pole in opamp's response, so it rollofs faster before phase response twists enough to make it oscilate. I'm sure others will explain nicely how to calculate having given opamp's parameters. If you have a scope, see if there's high frequency oscillating at opapms output. If there's none, your 100p is enough. Otherwise increase it and see what happens. If you don't have a scope, use diode+cap HF detector and a voltmeter. 
	The pots - for tuning knob I'd use 1:10 ratio of pot and load resistor. That gives unnoticeable (is that a word?) nonlinearity in knob scale. The values of load resistor you gave into coarse and fine are OK. They give +/-3 cotaves range in coarse, and +/-1 in tune. I'd make it 300k for +/-5oct, and 3M for +/-0.5oct respectively. 

	Answering to your last question: you have inverting summer there, so anything you connect there will not affect other pots, or anything. There's 0V potential at this point, so all pots 'think' they have load resistors connected to ground. 
	hope that was usefull and not too long 

	Roman 
---- Wiadomość Oryginalna ----
Od: ryan 
Do: synth-diy at dropmix.xs4all.nl
Data: Sun, 23 Nov 2003 17:04:29 -0600
Temat: [sdiy] VCO and expo converter question

   hi,   I've been studying expo converters for about 2 or 3 weeks now because I wanted to adapt an active CV summing stage to René Schmitz's 4069 VCO. It also is going to use a PTC tempco resistor instead of the NTC resistors.  What I've come up with seems like it should work, atleast mathematically, but I'm not sure about a few components which I've seen on other expo converters.   here is the schematic for the 4069 VCO with a modified expo converter. Everything after the MAT-02 was copied from the original 4069 VCO http://www.sdiy.org/destrukto/4069_VCO.png (http://www.sdiy.org/destrukto/4069_VCO.png) this seems to be the common way of making an expo converter using the MAT-02 as the differential pair. The part I'm unsure about is the capacitor in the feedback part of the 2nd opamp (the opa2277). I picked a 100pF capacitor here although I have no idea what it should be, or how to calculate what it should be.  I've been told that it's purpose is to prevent the opamp from oscillation but I just put it there because everyone else does...   so, does anyone have any comments, suggestions, explanations about this capacitor? I've seen it in many of your designs although the value seems to range from 1pF to 680pF in different designs.     one more question. I've chosen 100K pots for all the pots.because I'm unsure how to pick the values of pots in a voltage divider.  Is there some rule of thumb to follow when choosing pot values? It seems like the pot will affect the voltage seen at the input of the opamp so this could cause a problem?   thanks... ryan

 
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