About NATIONAL APP NOTE #299 VCO

gstopp at fibermux.com gstopp at fibermux.com
Wed Apr 24 18:41:20 CEST 1996


     Actually I did tie the 3046 substrate pin to V- in my testing. However 
     now I know that I had some solder flux contamination problems last 
     year that caused me to mis-judge several circuit designs. The 
     particular solder I was using was much worse than any I had used 
     before, so when I thought I was cleaning the boards I actually wasn't 
     doing a good enought job.
     
     Now of course I've discovered this and while I'm still using the same 
     solder, my cleaning methods are way overkill. No more problems, at 
     least.
     
     Yes - please try this VCO and tell us how it works. I'd like to do it 
     all over again now that I think I know what happened, but I haven't 
     found the time.
     
     - Gene


______________________________ Reply Separator _________________________________
Subject: About NATIONAL APP NOTE #299 VCO
Author:  "Mikko H." <MHELIN at tne01.tele.nokia.fi> at ccrelayout
Date:    4/24/96 4:06 AM


This is a kind of old thing, but as I was browsing a data sheets for 
CA3046 I found a note "The collector of each transistor of the CA3045 and 
CA3046 is isolated from the subsctrate by an integral diode. The substrate 
(terminal 13) must be connected to the most negative point in the external 
circuit to maintain isolation between transistors and to provide for 
normal transistor action." Maybe this explains why AN-299 VCO didn't
work as it should have, and why all 3046 based VCO won't oscillate 
at low frequencies (as Gene pointed out). EN #73 VCO option 1b in EN
Preferred Circuits Collections also mentions that sustrate pin should
be connected to -15V thru 10k resistor. With AN-299 VCO there's a problem 
as an absolute max. rating for collector-substrate voltage is 20 V, and 
the collector of Q6 is connected to 15 V, so maybe 20k/10k divider
should be used instead. I haven't build that VCO yet as I'd like to 
know if this feedthrough through substrate was the reason for original 
problems with that VCO design. 
     
-Mikko




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