[sdiy] summing amp v/oct trim

[THOMAS, Mervyn]

OK Obe-Wans, help me understand!

I've just got my head round how a summing amp works (with a little help from
Walt Jung's fine book) but now I'm confused over v/oct trimming.

In the ASM-1 vco there's a trim pot in series with the feedback resistor of
the summing amp. This is tweaked to get 18mV/V to the base of the first expo
tranny.

In other circuits I've seen a divider to ground on the output of the summing
amp feeding the expo.

and in other circuits I've seen the summing done without an op amp at all
and a trimmer/resistor between the summing point and ground with the pot
wiper feeding into the expo.

Question is: what's the difference (if any) in the methods?

Answers in words of one syllable please, I'm still learning...

Regards,


Merv Thomas
UNIX CAE SysAdmin - Portsmouth
*Email:	mervyn.thomas at astrium-space.com
*Tel.:	 +(0)23 9270 5394 
 * Fax.:	 +(0)23 9270 8663


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