[sdiy] Help with Timing Capacitor types.

[Ian Fritz]

At 02:05 AM 3/4/2007, Paul Schreiber wrote:

>Sure, a 100ppm tempco helps and in the MOTM-300 VCO, there is a specific 
>tempco loop for the cap.


Ah, that old, old, servo loop approach.

As I showed a couple of years ago, there is a much simpler way. This is to 
add a bit of dc offset at the base of the converter transistor.  Just takes 
a trimpot and a resistor.   This gives an adjustment that may be used to 
eliminate all linear temperature drifts, i.e., the correction is 
adjustable.  The old, old, servo loop method makes a fixed compensation and 
requires an opamp and several resistors.

I'm surprised that none of the early designers seems to have used this 
simpler method.  Although I developed it on my own, by studying the tempco 
math in detail, I later found a similar circuit in one of the app notes for 
matched pairs.

   Ian

http://home.comcast.net/~ijfritz/sy_cir7.htm
(discussion in paragraph above the last figure)

http://home.comcast.net/~ijfritz/sy_cir9.htm
(R13 and R14)