[sdiy] exponential vs linear vco cores

[David G. Dixon]

> Several quesions if you don't mind. Thanks!

I love questions!

> If I remember right you need to get a magic voltage good to 3 digits (down
> to mV) for your expo to work right.

The magic number (by theory) is 289.45 mV.  In practice, anything between
289 and 290 mV will be fine.

> Is this voltage used for the expo converter or the temp comp for the expo
> converter?

This voltage is fed to the VC pin of the tempco VCA, which is acting as a CV
summer prior to the CV gain opamp.

> How hard is it to get that tweaked in for proper operation with cheap test
> equipment (in this case for me cheap means < $50 DVM)?

I use an even cheaper DVM (a $25 Chinese "Victor" model -- it's great,
because it has frequency), and it's easy.  I can get 289.4 or 289.5, and
this is plenty close enough.

My VCO circuit includes a REF02 5V reference trimmed to exactly 5V in the
manner prescribed in the datasheet.  This reference voltage is used in about
half a dozen places around the VCO circuit, including the tempco voltage.  I
drop this voltage across a 4.7k 1% resistor, a 100-ohm multiturn trimmer,
and a 261-ohm 1% resistor to ground.  The VC pin of 2164 has a 5k input
impedance, and this, in parallel with the 261-ohm resistor, gives a parallel
impedance of 248 ohms, which gives a total effective input impedance of 5048
ohms to the 5V reference.  Now, 5048 * 289.45 / 5000 = 292 ohms, and 292 -
248 = 44 ohms.  Hence, the magic voltage should reside at or near 44% of the
trimmer's range, which is as close to 50% as I could get.  Also, the total
voltage range of the trimmer is 5 * 100 / 5048 = 0.099 = about 99 mV.  With
the 18-turn Bourns trimmers I use, each turn is worth about 5.5 mV.  This
allows one to dial in the magic voltage quite precisely.