New VCO,linearity!

[Ian Fritz]

Augusto --

----- Original Message -----
From: "Augusto Pinoche" <augustopinoche at hotmail.com>

> Talking about VCO's, what is the highest linearity
> percentage error one could accept over a 20Hz to 10KHz
> range? Is 0,4% bad? Talking only about the VCO
> not the expoconverter error introduced.

Well, there's room for personal taste and opinion here. The way I look at
it, it would be really nice to have frequency accurate to +/- 1Hz. That way
two oscillators playing together would only have slow beats. (If you want
beats, then you detune). This 1Hz criterion is not a fixed percentage, of
course. At 1kHz it is 0.1%, at 200Hz it is 0.5%, etc. I shoot for 0.1%
because that's about the limit of what you can measure without special
lab-grade test equipment. Also it's plenty hard to get to this level.

> And how about the highest PPM Temp value for
> the frequency of the VCO?

Again -- personal choice. Some people have been happy using a differential
pair converter without the compensation for the 1/T in the exponent. This is
3300ppm/K. If you just throw in a tempco resistor this drops to the range of
about 500-1000ppm/K, since the tempco is not exact compensation without
trimming and because there are other sources of drift in the circuitry. Many
people live with this level of drift, although it requires frequent
retuning. I've been trying to do better mainly because it's an interesting
technical challenge. It doesn't take too much work to get down to the
200-400ppm/K level, but it takes some time and careful measurements.

  Ian