Tempco measurements on ASM-1

[jorgen.bergfors at idg.se]

Hi DIYers.
I made some measurements this weekend on my ASM-1 clone VCO. I used two different tempco resistors and compared the compensation over an eight octave range. I also measured with an ordinary metal film resistor instead of the tempco. Measurements were done at 25 degrees and 38 degrees (centigrade). My CV quantizer was used to get repeatable control voltages.

Without compensation, the error is quite small att the high frequency range. At 7V CV (4186 Hz) it is 19 cents sharp. But with lower CVs the error increases linearly. At 0V CV (32,7 Hz) the error is a hefty 300 cent. No, that's not three dollars. It is three half note steps. Completely unacceptable, in other words.

With the 3000 ppm/C tempco, things looked completely different. Now it went slightly flat in the top range, when the temperature was rised. The error was -19 cent at 7V CV (4186 Hz). In this case the error didn't increase at lower frequencies. It got smaller. At the lowest frequency the error was so small that it couldn't be measured with mu frequency counter.

I also tried with a 3900 ppm/C tempco. This overcompensated slightly. At 7V CV (4186 Hz) the VCO went 30 cents flat. Lowering the frequency caused it to get flatter still. At the lowest frequency the error was -50 ppm. 

For the tests I used the 1k SMD tempcos that Farnell sells. The expo converters were BC847BS dual transistors. They were potted together with the tempcos to keep them at the same temperature. Without potting even the smallest movement of air caused the frequency to wander.

The data (error in cents):

Frequency 0 ppm 3000 ppm 3900 ppm
32,7 305 0 -51
65,4 255 0 -39
130,8 218 -3 -39
261,6 180 -3 -45
523,3 135 -6 -39
1047 97 -10 -35
2093 57 -12 -32
4186 19 -17 -30

So as you see, the 3000 ppm tempco gives the optimum results. It is not totally perfect, but very close. Increasing the coefficient slightly (to the theoretically ideal 3300 ppm) would only mean that it would go slightly flat in the entire range.

/Jorgen