[sdiy] XR2206 temeperature stability within +/- 5%

[René Schmitz]

Hi Jean-Pierre and all,

On 02.07.2020 17:09, Jean-Pierre Desrochers wrote:

> I need a pure sine wave generator adjustable from 10Hz to 60Hz  at  > around 5v peak. The adjusted frequency should stay stable within +/- 
 > 5% temperature wise. The project inside metal enclosure could have > 
temp of up to around +40deg C. I have a bunch of XR2206 VCO IC on > hand 
to use. I wonder if the best capacitor to be used here would be > A 
polystyrene type for temperature stability.. (??) I'll use 1% metal > 
film resistors for the rest of circuit..

The 2206 datasheet gives you a hint to the best value of the resistor 
for optimum temperature stability.

There is a graph of frequency variation vs resistance and temperature. 
(Figure 9) From that we can conclude that the

optimum is somewhere between 4k and 200k.


Now with your desired range, lets put 60Hz at the top and use 10k, this 
gives with 1/RC -> 1.65uF.

Practically you'd choose 1.5u or even lower and raise the resistor value 
somewhat. Distortion is best at about 30k.

As long as you stay near that corridor.



As for the dielectric, I'd say your requirements are rather modest, and 
can easily be met.



For a ball park figure:

https://en.wikipedia.org/wiki/Film_capacitor#Frequency_and_temperature_changes_in_capacitance

The basic properties of some of the films used for film caps. (Ceramics 
COG or NP0 may

work, but large values might not be available. Some other films exist, 
but are not given in the plot.)


You can see that PP and PPS have excellent properties.


At 60Hz you don't have to worry much about inductance, frequency or 
other effects either.


Polystyrene and Polycarbonate were once the best choice for timing caps, 
but afaik are out of

production. And it is difficult to find a PS cap in that capacitance 
range anyway.



But even PET and PEN would be well within 2% over a practical 0..70°C range.

If your enclosure narrows this temperature range by shielding it from 
the environment, it may even help.


So you can make this with cheap and easy to get polyethylene caps. You'd 
have to confirm with the

datasheet of the cap chosen of course.


The largest contributors are the chip with worst below 1% and the cap 
with 2%. Metal film resistors

would add a mere 100-200ppm and are a good choice.




Best,

  René


--
synth at schmitzbits.de
http://schmitzbits.de

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