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<p>Hi Jean-Pierre and all,<br>
</p>
<p>On 02.07.2020 17:09, Jean-Pierre Desrochers wrote:</p>
<span style="white-space: pre-wrap; display: block; width: 98vw;">> 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..
</span><br>
<br>
The 2206 datasheet gives you a hint to the best value of the
resistor for optimum temperature stability. <br>
<br>
<p>There is a graph of frequency variation vs resistance and
temperature. (Figure 9) From that we can conclude that the <br>
</p>
<p>optimum is somewhere between 4k and 200k.</p>
<br>
Now with your desired range, lets put 60Hz at the top and use 10k,
this gives with 1/RC -> 1.65uF.<br>
<br>
<p>Practically you'd choose 1.5u or even lower and raise the
resistor value somewhat. Distortion is best at about 30k. <br>
</p>
<p>As long as you stay near that corridor. <br>
</p>
<br>
<br>
As for the dielectric, I'd say your requirements are rather modest,
and can easily be met.<br>
<br>
<br>
<br>
For a ball park figure:<br>
<br>
<a class="moz-txt-link-freetext" href="https://en.wikipedia.org/wiki/Film_capacitor#Frequency_and_temperature_changes_in_capacitance">https://en.wikipedia.org/wiki/Film_capacitor#Frequency_and_temperature_changes_in_capacitance</a><br>
<br>
<p>The basic properties of some of the films used for film caps.
(Ceramics COG or NP0 may <br>
</p>
<p>work, but large values might not be available. Some other films
exist, but are not given in the plot.)</p>
<br>
<p>You can see that PP and PPS have excellent properties.</p>
<p><br>
At 60Hz you don't have to worry much about inductance, frequency
or other effects either.</p>
<br>
<p>Polystyrene and Polycarbonate were once the best choice for
timing caps, but afaik are out of <br>
</p>
<p>production. And it is difficult to find a PS cap in that
capacitance range anyway.</p>
<br>
<br>
But even PET and PEN would be well within 2% over a practical
0..70°C range.<br>
<br>
If your enclosure narrows this temperature range by shielding it
from the environment, it may even help.<br>
<br>
<br>
<p>So you can make this with cheap and easy to get polyethylene
caps. You'd have to confirm with the <br>
</p>
<p>datasheet of the cap chosen of course. <br>
</p>
<p><br>
</p>
<p>The largest contributors are the chip with worst below 1% and the
cap with 2%. Metal film resistors <br>
</p>
<p>would add a mere 100-200ppm and are a good choice. </p>
<br>
<br>
<br>
Best,<br>
<br>
René<br>
<br>
<br>
--<br>
<a class="moz-txt-link-abbreviated" href="mailto:synth@schmitzbits.de">synth@schmitzbits.de</a><br>
<a class="moz-txt-link-freetext" href="http://schmitzbits.de">http://schmitzbits.de</a><br>
<br>
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