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10th September 2010, 11:30 AM  #1 
diyAudio Member

Resonant circuit design?
I wasn't sure which area this question belongs so I guess this one is as good as any...
When designing a capacitive resonant circuit using the formula f=1/2*pi*R*C does it matter about the relative impedances of the resistors and capacitors? I mean, as long as the combination arrives at the resonant frequency you choose, can you just put any combination of resistor and capacitor you want?
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10th September 2010, 11:51 AM  #2 
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There's a bit more to it than that... not least the practicalities and properties of real world components.
Look up "Q" with regard to resonance... it's a measure of the "goodness" of a resonant circuit and is affected by real world properties of components. To answer the question, yes, if the impedance of a cap deviates from the ideal then it affects the resonant circuit. This is particularly true of coils as well. The formula doesn't take these into consideration, it assumes ideal.
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10th September 2010, 12:45 PM  #3 
diyAudio Member
Join Date: May 2007

RC is not a resonant circuit, you need LC or an active circuit for resonance. The value given by the RC formula is a corner frequency, not a resonance.
The impedances will give the stated frequency whatever their individual values  but you have to realise that the resistance/capacitance which appears in the formula is not the value of a component which you have just soldered in, but the effective value after the loading of the rest of the circuit is considered. People often forget this. 
10th September 2010, 05:05 PM  #4 
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10th September 2010, 07:03 PM  #5 
diyAudio Member

Ok, I see that a picture is worth a thousand words. In the attachment, if I just change the capacitors by an order of magnitude, will I get approximately an order of magnitude change in the oscillation frequency?
This was taken from Linear Technology's app note AN67.
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10th September 2010, 08:16 PM  #6 
diyAudio Member
Join Date: May 2007

Yes, because they are already significantly larger than any likely circuit stray capacitances. However, if you dropped them by a factor of 100 then the frequency might not change by quite that much as a few hundred pF is starting to get near stray values.

10th September 2010, 08:19 PM  #7  
diyAudio Member
Join Date: Sep 2006

Quote:
If it's the latest LTCxxx or ADzzz, your chances are better. 

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