RC Oscillators: relationship of frequency to R, C

Thread Starter

jocavian

Joined Oct 24, 2007
2
My textbook asks: "In what type of oscillator will a change by a factor of ten in resistor values NOT result in a frequency change by a factor of ten? Choices: (a) Wien bridge; (b) Phase-shift; (c) Twin-T

My attempts at solving consisted of constructing each of these circuits on a breadboard and replacing the resistors and capacitors with different values of same. In each case, the frequency change was inversely proportional to the values of R and C.

All my research states the same theory.


Please provide guidance. I have no electronics background and am teaching myself via a self-paced course.
 

thingmaker3

Joined May 16, 2005
5,084
The book seems to have an error in it. All three of those circuits have frequency inversely proportional to R. In both the Wien-bridge and twin-T, f=1/(2\(\pi\)RC). In the phase-shift oscillator, f=1/(2\(\pi\)RC\(\sqrt{6}\))
 

Thread Starter

jocavian

Joined Oct 24, 2007
2
On 10/25 you responded to my ? (In what typ of oscillator will a change by a factor of 10 in resistor values not result in frequency change by a factor of 10 . .. Wein bridge, phase-shift or twin-t)

I wrote to the instructor asking if the textbook could be in error. they responded: "To answer this question, you need to examine the formula for frequency for each oscillator type. Each formula will contain a resistance term. Observe which of the formulas is similar and which is different. In addition, a change by a factor of 10 can be representedby multiplication 10 or by 1/10"

So, evidently, the correct answer would be the "phase shift". Just wanted to let you know since you were kind enough to respond to my inquiry.

Thanks again,
jocavian
 

thingmaker3

Joined May 16, 2005
5,084
I still think they're wrong. Changing R by any proportion will change the frequency by the inverse proportion. x*10 = x*10, regardless of whether x=R*1 or x=R*\(\sqrt{6}\).
 

recca02

Joined Apr 2, 2007
1,214
yeah i check that too, back then it seemed to me the book is at fault then too.
but i did not reply since i wasnt sure myself.
 
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