# RC Oscillators: relationship of frequency to R, C

Discussion in 'Homework Help' started by jocavian, Oct 24, 2007.

1. ### jocavian Thread Starter New Member

Oct 24, 2007
2
0
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.

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
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$\sqrt{6}$)

3. ### jocavian Thread Starter New Member

Oct 24, 2007
2
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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

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
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*$\sqrt{6}$.

5. ### recca02 Senior Member

Apr 2, 2007
1,211
0
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.