# RC Oscillators: relationship of frequency to R, C

#### 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$$\sqrt{6}$$)

#### 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.