# Calculating RC oscillator frequency

I'm working through an old Popular Electronics article, trying to apply and learn some of the math behind the project. Math for the clock pulse generator frequency has me stumped; wondering if anyone can provide a hint or might know what I'm doing wrong.

According to the description, the clock pulse generator should generate a square wave with a period of 8.1 microseconds or 122.8 khz. It uses part of a 4049 CMOS hex inverter, and the project runs on 9VDC; this feeds a counter chain which provides other multiples of the clock. In the diagram below, C1 is 100pf, R1 is 100k, R2 is 15K, and R24 5K variable/pot. Datasheets and other online discussion points to the formula as freq = 1 / 2.2RC, which I find gives me about twice the frequency I'm expecting here (2.2 x 17.5k x (100 x 10^-12)?). Am I trying to apply the wrong formula? Thanks.

Edit: I have not built the project, just working on paper.

• What formula are you trying to apply? – user207421 May 24 '15 at 23:28
• Formula I tried: frequency = 1 / 2.2 x R x C – lrom May 25 '15 at 0:58

The resistance is probably still low in comparison with the output resistance of the 4049 (probably < 100$\Omega$), but the input capacitance of IC1A is 15pF typically, so there's another RC in there (charge and discharge) with the 100K which will cause it to oscillate much more slowly.