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.