# MATH/S FREQUENCY help!

Discussion in 'The Projects Forum' started by tibbles, Jul 22, 2014.

1. ### tibbles Thread Starter Active Member

Jun 27, 2008
249
3
hi
im hoping to build this http://www.slottechforum.com/slotin...ltage Comparators/LM339/LM339 oscillators.pdf bottom left
unfortunately ive met my nemesis- math/s.

r12 and c1 what units to use?
1/f intermediate frequency or more likely 1/f ?
>> much more than?

if i assume say r12 =25k=25000 ohms
" " c1 = 1n =.000001 farads
= .025
1/.025 =40 hz - when i should be looking for a few thousand hz on these figures, according to a similar example.

the xtal is 2 MHZ, i'm using a high speed comparator.

hope someone can help

doug

2. ### Dodgydave Distinguished Member

Jun 22, 2012
4,786
708
R is in Ohms, C is in Farads, 1uF is 0.000001, 1nF is 0.000000001, 1pF is 0.000000000001. , work backwards so 1/F = RxC

So your values R 25k x 1nF=0.00025Sec =40Khz

2Mhz is a 10K resistor and 50pF cap.

Last edited: Jul 22, 2014
3. ### tibbles Thread Starter Active Member

Jun 27, 2008
249
3
thanks DODGYDAVE youve saved my life,i'm glad you did it with zeros rather than powers of ten, which i never could get my head round,after all its easy to cancel most of the zeros.
thanks again,
doug

Dodgydave likes this.
4. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
the powers of ten tell you how many zeros to cancel or add. 10 to the 6th, 1 with 6 zeros. one millionth, 1 times 10 to the -6, put the 1 in the sixth place to the right of the decimal point.

5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,716
792
Errr...

1 times 10 to the -6 (minus 6), you would put the 1 in the 6th place to the right of the decimal point.