help - 3-inverter CMOS oscillator frequency

[MR COFFEE]

Can anyone here tell me what frequency the standard 3-inverter HC-CMOS oscillator runs at with a 10K resistor and 120 pf capacitor (1K on the input to the 2-inverter-in-a-row section)?

By standard I mean a string of 3 inverters in series, with a 10K resistor coming off the output, and the other end of the 10K goes to a junction with the 120 pf cap which goes back to the input of the last inverter, and the junction of the 10K and 120 pf goes to a 1K resistor which feeds back to the first inverter in the string (which is two inverters in series).

I get so screwed up trying to figure out all the decimal places I don't trust my answer.

Thanks!

[MR COFFEE]

FWIW,
I know it's "6", but I don't know if it's 600kHz, 60 kHz, or 6 MHz. So were talking orders of magnitude, not calculations.  Somebody here with digital chops probaby knows off the top of their head, right?

[Paul Perry (Frostwave)]

We should run a sweep on this! My entry: 600Khz.

[gez]

I'm confused by your post.  Could you link to a schematic?

[MR COFFEE]

Hi Paul,
Thanks for reply. I'm not sure, but it's seems likely to me.
Yours is a guess, too? It's used to clock a Sigma-Delta modulator for audio.

Hi Gez,
Thanks for reply, too.
Here's a link to the Application Note that has formulae and all.
Schematic is on page 2.

http://www.fairchildsemi.com/an/AN/AN-118.pdf

[gez]

New one on me, but according to the formula given I make it 571kHz.

[Paul Perry (Frostwave)]

That application note brings back memories! I suspect it might have been written by Jim Williams while he was at National Semi. The following certainly sounds like him:
"A popular oscillator is shown in fig.5a. The only undesirable feature of this oscillator is that it may not oscillate." They don't write app notes like that anymore.....

http://www.oricomtech.com/misc/teklink1.htm
For anyone without enough links.

[MR COFFEE]

Thanks guys. That's 3 of us thinking 600 khz (571 Khz per Gez with all the decimal places). I think I can trust my answer now.

Those old app notes really came with a bit of personality, didn't they? Alot more fun to read ithan today's terse pidgeon English

Anyone else remember grinning over the old National app note (or was it on the application part of the data sheet?) for the "damnfast buffer" with "Achtung!" in big letters at the top? I knew so little German I had to look it up

Thanks again for the assist.

[PharaohAmps]

Anyone else remember grinning over the old National app note (or was it on the application part of the data sheet?) for the "damnfast buffer" with "Achtung!" in big letters at the top? I knew so little German I had to look it up

Haha!  Yeah, I know that one - a bit of googling turned it up:

http://groupdiy.twin-x.com/displayimage.php?album=2&pos=1

Funny.  But look at the slew rate on that thing - 6000V / us !!!!!

That IS damnfast!

Matt Farrow