Feedback resistor in CMOS Xtal oscillator.

Scary. These oscillators are notoriously flakey. I'd worry if any part in the circuit didn't have a 3:1 margin in both directions. And the "no specs" crystal makes things worse.

John

Like any other shunt feedback circuit, too low a "feedback" resistor make the input impedance too low, make RF 10-20Meg like the book says and it'll be fine.

Larkin wouldn't know how to evaluate a crystal oscillator if you gave him a full-term course in the subject... I suspect too much spanking at Tulane Kindergarten :-) ...Jim Thompson

:-) Thanks for the replies, both of you. ATM I have nothing higher than 6.8Meg in stock, so I guess I'll just have to use two or more in series.

Quite a while ago, i posted a "semi-universal oscillator" which worked for "standard" crystals that take a "normal" drive, and "tuning fork" crystals that demand a rather low drive (level). As i vaguely remember it, the input-to-output resistor for the CMOS gate was rather high - maybe 10Megs and i did not try to fiddle with that, thinking that substantially lower values would make for excessive load on the crystal, creating a multiplicity of un-intended consequences (lower Q, frequency shift for starters). I would say to use 10Megs and not look back.

...how many Lanes?

OK. 10Meg it is (2x4.7M). The 6.8Ms I have are big - legacies from vacuum tube days, and I wouldn't want to squeeze them in unless I have to.

JT is getting senile, so eager to insult me that he's willing to not make sense to do it. He thinks because he went to MIT it makes him important somehow. It doesn't.

My wife went to BU and used to wait tables in Cambridge. All the waitresses made fun of the MIT geeks who buried their heads behind books are were afraid to make eye contact with girls. We didn't have problems like that at Tulane, and we tended to contact lots more than with eyes.

John

John

You aren't even very good at _attempting_ to be insulting.

AIRI all the BU girls used to parade naked before their open dorm room windows advertising their "wares" :-)

The frat houses along Memorial Drive used to feature "dark rooms" with telescopes ;-)

No, I wasn't a member.

I was already married and living in a house on Magazine Street near Stop & Shop :-)

I have trouble understanding why you, as a mediocre circuit designer, think you have to get personally insulting. You _don't_ know crystal oscillators all that well... while I am generally designing one, at chip-level, every few months.

Latest one was on that photo of the video processing chip I recently posted... carefully designed for minimum power drive at 27MHz.

But you are _really_ good at purchasing :-) ...Jim Thompson

I believe this putrid asshole just tried to insult my wife. I have not language to express my contempt.

John

A 0603 5% SMD is the smallest available but 1/4W are also available; both may be slightly smaller than what you are talking about..

Or, if the OP has only 1..2M resistors, use two in series and put a 1nF cap from the connection to GND.

Arie

Keep trying. You're making a bigger and bigger fool of yourself at every attempt.

BTW: At what "wait tables" restaurant? I know Cambridge pretty well, and ate out a lot... of course this was 1958-1962. ...Jim Thompson

At 32 kHz, 15 pF holds very little energy; you probably want more than that. RCA's old ICAN-6086 document suggests the use of 100 k resistor and 33 pF capacitor on the gate output/crystal input side.

A transistor might be just as easy to design in as a CMOS inverter. Resistors at the high range are no longer common items, you have to find a carbon-film supplier (most resistors in the catalog are metal film).