Feedback resistor in CMOS Xtal oscillator.
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From: Robert Baer on 15 Apr 2010 01:46 Jim Thompson wrote: > On Wed, 14 Apr 2010 17:22:58 -0700, John Larkin > <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 15 Apr 2010 02:24:36 +0530, "pimpom" <pimpom(a)invalid.invalid> >> wrote: >> >>> I'm experimenting with a 32.768kHz oscillator using a watch >>> crystal (no specs) and a CMOS inverter. The current limiting >>> resistor is 330k and the caps are both 15pF. It oscillates with >>> 4.7-10Meg feedback resistor but not with 3.3M. I increased the Rf >>> in steps from 3.3M and it reluctantly starts up with 3.8M, taking >>> about 2 seconds to build up to full amplitude. Do you think 4.7M >>> will be enough to ensure oscillation with different samples of >>> the same type of crystal and under different environmental >>> conditions? The power supply is regulated. >>> >>> (I have both practical considerations and an academic interest >>> for wanting to know if 4.7M is enough). >>> >> 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 ....how many Lanes?
From: pimpom on 15 Apr 2010 06:48 Robert Baer wrote: > pimpom wrote: >> I'm experimenting with a 32.768kHz oscillator using a watch >> crystal (no specs) and a CMOS inverter. The current limiting >> resistor is 330k and the caps are both 15pF. It oscillates >> with >> 4.7-10Meg feedback resistor but not with 3.3M. I increased the >> Rf >> in steps from 3.3M and it reluctantly starts up with 3.8M, >> taking >> about 2 seconds to build up to full amplitude. Do you think >> 4.7M >> will be enough to ensure oscillation with different samples of >> the same type of crystal and under different environmental >> conditions? The power supply is regulated. >> >> (I have both practical considerations and an academic interest >> for wanting to know if 4.7M is enough). >> >> > 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. 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.
From: John Larkin on 15 Apr 2010 10:15 On Wed, 14 Apr 2010 22:46:16 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >Jim Thompson wrote: >> On Wed, 14 Apr 2010 17:22:58 -0700, John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Thu, 15 Apr 2010 02:24:36 +0530, "pimpom" <pimpom(a)invalid.invalid> >>> wrote: >>> >>>> I'm experimenting with a 32.768kHz oscillator using a watch >>>> crystal (no specs) and a CMOS inverter. The current limiting >>>> resistor is 330k and the caps are both 15pF. It oscillates with >>>> 4.7-10Meg feedback resistor but not with 3.3M. I increased the Rf >>>> in steps from 3.3M and it reluctantly starts up with 3.8M, taking >>>> about 2 seconds to build up to full amplitude. Do you think 4.7M >>>> will be enough to ensure oscillation with different samples of >>>> the same type of crystal and under different environmental >>>> conditions? The power supply is regulated. >>>> >>>> (I have both practical considerations and an academic interest >>>> for wanting to know if 4.7M is enough). >>>> >>> 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 >...how many Lanes? 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
From: John Larkin on 15 Apr 2010 10:16 On Wed, 14 Apr 2010 22:45:04 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >pimpom wrote: >> I'm experimenting with a 32.768kHz oscillator using a watch >> crystal (no specs) and a CMOS inverter. The current limiting >> resistor is 330k and the caps are both 15pF. It oscillates with >> 4.7-10Meg feedback resistor but not with 3.3M. I increased the Rf >> in steps from 3.3M and it reluctantly starts up with 3.8M, taking >> about 2 seconds to build up to full amplitude. Do you think 4.7M >> will be enough to ensure oscillation with different samples of >> the same type of crystal and under different environmental >> conditions? The power supply is regulated. >> >> (I have both practical considerations and an academic interest >> for wanting to know if 4.7M is enough). >> >> > 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. 3:1 up from where it quits. Bingo. John
From: Jim Thompson on 15 Apr 2010 13:23
On Thu, 15 Apr 2010 07:15:06 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 14 Apr 2010 22:46:16 -0700, Robert Baer ><robertbaer(a)localnet.com> wrote: > >>Jim Thompson wrote: >>> On Wed, 14 Apr 2010 17:22:58 -0700, John Larkin >>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> On Thu, 15 Apr 2010 02:24:36 +0530, "pimpom" <pimpom(a)invalid.invalid> >>>> wrote: >>>> >>>>> I'm experimenting with a 32.768kHz oscillator using a watch >>>>> crystal (no specs) and a CMOS inverter. The current limiting >>>>> resistor is 330k and the caps are both 15pF. It oscillates with >>>>> 4.7-10Meg feedback resistor but not with 3.3M. I increased the Rf >>>>> in steps from 3.3M and it reluctantly starts up with 3.8M, taking >>>>> about 2 seconds to build up to full amplitude. Do you think 4.7M >>>>> will be enough to ensure oscillation with different samples of >>>>> the same type of crystal and under different environmental >>>>> conditions? The power supply is regulated. >>>>> >>>>> (I have both practical considerations and an academic interest >>>>> for wanting to know if 4.7M is enough). >>>>> >>>> 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 >>...how many Lanes? > > >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 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 -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy |