Feedback resistor in CMOS Xtal oscillator.
in [Design]
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From: pimpom on 14 Apr 2010 16:54 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).
From: John Larkin on 14 Apr 2010 20:22 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
From: Jim Thompson on 14 Apr 2010 20:29 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 -- | 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
From: pimpom on 15 Apr 2010 01:11 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 :-) > :-) 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.
From: Robert Baer on 15 Apr 2010 01:45
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. |