Feedback resistor in CMOS Xtal oscillator.
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From: Robert Baer on 16 Apr 2010 04:17 pimpom wrote: > 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. > > 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..
From: Arie de Muynck on 16 Apr 2010 09:04 "Robert Baer" <robertbaer(a)localnet.com> wrote in message news:-I2dnTB9-q3VNVvWnZ2dnUVZ_jOdnZ2d(a)posted.localnet... > 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. Or, if the OP has only 1..2M resistors, use two in series and put a 1nF cap from the connection to GND. Arie
From: whit3rd on 16 Apr 2010 14:36
On Apr 14, 1:54 pm, "pimpom" <pim...(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. 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). |