Optimum RC values for crystal oscillator?
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Posted by Bill Bowden on May 9, 2008, 9:51 pm
Please log in for more thread options inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large crystal package that measures about 3/4 by 3/8 inch from the 1970s. The first inverter feeds the second, and the output of the second connects the crystal back to the input of the first inverter. The first inverter has a 100K resistor connected from input to output and there is a 470pF cap from the input to ground. Works well, but I'm wondering what the best RC values should be? I was thinking the reactance of the cap should be about equal to the resistor, but that works out to about 50pF which doesn't work. Lots of different combinations will work, such as 22K and 1000pF. How does one determine the best values? -Bill | |||||||||||||
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Posted by Andrew Holme on May 10, 2008, 5:38 am
Please log in for more thread options The resistor controls loop gain. I would find the lowest value for which oscillation starts reliably and multiply it by 10. The loading capacitance will affect frequency. Since you are using a crystal, presumably you care about frequency (if not, you could use a simple RC oscillator) so choose the capacitor which gets you closest to 31.500 KHz. I was recently playing with miniature 32.768 KHz tuning-fork watch crystals, which require extremely low drive levels; but I presume your large 1970s crystal is not of this type. | |||||||||||||
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Posted by Bill Bowden on May 10, 2008, 10:16 pm
Please log in for more thread options >
> > > > >I made a low frequency crystal oscillator using a couple CMOS
> > inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large > > crystal package that measures about 3/4 by 3/8 inch from the 1970s. >
> > The first inverter feeds the second, and the output of the second
> > connects the crystal back to the input of the first inverter. The > > first inverter has a 100K resistor connected from input to output and > > there is a 470pF cap from the input to ground. Works well, but I'm > > wondering what the best RC values should be? I was thinking the > > reactance of the cap should be about equal to the resistor, but that > > works out to about 50pF which doesn't work. Lots of different > > combinations will work, such as 22K and 1000pF. >
> > How does one determine the best values?
>
> > -Bill
>
> The resistor controls loop gain. I would find the lowest value for which > oscillation starts reliably and multiply it by 10. > > The loading capacitance will affect frequency. Since you are using a > crystal, presumably you care about frequency (if not, you could use a simple > RC oscillator) so choose the capacitor which gets you closest to 31.500 KHz. > > I was recently playing with miniature 32.768 KHz tuning-fork watch crystals, > which require extremely low drive levels; but I presume your large 1970s > crystal is not of this type. Thanks for the reply. Yes, the little 32768 tuning fork watch crystals work well with little drive. I'm just playing around with this old "heavy duty" crystal, I really don't need to use. My understanding of crystals is they look like a high Q parallel LC circuit at resonance, and to excite them, they just need feedback at 180 degrees. But that doesn't seem to work, and some phase shift is needed for reliable operation, and so the addition of the RC components to shift the phase slightly so the feedback is not exactly 180 degrees. Using 100K and 470pF yields a frequency of 31493 which is only a 7 cycle error out of 31500. -Bill | |||||||||||||
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Posted by John Fields on May 11, 2008, 8:51 am
Please log in for more thread options On Sat, 10 May 2008 19:16:49 -0700 (PDT), Bill Bowden
>>
>> >> >> >> >I made a low frequency crystal oscillator using a couple CMOS
>> > inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large >> > crystal package that measures about 3/4 by 3/8 inch from the 1970s. >>
>> > The first inverter feeds the second, and the output of the second
>> > connects the crystal back to the input of the first inverter. The >> > first inverter has a 100K resistor connected from input to output and >> > there is a 470pF cap from the input to ground. Works well, but I'm >> > wondering what the best RC values should be? I was thinking the >> > reactance of the cap should be about equal to the resistor, but that >> > works out to about 50pF which doesn't work. Lots of different >> > combinations will work, such as 22K and 1000pF. >>
>> > How does one determine the best values?
>>
>> > -Bill
>>
>> The resistor controls loop gain. I would find the lowest value for which >> oscillation starts reliably and multiply it by 10. >> >> The loading capacitance will affect frequency. Since you are using a >> crystal, presumably you care about frequency (if not, you could use a simple >> RC oscillator) so choose the capacitor which gets you closest to 31.500 KHz. >> >> I was recently playing with miniature 32.768 KHz tuning-fork watch crystals, >> which require extremely low drive levels; but I presume your large 1970s >> crystal is not of this type. >
>Thanks for the reply. > >Yes, the little 32768 tuning fork watch crystals work well with little >drive. I'm just playing around with this old "heavy duty" crystal, I >really don't need to use. My understanding of crystals is they look >like a high Q parallel LC circuit at resonance, and to excite them, >they just need feedback at 180 degrees. But that doesn't seem to work, >and some phase shift is needed for reliable operation, and so the >addition of the RC components to shift the phase slightly so the >feedback is not exactly 180 degrees. > >Using 100K and 470pF yields a frequency of 31493 which is only a 7 >cycle error out of 31500. --- Crystals can look like either series or parallel resonant circuits, depending on how they're driven. Here's a pretty good explanation: http://download.intel.com/design/mcs51/applnots/23065901.pdf JF | |||||||||||||
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Posted by Bill Bowden on May 12, 2008, 9:03 pm
Please log in for more thread options > On Sat, 10 May 2008 19:16:49 -0700 (PDT), Bill Bowden
> > > > > > >> >I made a low frequency crystal oscillator using a couple CMOS
> >> > inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large > >> > crystal package that measures about 3/4 by 3/8 inch from the 1970s. >
> >> > The first inverter feeds the second, and the output of the second
> >> > connects the crystal back to the input of the first inverter. The > >> > first inverter has a 100K resistor connected from input to output and > >> > there is a 470pF cap from the input to ground. Works well, but I'm > >> > wondering what the best RC values should be? I was thinking the > >> > reactance of the cap should be about equal to the resistor, but that > >> > works out to about 50pF which doesn't work. Lots of different > >> > combinations will work, such as 22K and 1000pF. >
> >> > How does one determine the best values?
>
> >> > -Bill
>
> >> The resistor controls loop gain. I would find the lowest value for which
> >> oscillation starts reliably and multiply it by 10. >
> >> The loading capacitance will affect frequency. Since you are using a
> >> crystal, presumably you care about frequency (if not, you could use a simple > >> RC oscillator) so choose the capacitor which gets you closest to 31.500 KHz. >
> >> I was recently playing with miniature 32.768 KHz tuning-fork watch crystals,
> >> which require extremely low drive levels; but I presume your large 1970s > >> crystal is not of this type. >
> >Thanks for the reply.
>
> >Yes, the little 32768 tuning fork watch crystals work well with little
> >drive. I'm just playing around with this old "heavy duty" crystal, I > >really don't need to use. My understanding of crystals is they look > >like a high Q parallel LC circuit at resonance, and to excite them, > >they just need feedback at 180 degrees. But that doesn't seem to work, > >and some phase shift is needed for reliable operation, and so the > >addition of the RC components to shift the phase slightly so the > >feedback is not exactly 180 degrees. >
> >Using 100K and 470pF yields a frequency of 31493 which is only a 7
> >cycle error out of 31500. >
> --- > Crystals can look like either series or parallel resonant circuits, > depending on how they're driven. > > Here's a pretty good explanation: > > http://download.intel.com/design/mcs51/applnots/23065901.pdf > > JF Yes, it looks like a series resonant circuit needs 360 degree feedback (two inverters) and the parallel circuit with 180 degree feedback needs only one inverter with some bias. But, I broke my crystal when I swiped my hand across the table and the crystal went flying, hit the wall, and landed on the floor, and doesn't work anymore. So, I tried another crystal of larger size in the parallel configuration using just one inverter (parallel resonant) with good results. This thing is huge and measures 1.5 by 3/4 inch. It's 50KHz and works well using 47K and 470pF. I found I could adjust the duty cycle with the resistor value. -Bill | |||||||||||||
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> inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large
> crystal package that measures about 3/4 by 3/8 inch from the 1970s.
>
> The first inverter feeds the second, and the output of the second
> connects the crystal back to the input of the first inverter. The
> first inverter has a 100K resistor connected from input to output and
> there is a 470pF cap from the input to ground. Works well, but I'm
> wondering what the best RC values should be? I was thinking the
> reactance of the cap should be about equal to the resistor, but that
> works out to about 50pF which doesn't work. Lots of different
> combinations will work, such as 22K and 1000pF.
>
> How does one determine the best values?
>
> -Bill