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RE : [Electronics_101] Crystals and Capacitance - confused by the specs...

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  • Robert Hedan
    Two crystals? I only have one crystal on my microprocessors. I do have 2 capacitors connected to the crystal though, but they are in parallel, not series.
    Message 1 of 11 , Feb 1, 2006
      Two crystals? I only have one crystal on my microprocessors.

      I do have 2 capacitors connected to the crystal though, but they are in
      parallel, not series.

      I've also started adding a 1 Mohm 1/4 W resistor across the crystal too.
      Don't ask me why, 'cause someone that knows more than me said he does and
      that was good enough for me. I haven't had a problem with my MCUs either,
      well none except when I short-circuited a few 16F877. POOF!!!
      sssssssss......

      I always used 22pF ceramic capacitors on my 20 MHz crystals that I got at
      surplus. Don't ask me what specs they have, I never bothered to check up a
      datasheet. All the schematics I happened to find used 22pF for 20 MHz, I
      never argued.

      Robert
      :)



      -----Message d'origine-----
      De : Electronics_101@yahoogroups.com
      [mailto:Electronics_101@yahoogroups.com] De la part de Leon Heller
      Envoyé : février 1 2006 13:05
      À : Electronics_101@yahoogroups.com
      Objet : Re: [Electronics_101] Crystals and Capacitance - confused by the
      specs...


      It depends on the crystal load capacitance. The two crystals are effectively

      in series, and the load capacitance is often 20 pF, so a pair of 33pF
      capacitors would be about right, if stray capacitance is taken into account.

      18 pF will probably work, the oscillator will often work without any added
      capacitance.

      Leon



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    • Leon Heller
      ... From: Robert Hedan To: Sent: Wednesday, February 01, 2006 8:47 PM Subject: RE :
      Message 2 of 11 , Feb 1, 2006
        ----- Original Message -----
        From: "Robert Hedan" <robert.hedan@...>
        To: <Electronics_101@yahoogroups.com>
        Sent: Wednesday, February 01, 2006 8:47 PM
        Subject: RE : [Electronics_101] Crystals and Capacitance - confused by the
        specs...


        Two crystals? I only have one crystal on my microprocessors.

        I do have 2 capacitors connected to the crystal though, but they are in
        parallel, not series.

        I've also started adding a 1 Mohm 1/4 W resistor across the crystal too.
        Don't ask me why, 'cause someone that knows more than me said he does and
        that was good enough for me. I haven't had a problem with my MCUs either,
        well none except when I short-circuited a few 16F877. POOF!!!
        sssssssss......

        I always used 22pF ceramic capacitors on my 20 MHz crystals that I got at
        surplus. Don't ask me what specs they have, I never bothered to check up a
        datasheet. All the schematics I happened to find used 22pF for 20 MHz, I
        never argued.


        I meant capacitors, not crystals!

        If one end of the capacitors is grounded, they are in series - it's a Pierce
        oscillator. A resistor is sometimes required, it depends on the actual chip
        spec.

        Leon
      • Steve
        Do some web searches like: http://www.ecliptek.com/crystals/glossary.html You need two resistors, one (~1M) for input DC bias, and another (~100) in series
        Message 3 of 11 , Feb 21, 2006
          Do some web searches like:
          http://www.ecliptek.com/crystals/glossary.html

          You need two resistors, one (~1M) for input DC bias, and another
          (~100) in series with the output to limit the drive power. The caps
          and the crystal give you 180° phase shift, and a lot of voltage gain
          if driven too hard. You can pull the frequency by changing the caps a
          bit. Series vs parallel resonant frequencies are very close together,
          but you buy xtals spec'd at one or the other. Have a look at the Z
          graphs at the vendors web page.

          Steve



          --- In Electronics_101@yahoogroups.com, "lcdpublishing"
          <lcdpublishing@...> wrote:
          >
          > I ordered some Atmel chips along with some 20 Mhz crystals and some
          > small caps (18 pF I believe). Anyway, I remember looking at the
          > choices from the Digikey website while picking the crystal.
          >
          > One of the attributes was the capacitance of the crystal and I
          > rememebered selecting the same value as I was planning on ordering
          for
          > the Caps. As you all know by now, I really have no idea what I am
          > doing and didn't even think about this until late last night (I
          > ordered the parts last week).
          >
          > Anyway, while thinking about that, I got to thinking if perhaps by
          > luck I did the right thing. Is it normal (or proper) to select the
          > crystal with the capacitance rating in the way I did it?
          >
          > Chris
          >
        • Robert Hedan
          Message 4 of 11 , Feb 22, 2006
            <The rate of aging is typically greatest during the first 30 to 60 days
            after which time the aging rate decreases. The following factors effect
            crystal aging: adsorption and desorption of contamination on the surfaces of
            the quartz, stress relief of the mounting and bonding structures, material
            outgassing, and seal integrity.>

            Wow, so unusual. "The rate of aging is typically greatest during the first
            30 to 60 days after which time the aging rate decreases." It's the first
            time I see a part age 'less fast' with age.

            Robert
            :)


            -----Message d'origine-----
            De : Electronics_101@yahoogroups.com
            [mailto:Electronics_101@yahoogroups.com] De la part de Steve
            Envoyé : février 22 2006 01:04
            À : Electronics_101@yahoogroups.com
            Objet : [Electronics_101] Re: Crystals and Capacitance - confused by the
            specs...


            Do some web searches like: http://www.ecliptek.com/crystals/glossary.html

            You need two resistors, one (~1M) for input DC bias, and another
            (~100) in series with the output to limit the drive power. The caps
            and the crystal give you 180° phase shift, and a lot of voltage gain
            if driven too hard. You can pull the frequency by changing the caps a
            bit. Series vs parallel resonant frequencies are very close together,
            but you buy xtals spec'd at one or the other. Have a look at the Z
            graphs at the vendors web page.

            Steve





            Yahoo! Groups Links
          • Leon Heller
            ... From: Robert Hedan To: Sent: Wednesday, February 22, 2006 10:49 AM Subject:
            Message 5 of 11 , Feb 22, 2006
              ----- Original Message -----
              From: "Robert Hedan" <robert.hedan@...>
              To: <Electronics_101@yahoogroups.com>
              Sent: Wednesday, February 22, 2006 10:49 AM
              Subject: [Electronics_101] Re: Crystals and Capacitance - confused by the
              specs...


              <The rate of aging is typically greatest during the first 30 to 60 days
              after which time the aging rate decreases. The following factors effect
              crystal aging: adsorption and desorption of contamination on the surfaces of
              the quartz, stress relief of the mounting and bonding structures, material
              outgassing, and seal integrity.>

              Wow, so unusual. "The rate of aging is typically greatest during the first
              30 to 60 days after which time the aging rate decreases." It's the first
              time I see a part age 'less fast' with age.


              Not quite right! The *rate* of ageing is slowing, not the ageing. 8-)

              Leon
            • Leon Heller
              ... From: Leon Heller To: Sent: Wednesday, February 22, 2006 2:11 PM Subject: Re:
              Message 6 of 11 , Feb 22, 2006
                ----- Original Message -----
                From: "Leon Heller" <leon.heller@...>
                To: <Electronics_101@yahoogroups.com>
                Sent: Wednesday, February 22, 2006 2:11 PM
                Subject: Re: [Electronics_101] Re: Crystals and Capacitance - confused by
                the specs...


                > ----- Original Message -----
                > From: "Robert Hedan" <robert.hedan@...>
                > To: <Electronics_101@yahoogroups.com>
                > Sent: Wednesday, February 22, 2006 10:49 AM
                > Subject: [Electronics_101] Re: Crystals and Capacitance - confused by the
                > specs...
                >
                >
                > <The rate of aging is typically greatest during the first 30 to 60 days
                > after which time the aging rate decreases. The following factors effect
                > crystal aging: adsorption and desorption of contamination on the surfaces
                > of
                > the quartz, stress relief of the mounting and bonding structures, material
                > outgassing, and seal integrity.>
                >
                > Wow, so unusual. "The rate of aging is typically greatest during the
                > first
                > 30 to 60 days after which time the aging rate decreases." It's the first
                > time I see a part age 'less fast' with age.
                >
                >
                > Not quite right! The *rate* of ageing is slowing, not the ageing. 8-)

                Sorry, I see now that was what you actually said - "ageing 'less fast'".

                Leon
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