> Correct again. It may also be worth remembering that the old > oscillator IC for the 8080 series micros had a tank circuit to ensure > the crystal started in the correct mode. > > Typically to have a crystal work in series mode will require a series > tank circuit (both C and L) to force it into series mode - especially > running in overtone mode. Not quite right. The tank circuit is to supress the fundamental (or rather, prevent the total circuit from providing the required phase shift at the fundamental resonance). That's unrelated to the series- parallel issue. When a crystal is operating at its series resonant point it has no reactive component and appears purely resistive. To make an oscillator with one, requires that all of the required 360 degree phase shift is provided by the amplifier. As the speed goes up, that becomes more difficult as any delay is a phase shift. Also, because the impedance curve is close to horizontal at the series resonant point, it only takes a small amount of capacitance to move the frequency around. This is good if you want it to be (tuning RTC crystals) and bad if you don't want it to be (flux residues, fingers, etc). When a crystal is operating at its parallel resonant point, it appears as a huge inductor, which together with the capacitors, provides 180 degrees of phase shift. Add another 180 degrees from your amplifier and away it goes. The tank in the 8080 circuit would allow you to use an overtone crystal which would have been cheaper. The fundamental frequency of a crystal is inversely proportional to its thickness so high frequency parts are harder to make. Steve. ====================================================== Steve Baldwin Electronic Product Design TLA Microsystems Ltd Microcontroller Specialists PO Box 15-680, New Lynn http://www.tla.co.nz Auckland, New Zealand ph +64 9 820-2221 email: steveb@tla.co.nz fax +64 9 820-1929 ====================================================== -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.