Peter L. Peres wrote:
>
> On Sun, 18 Oct 1998, Steve Baldwin wrote:
>
> > Me:
> > > I always thought a ceramic resonator works on the SAW principle (or
> > > compression wave) using a disturbance structure to set the frequency.
> >
> > As does a crystal. They are both mechanical resonance devices. The
> > RLC "components" are analogous (?) to the mechanical properties
> > damping, mass and compliance.
>
> ? I understand that a ceramic resonator works by channeling a wave through
> a (simple) structure. The wave channel properties and not the terminals
> determine the frequency. Thus there is no such thing as a (three terminal)
> ceramic resonator in series mode. Am I way off or are my books too old ?
> Of course there is a mechanical equivalent to it. I think that the ceramic
> resonator is modeled as a weakly coupled (on both sides) pendulum or such.
>
> > What seems to work for you ? Earlier you were talking of leaving off
> > the capacitors on a PIC oscillator. Now you refer to two-inverter
> > oscillators. One inverter = parallel mode, Two = series mode.
>
> What seems to work for me is:
> 1) PIC16C54 and NS16550A with 4MHz, resp. 8MHz, no caps, and a 1M5
> resistor in series with the crystal. The crystal was about 2.5 cm away
> from the chip in both cases.
> 2) Trials with R down to 220K on PIC16C54 with 4MHz, with specced pi caps,
> at which point (220K) it stopped starting reliably. 1M was the lowest R
> that made it work without caps at all. The PIC was a JW in XT mode every
> time.
>
> I guess, all you have to do, is try it out. Leave the caps out when
> populating the board on the next project and find out ;)
>
> > A typical crystal has an ESR at resonance of <100 ohms. The influence
> > of the feedback resistor is minimal once oscillation has started so
> > it can actually be quite low. Startup is a different situation and
> > the resistor needs to be high enough that the first signs of life
> > from the crystal will overdrive the signal from the resistor.
>
> I always thought that the device with the highest noise at the input of
> the oscillator starts the show (the MOS transistors probably). Anyway, an
> ESR or 100 ohms is an Equivalent >Series< Resistance. This, and the
> extremely high impedance of a parallel resonating crystal make me believe
> that putting the resistor there puts the crystal in series mode somehow.
>
> Peter
>
> PS: This thing is exclusively trial stuff. I did not produce any large
> numbers of anything using this method. BUT I have to add that older micros
> in equipment which develop crystal start-up problems seem to benefit from
 the same modification (without removing the original pi caps).