On Sun, 18 Oct 1998, Steve Baldwin wrote:

> You guys are starting to get way off target here. Removing the caps
> from your circuit doesn't make it series resonant. The resistor
> across the amplifier terminals is selected so that it has minimal
> effect on the crystal parameters while still biasing the amp. A
> ceramic resonator works in the same way as a crystal, has parallel &
> series resonant points, but all parameters are much looser than a
> quartz crystal.

I always thought a ceramic resonator works on the SAW principle (or
compression wave) using a disturbance structure to set the frequency.

- snip -

> To use a crystal at the series resonant point you must provide an
> amplifier with the 360 degree phase shift needed. The faster you go,
> the harder this gets because any time taken to get around the circuit
> equates to a phase shift.

This is correct, but it somehow works for me. It is possible that there is
enough phase shift in the oscillator and stray capacitances to allow
enough gain nevertheless. And the 1M5 resistor I usually use for this is
too low to allow parallel mode imho. I went down to 220K without stopping
the oscillation with this, at 5V, with PIC16C54XT and JW in XT mode. The
pi caps were connected when I went down to 220K.

BTW the other commonly used two-inverter oscillator (LS, HC, CMOS etc) is
a series oscillating one too. This is one of the more robust and
un-demanding simple oscillators that exist. I mean:

   R1
+-/\/\/\-+
|        |
|  |\    |  |\
+--+ O---+--+ O--+--O Out
|  |/       |/   |
|                |
+-|X|-/\/\/\--||-+

  Xtal  R2    C1

there is another variation with feedback on the second gate.

Peter