RE: Asteroids Deluxe 12.096MHz xtal

>I've also got a question about ordering crystals. I usually order my parts
>from Mouser and in their catalog, most of the crystals have two choices:
>series or 20pf. Which one should be used on Atari boards or does it really
>matter?

I'll take a shot at this, but I can quickly get out of my depth on this
analog stuff, so anyone else feel free to correct me if I'm wrong. ;-)

Crystals are usually specified as being either Series or Parallel resonant.
Some will also operate in fundamental mode, or an "overtone" mode (a
harmonic of the fundamental frequency).

Generally speaking, most crystals under 20MHz are probably fundamental mode.
  (The frequency they run at is the primary frequency that the cut in the
crystal achieves when current is passed through it.)

Series mode crystals generally do not require any additional capacitance to
oscillate at their given frequency (they act more like a resistor
electrically). Parallel mode crystals do require extra load capacitance to
oscillate-- the crystal acts more like an inductor in the crystal circuit.
(The load capacitance is that 10-20pF you see in the catalogs-- it's usually
a little cap hanging off each leg of the crystal when you look at the
schematic.)

In a overtone-mode (non-fundamental frequency) there's usually another
inductor in the crystal tank circuit that tunes the circuit off of the
fundamental (and usually just over or under the desired overtone). This
helps the resonance to start and selects the harmonic instead of the
principle frequency. Overtone crystals are usually what you find for
frequencies over 33MHz-- it can be tricky to design the tank circuit to go
with them. Oscillators are a good choice if you don't want to deal with it.
;-)

Anyway, the good news is that most old games use relatively low frequency
crystals, and for all intents and purposes you can probably get away with
using either a series or parallel resonant crystal in an old game-board
repair.

-Clay
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Received on Thu Jan 13 14:55:14 2000