Jacques said:
Hi,
Anyone can explain to me the difference in configuration require for a serie
or parallel crystal pierce oscillator. I wish to use a SB74LVC1404 chip to
build a oscillator using a 3.000Mhz crystal. The application sheet use a
parallel load crystal, but the only 3Mhz crystal I can found is serie. I did
try it, it look to work properly, but I am not sure it will work all the
time. In the past, I observe that using a serie crystal often result in
instable oscillator. So what can I do to make sure the oscillator will alway
start at proper frequency. I wish I can find a proper crystal, but I can not
at this point, and further more, the right frequency for me should be
1.5mhz.
Bye
Jacques
There are two issues here. One is the series/parallel distinction. All
crystals looks like a capacitor below its series resonance and also
above its parallel resonance. In the very narrow region between, it
looks like an inductor. So if you make an ordinary LC oscillator, and
replace the inductor with a crystal, you'll get a crystal oscillator
running at a frequency somewhere between the two resonances. (This
assumes that the capacitors are in the right range of values--20 to 50
pF usually.)
The frequency marked on a 'parallel resonant' crystal is the resonant
frequency of the tank circuit built from the crystal and the specified
capacitance. (Parallel resonant crystals always have a capacitance spec
as well as a frequency spec, for this reason.)
The second issue is startup. It's very possible for a poorly-designed
crystal oscillator not to start reliably, or to start up at the wrong
frequency. There are two classes of wrong frequencies: (a) overtones,
and (b) LC resonances.
Crystals, like guitar strings, have more than one resonance. Generally
the higher overtones are harder to use, because the parallel capacitance
of the electrodes in the crystal tends to swamp the inductive reactance
more and more at higher and higher overtones. So if you design your
oscillator so that it needs a decent amount of inductance, and doesn't
have too much gain, it'll usually work reliably at the fundamental.
But you have to really design it. Calculate how inductive your crystal
gets (minimum and maximum Q specs apply); choose the capacitor values
accordingly; and calculate the losses, so you can choose the right
amount of gain in the active device: enough to start reliably from zero
signal, but not much more than that.
If you don't do the design carefully, you're liable to find that your
gizmo can oscillate at many frequencies--a few crystal resonances, the
LC resonance of a poor layout, or the (much higher) frequency where the
propagation delay and phase shift due to the capacitive load add up to
180 degrees. (Low and stable gain is your friend.) Crystal oscillator
startup is one place where SPICE isn't that much help, so do the
algebra. If you don't know where to begin, find the crystal parameters
from the data sheet and post them. One of us will probably be able to help.
If you have a 3 MHz crystal, the easiest way to get 1.5 MHz is to use a
flipflop as a divide-by-2.
Cheers,
Phil Hobbs