cdb wrote:
> ( in some circumstances ) be replaced with a 250uF/35v, so why not use 
> a 200MHz  xtal to run a 40MHz Pic? 
>
> Further confusion or query could ensue from properties of crystals 
> when used to derive lower frequencies or when using a crystal via it's 
> harmonic frequencies.
>
>
>   
In fact are there *ANY* real crystals much above 40MHz, or are they 
mechanical overtones (confusingly not integer related to the 
"fundamental" mode frequency in the sense that frequency multiplier of  
an electronic frequency, or selecting a harmonic of a non-perfect 
sinewave is)?

I think a 101.5MHz crystal I have will give approximately 1/5th 
frequency (but actually 1/5th) in a basic oscillator circuit rather that 
one designed for 5th overtone.

So crystals, being slivers of mechanically resonant "glass" in a 
feedback loop of an amplifier are very complicated in one sense compared 
to resistors and capacitors. Inductors also of course behave very 
strangely if printed foil for 100MHz and you stick 1000MHz in as they 
are then striplines rather than a spiral coil (model approx 80nH spiral 
track for a 100MHz VCO with 3 or 4 turns on FR4 0.8mm to see what I 
mean, no ground plane on rear, stick ground plane at PCB coil and it 
won't work. However a properly designed stripline filter usually is 
above a ground plane).

And even Ohms law with 12V car headlamp light bulbs instead of a good 
resistor gives a different answer for the resistance at 12V and 1.2V (An 
early HP oscillator used this PTC effect as negative feedback to improve 
purity of a signal generator, PTCs have been used for degauss coils and 
NTC should be on input of direct rectified SMPSU with significant 
capacitors).

Lots of things self evident to the experienced  can  be baffling  to  
the  newcomer, even if armed with datasheets, ohm's law and a calculator.


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist