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Crystal Help

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bryan

Member
Looking to purchase a few crystals for some projects, one of which is a PIC project. While browsing through the digikey catalog there is specs for the crystals called "load capacitance". Specs for load capacitance for most is 12pf-32pf or series??

What does load capacitance mean?
 

richard.c

New Member
Put simply, the load capacitance values are those the crystal manufacturers say are required to run their crystals at the specified frequency.

You need those values to ensure you get precise timing on you projects.

You can use different cap values perhaps given in circuit diagrams, but this may not result in the exact frequency being given, although it may not be noticeable in many circuits.

Have a look at any Microchip Pic chip manual - section Oscillator Configurations for a good practical explanation .
A good example is a 32k xtal running as a real time clock, if you fit say, a 15pf instead of the recommended 27pf you can find the clock will be out by several seconds per day.

HTH
 

bryan

Member
Hiya Richard:

Thanks for the help. So there should be no problems substituting the caps in your temp controller schematic with the recommended caps from the mfg.

But what do the mfg mean when they specify series in the "load capacitance".

Cheers

-=Bryan=-
 
Last edited:

richard.c

New Member
Hi Bryan - small world,

Theoretically using the manufacturers recommended values should work, although in practice you may still find you have to change them to get very accurate timing due to the effects of the pcb and microchip using the xtal. (22 to 33 pf will still work fine)
Also will the xtals you get actually come from the same manufacturer as the datasheet show - not always in my experience.

Series Capacitance - have a look at, for I better description than I can give. http://www.giangrandi.ch/electronics/crystalfilters/xtaltest.html

cheers,

Richard
 

bryan

Member
Hi Richard:

I let my fingers do some googling and came up with this

Series vs. Parallel:

<<<Parallel resonant crystals are intended for circuits which contain reactive components (capacitors) in the oscillator feedback loop. These circuits depend on the reactive components and the crystal to achieve the phase shift needed to start and maintain ocsillation at a specified frequency. Series resonant crystals are intended for circuits that contain no reactive components in the oscillator feedback loop. >>>

Another little project I wanted to tackle was a basic frequency counter using a pic. It uses a crystal with a cap on one leg and a variable trimmer cap on the other leg. Trimmer cap is to adjust for accuracy.

Cheers
-=Bryan=-
 
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