Go Down

Topic: Building arduino on a breadbord (Read 709 times) previous topic - next topic

jonrobertd

I have everything I need except the 22pf caps, I have several 12, 15, and some various others from salvaging crystals. Can any other cap value work or does it have to be 22pf? Thanks.

I think 15 would be OK, there would be capacitance in the breadboard traces.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics


Grumpy_Mike

Two 12pF caps in parallel would give you 24pF if 15 did not work.

mrburnette

Quote
and some various others from salvaging crystals


Various crystals use different load capacitor values.  It is a function of the crystal cut

From: http://www.foxonline.com/technical-3031
Quote
Load capacitance is one of the most overlooked parameters when specifying crystals. In the popular Pierce oscillator circuit, which has a capacitor to ground on either side of the crystal, the load capacitance is equal to the series combination of the two capacitors plus Cstray. (Cstray is the sum of capacitances that are contributed to the circuit by the layout, board material, and the input and output impedance of the active device.)

A good rule of thumb for Cstray is 5pF. If one of the capacitors is replaced with a varactor, the frequency can be "pulled" by applying a tuning voltage on the varactor. This configuration can be the basis for a simple VCXO. If the tuning voltage is derived from a thermistor network, the frequency can be adjusted to null out the effects of temperature. This configuration is an approach for a TCXO. A combination of both features is known as a TCVCXO.

A pullability specification for an HC49U crystal used in a fundamental mode VCXO might have the following form:

CL = 20 to 45pF, pullability = -100ppm max, CL = 20 to 10pF, pullability = +100ppm min.

Smaller crystals have about half the pullability of the HC49U. The pullability of overtone crystals is reduced by 1/n2, where n is the overtone mode (i.e. 1, 3, 5, etc.).

SirNickity

If it's just a dev platform (not something that needs to work reliably or accurately), the cap selection is fairly tolerant.  I tend to use whichever two-digit-picofarad cap I happen to grab from my parts bin.  I've heard some people have been lucky enough for the XO to oscillate without a load cap at all (although I haven't tried it, and I wouldn't count on it.)

The result of incorrect cap sizing is an error in clock speed.  Usually something you can live with, although timing-sensitive things (like RS232) will potentially suffer.

Go Up