EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Circlotron on September 04, 2014, 01:42:30 pm
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I use a 4MHz xtal on a micro for a board I make regularly. I used to use the taller HC49U style and a while back went to the shorter HC49S. Then just the other day I notice a difference in the oscillator signal level between the two. On a 5V supply the tall xtals gives about 2V p/p and the short ones about 1.5V p/p. The taller ones have a disc inside of about 7mm diameter IIRC so the short ones must be really tiny.
Is this difference in output to be expected, and are there other things to watch out for like how easy or quickly one or the other starts up?
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I've heard from ham radio guys building ladder filters that the HC49S tend to have lower Q. You ought to be able to see this parameter in the crystal datasheet.
Also see:
http://www.foxonline.com/techfaqs.html#a12 (http://www.foxonline.com/techfaqs.html#a12)
12. I have an existing design that uses a HC49U. What should I consider if I change to the smaller HC49S crystal, or an SMD crystal?
Drive Level - HC49U crystals are typically rated 1.0mW max, while the HC49S and most SMD crystals are rated at only 0.5mW or 0.1mW maximum. See question 11 for more information on exceeding the maximum drive level of a crystal.
Pullability - some applications require tuning the frequency of the oscillator, either by mechanically or electrically changing the value of load capacitance. The HC49S and surface mount crystals have less pullability than the HC49U. It is important to verify whether the HC49S or the surface mount crystal will have sufficient pull range for the application. Generally, an HC49S has one-half the pullability as an HC49U.
ESR - The Equivalent Series Resistance (ESR) is generally higher with the HC49S and surface mount crystals, and can cause a problem if the oscillator circuit does not have sufficient loop gain.