Hello,
I'm working on a PCB on which I have to put a 17,72 (4fsc) oscillator. I have one unused schmitt trigger inverting gate. Can it be used with a crystal for making an oscillator (in order to keep it cheap and compact), or do I have to use a "regular" gate ?
-- Jerome
-- What\'s "17,72 (4fsc)"?
fsc = European TV color burst?
If this is the case, then an oscillator based on an unbuffered inverter may not be nearly accurate enough -- or it may be just fine. The broadcast spec is very very tight (down in the ppm), pro video equipment such as cameras and recorders have tight requirements (the VXCOs that are commonly used for this have +/-0.25% ranges), home VCRs and such have moderately tight requirements, and monitors care little -- but they still have VXCOs that must be locked, so you can't stray too far from their range.
I won't say that one _can't_ get good enough precision using an unbuffered gate if you're really going to use that for a colorburst signal, but if the OP has to ask, he may find it a challenge.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
If it's an RF or video application, I'd avoid it. A company I worked for spec'd a Schmitt by mistake in a clock circuit. Everything functioned, but jitter in the clock showed up as spurious noise all across the spectrum. The techs trying to troubleshoot the things were almost in open revolt before it was dealt with.
Maybe you can get away with a single 7404-type inverter in a SOT-353 package? They're small & cheap. Try a Digikey search on "sot 353".
sorry, I ate the units, that is 17.72 MHz (4 times the freq of the color sub-carrier for most PAL video signals)
OK, thanks
-- Jerome
A regular gate will work much better in terms of generating a "nice" (low harmonics and phase noise) output. You might use a single-gate IC for the inverter, or you could just use a regular old transistor... the later being cheaper, the former probably not requiring as many external components for biasing, etc.
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