Quartz oscillator

[Jerome]

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 ?

 

[John Fields]

Hello,

I'm working on a PCB on which I have to put a 17,72 (4fsc)
oscillator.

---
What's "17,72 (4fsc)"?
---

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)

---
You should use an unbuffered CMOS inverter:

http://focus.ti.com/lit/ds/symlink/sn74ahc1gu04.pdf
http://focus.ti.com/lit/ds/symlink/sn74auc1gu04.pdf
http://focus.ti.com/lit/ds/symlink/sn74lvc1gu04.pdf

 

[Tim Wescott]

---
What's "17,72 (4fsc)"?
---


---
You should use an unbuffered CMOS inverter:

http://focus.ti.com/lit/ds/symlink/sn74ahc1gu04.pdf
http://focus.ti.com/lit/ds/symlink/sn74auc1gu04.pdf
http://focus.ti.com/lit/ds/symlink/sn74lvc1gu04.pdf

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

 

[Randy Day]

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 ?

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".

 

[Jerome]

sorry, I ate the units, that is 17.72 MHz (4 times the freq of the
color sub-carrier for most PAL video signals)

You should use an unbuffered CMOS inverter:

http://focus.ti.com/lit/ds/symlink/.../focus.ti.com/lit/ds/symlink/sn74lvc1gu04.pdf

OK, thanks

 

[Joel Koltner]

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 ?

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.