Metal Clock Oscillator: under the hood - diyAudio
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Old 3rd January 2013, 08:26 PM   #1
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Default Metal Clock Oscillator: under the hood

Those ubiquitous box-like clock osc. devices ... you've all seen them...
Click the image to open in full size.
... but what's exactly INSIDE the box ... uh ... under the hood? Equiv. circuit?
Is it the crystal plus decoupling caps, a resistor, and some pwr-regulation components? Like here...
Click the image to open in full size.
For reference, we know what the SIMPLE crystal can looks like w/o its hood:
Click the image to open in full size.

FWIW ...
I searched the patent database for "TCXO" and came up with schemas like this ...
Click the image to open in full size.
Thx!
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Last edited by hollowman; 3rd January 2013 at 08:52 PM.
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Old 3rd January 2013, 10:55 PM   #2
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Buy a cheap one and cut it open, it's instructive.

You'll probably find a picogate configured as Pierce oscillator, a small crystal, maybe a very small supply decoupling cap also. That's about all.
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Old 3rd January 2013, 11:16 PM   #3
FoMoCo is offline FoMoCo  United States
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Generally it's a crystal (or resonator) and logic inverter and or buffer. Also includes the caps for the crystal.
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Old 4th January 2013, 12:00 PM   #4
oshifis is offline oshifis  Hungary
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I opened several oscillators with a jigsaw. All they were different. Some use a thick film transistor oscillator circuit, some use a 74U04 inverter, and some use a programmable divider. Also the output signal waveform varies: some have a pretty fair squarevawe, but some are more like triangle shaped...
I also used to "repair" an exotic frequency oscillator by carefully opening it, cleaning off some tiny metal debris with a fine brush, and soldering back the cup
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Old 4th January 2013, 12:54 PM   #5
DF96 is offline DF96  England
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What is inside will depend on things like the degree of temperature compensation (from 'none' to 'near perfect'). For example, the TCXO circuit in post 1 includes varicaps driven from a diode to provide temperature compensation. Note that good temperature stability is usually not required for audio purposes, as a plain quartz crystal is plenty good enough on its own. Low jitter is our requirement, and a bad temperature compensation circuit could make this worse by injecting supply rail noise (as that circuit does!).
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Old 4th January 2013, 04:18 PM   #6
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Just glancing at some of the selection parameters at DigiKey/Mouser/Farnell-Newark, I note:
Output: HCMOS, CMOS, SineWave, etc.
Type: TXCO, XO (std.), etc.

Yeah, I've got several of those in my bin -- and they ain't the el cheapo variety -- but they are now 5-6 yrs old (back in the days of CD and Uni players!) and my DIY priorities have changed (almost entirely for the better). I also feel that those $$ osc.'s (Tent, etc.) from yesteryear have mostly been superseded with new offerings from Crystek, Vanguard, Valpey, etc...
I have no idea how Type (XO, VCXO, TCXO, etc.), Frequency Stability (ppm), Output type, etc., all come together in some sort of optimum/synergistic way to result in, e.g., low jitter -- I guess that's what tweaking, o'scopes and DIY PSU's are for.
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Old 4th January 2013, 06:33 PM   #7
oshifis is offline oshifis  Hungary
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I just read somewhere that OCXOs use SC-cut crystals (in contrast to the most common AT-cut crystals), and that is for some reason good for low jitter... Unfortunately they are available only in big cans.
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