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Another ? from a dumb RF newbie

Is it possible to "burn out" a crystal by hooking it up incorrectly? Does it matter which way you hook it up?

I am attempting to build the 10 MHz crystal oscillator displayed on p. 246 of Joe Carr's Secrets of RF Circuit Design, and must have done something wrong. My 100 MHz scope and my DMM give me nothing but anomylous readings which I can't make any sense out of. Thus my question, above. Decided to consult those more in-the-know than myself, before I take out the crystal and install the spare I bought.

If desired, I think I can probably figure out how to upload the schematic to alt.binaries.schematics.electronic, if anyone wants to see it. It's a simple colpitts crystal oscillator making use of six disc capacitors and a couple of resistors, plus, of course, the crystal and a 2N3904. I am using a LM7812 to drive it, with the heatsink screwed down tight to the metal enclosure, and a 12V wall-wart feeding that (it puts out 18V, and my circuit voltage is +11.91 volts. The scematic says anything between 9 and 12 volts should work.) I did have to make a couple of corrections to misconfigured capacitors, but that is all. The only trace I can get that looks like any kind of sine-wave is approx 10 mV at maybe 1.5 MHz.

Any feedback on proper use and installation of crystals is welcomed. I'm about to scrap the whole thing and start over.

Thanks,

Dave

Reply to
Dave
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It is possible to kill a crystal with too much RF current, but that seems unlikely with a 2N3904 circuit. I'm just guessing, but I suppose the 1.5MHz you're seeing could be from a local AM broadcast station. It's possible that you have a bad crystal; I've bought some that had "low activity" and didn't want to oscillate as well as a different batch. Do you have any way to make sure that the capacitor values are correct?

What is the current in the 2N3904? I would expect it would be happy to oscillate with anywhere between about one and ten milliamps of current in the collector.

Cheers, Tom

Reply to
Tom Bruhns

Dave first check the DC voltages and currents.

If they are all correct then check the caps for silly errors like pf vs uF or other wiring errors.

Mark

Reply to
Mark

Check your caps carefully. If you are doing this on a solderless breadboard, remember the parasitic capacitance of the board (~10p probably depending on the board). If you have too much load on the crystal the oscillator won't start or it won't oscillate at the marked frequency.

--
Ben Jackson AD7GD

http://www.ben.com/
Reply to
Ben Jackson

Thanks, all, for the replies. Unfortunately I have stared at this stupid thing until I no longer trust myself to see straight. Setting it aside for a day or two, then will probably disassemble and rebuild. Your comments are much appreciated however, and I take them to heart.

I'll be back in a day or two if I still can't get it working. Right now I have stared at it so long I can no longer see it. Instead I see the image in my mind, which is hopelessly tangled.

Thank you though, for your time and the consideration of replying.

Dave

Reply to
Dave

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