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> Not Gate Crystal Oscillator, not working
atom
Posted: February 23, 2012 05:56 pm
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I'm trying to get this circuit to work with this 38kHz crystal and a CD4069. The datasheet states 11pF load capacitance, so I used 22pF caps to ground at either end of the crystal.

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Whatever I try, I can't get it to oscillate properly. I only get random oscillations at around 200kHz, rapidly changing in frequency. The crystal is a tiny low drive type, so I initially used a 220k series resistor. When that didn't work I tried 22k and 2k2, but neither made it oscillate.
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CWB
Posted: February 24, 2012 12:39 am
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these might help :
http://www.dieelektronikerseite.de/Datashe...8Philips%29.pdf
http://www.seekic.com/circuit_diagram/Sign...cuitCD4069.html

have you used decoupling caps (say .01/.001) on the power and ground leads very close to the IC ?

as shown in the second link , you may have to tweak the value of a cap to get the thing to whistle reliably .
a small trimmer cap is about the easiest to use .


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Village Idiot
Posted: February 24, 2012 03:05 am
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These tiny tuning fork crystals require extremely low current levels that don't work well with CMOS inverter type oscillators. Currents in the low microamp range are difficult to keep stable in a CMOS inverter.

Even when the circuit does work, the crystals have super high Q, and can take several seconds to start oscillating after the power is applied. So you may have to wait a while to make sure it's not just slow starting.

I had a lot of problems trying to get these things to oscillate with a CMOS invertor and eventually went looking for a different circuit. It was quite a while ago, but I think this is the circuit I fould that works well:

http://www.discovercircuits.com/DJ-Circuit...wpwrxtlosc1.htm

Even with this circuit, I recall that the oscillator sometimes took about 30 seconds to start oscillating.

Also be aware that these crystals can easily be destroyed if you overdrive them. So, it's possible that when you reduced the series resistor below 220k you may have damaged it.
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atom
Posted: February 24, 2012 08:48 am
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@CWB - thanks for the links. I've seen the datasheet already but hadn't thought of using another NOT gate as a linear amp. Maybe the circuit is oscillating but the output's too weak to be picked up by my cheap frequency counter (no oscilloscope unfortunately), so this might help.

QUOTE
Even when the circuit does work, the crystals have super high Q, and can take several seconds to start oscillating after the power is applied. So you may have to wait a while to make sure it's not just slow starting.

Funny you should mention that - when I first power it up I actually get no output at all. Only after a few seconds it starts oscillating, but on the wrong frequency.

QUOTE
Also be aware that these crystals can easily be destroyed if you overdrive them. So, it's possible that when you reduced the series resistor below 220k you may have damaged it.

Yeah that was what I was worried about. I'd much rather use a normal sized crystal but 38kHz crystals must be really rare - this was the only one I could find for sale. I also looked for a 76kHz crystal to divide down with a flipflop (I'm already using one half of a 4013 to divide 38kHz down to 19kHz, with the other half unused), but I can't find one at all. My other idea was to use higher frequency crystal and divide down to 38kHz with a 4060, but I couldn't find any crystals at power of two multiples of 38kHz either. I'll try the circuit you linked to - if I can't get that to work then I'll probably have to go with an RC circuit or something.
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CWB
Posted: February 24, 2012 01:01 pm
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"Only after a few seconds it starts oscillating, but on the wrong frequency."
the circuit is female .


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Geek
Posted: February 24, 2012 02:46 pm
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QUOTE (CWB @ February 24, 2012 05:01 am)
"Only after a few seconds it starts oscillating, but on the wrong frequency."
the circuit is female .

laugh.gif laugh.gif

Ever open a tuning fork "crystal"? They ain't crystals!

There's dedicated chips for them suckers, but that gate oscillator thingy won't work with them dunno.gif

Cheers!


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draget
Posted: February 24, 2012 02:49 pm
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I've used those with a 74HC14 just fine. Use the extra sections to clean up the waveform.
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Geek
Posted: February 24, 2012 03:24 pm
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Ooooh, HC... that might work smile.gif

Cheers!


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Colt45
Posted: February 25, 2012 10:17 am
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I have a vague recollection that you need the CD4069UB (Unbuffered), if you want it to work with a 4069.


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zonks
Posted: March 01, 2012 07:22 pm
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Agreed, and I've frequently seen the 74HCU04 used for this purpose too.
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