Crystal Oscillator

[mrydhan]

I have the following crystal oscillator circuit. It does not oscillate. Any help?

------/\/\/\------------
| 10 M |
| |
| ---------- |
| | | |
|----| 74LVC04|----- |
| |-------- | /
| | / 330K
| | |
|--------|[]|----------|
| 32.768KHZ XTAL |
--- ---
--- ---
| |
| |
GND GND

 

[Lewish]

What are the values of your 2 capacitors?

What is the logic high output voltage of that gate?

 

[melone]

Why do you have the 330K in there? You are loading the crystal in a non-sysmetrical fashion.

 

[melone]

Are you trying to get your inverter (7404) to oscillate at 32.768kHz? If so, this circuit will not work. Crystals will not drive the inputs to a CMOS gate, and require special input (and output) circuitry. Try looking up some crystal oscillator circuits on the web.

If this is not your intention, please give a description of what you are trying to do with this circuit.

 

[mrydhan]

The 2 capacitors are 22 pf.
This circuit is per recommended by crystal vendor ECS at:

http://www.ecsxtal.com/pdf/ecx-3ta.pdf

The difference is the inverter. They specified TC4069 versus in my circuit it is 74LVC04.

 

[melone]

1) Try taking out the 330K.

2) Try a differenct crystal (you might have fractured it).

3) Place a scope on it and monitor it from power up (see if it even tries to oscilate).

4) Try the TC4069. Perhaps it has different input/output capacitance, drive strength, speed, etc.

 

[IRstuff]

You don't show what pins are connected on the '04. Are you sure that part is wired correctly? TTFN

 

[Lewish]

These configuration oscillators are notoriously hard to get to work. They are very sensitive to the parasitic capacitance of the layout. Also, the output capacitance of the logic gate is important, or it may cause enough phase shift to prevent oscillation.
The 74LVC04 has an output of only about 2.0 volts when Vdd is 3.0, whereas the TC4069 has an output of 2.95 volts. This may be enough gain difference to prevent oscillation.
To oscillate, a circuit must have a gain of more than unity, and a phase shift of 0 degrees. But to guarantee that it will start oscillating, the circuit must have a gain of about 3.

 

[felixc]

A couple of things:
- the 330K is there to limit the loop gain. It is usually tuned to the type of inverter and the series resonance of the crystal that you use. You will have to tune yours, but as it does not oscillate now, take it off for now.
- the bandwidth of the loop has to match somehow the frequency at which the crystal works. Your gate can drive up to 100MHz. It sure is a problem! If the bandwidth capability of the inverter is high, the probabilities are high that if it oscillates it will happen in an overtone mode. Big potential for problems there.
- many of the recent CMOS families have a little intrinsic schmitt-trigger behaviour. That may just prevent the oscillator loop from starting, as the background noise is what starts the crystal.
- the suggested old CMOS 4000 series is normally the way to drive an oscillator at such a low frequency, as its bandwidth capability is low. You may want to try a 74HC too.
- many designs like this also add an L-C tank in series with the crystal, in order to prevent the crytal to oscillate in overtone modes.
- you may have to try slightly different capacitor values. My oscillator uses 15pF and 33pF, after I saw that it started oscillating after I touched it with a scope probe.

Hope this helps!

 

[mrydhan]

I am going to try TC4069. I am waiting for the samples.
Thanks.

 

[melone]

Can you post the layout so we can take a look?

 

[dumbo]

power the IC.
10M across first gate only.
10 pF on the 330 K side 22 pF on the other.

 

[jbartos]

Suggestion: Reference:
1. John Markus "Electronic Circuit Manual," McGraw-Hill Book Co.

 

[jbartos]

Sorry, a small correction: "Electronic Circuits Manual"

 

[mrydhan]

I have ordered the book.
Thanks for the recommendation.
I am still waiting for the samples on TC4069.

 

[rodneyknaap]

Hello,
I know it is late since 2003, but for those who ended up here recently by a search engine or by browsing the messages: by coincidence I am also trying to use the same IC for an oscillator, coincidentally also for 32.768 Khz. I am having success on a breadboard(with an smd to dil adapter) with only an input capacitor of 47pF. Also with a 10M and 330k resistor. before I used a 2k2 instead of the 330k and got no oscillation at all. Now with the 330k as in this article I do get oscillation. One note, at power on, it takes about 2 seconds for the oscillator to achieve full amplitude. For my design this is acceptable. It seems reasonably stable as long as I don't touch the components. Granted, yes, a 40106 for example is more suitable for this frequency, however I alreade implemented a 74LVC04 for my design because the other gates need to be fast. In this way, I can save an IC in the design. Finally, when I include the capacitor on the right, the signal is less smooth so I left it out.

 

[rodneyknaap]

one small correction, I used a 47pF as in the artical above, on the right, and left out the one on the left