Crystal Oscillator problem

I encountered with very strange problem with a ceramic crystal oscillator. It took a long time to recreate the problem. When a power supply voltage is turned on first time there is no output frequency. When the power supply is turned off and on again the frequency appears. You can see it on the video here.

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Any suggestions? The vendor doesn't know why it happens. :(

Reply to
Mig
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Post the schematic somewhere. Too little gain or feedback.

Reply to
Don Bowey

Ceramic or crystal? They're different animals.

It could be an analog problem with the start-up (eg. load capacitors wrong or not enough gain, or biasing not present) or it could be something else entirely like your circuit (if it's some complex chip) going into sleep mode or whatever. Best show us the complete circuit-- and don't expect oscillators built on plug breadboards (if that's what you have) to work reliably.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Sorry my mistake. This is no a crystal. It is a ceramic. There is no schematic. The part comes with 4 pins, Power, GND, Enable and Frequency output. I'm just testing the bad part that malfunctioned on the board. I connected just a power and hooked the oscilloscope to the output. That the first time ever I have encountered with the problem with the ceramic oscillator!

Reply to
Mig

Desiging an oscillator that will start up reliably 100 percent of the time is far more difficult than designing an oscillator that works most of the time.

This is a pre-canned oscillator, right? Or wrong?

Most of the pre-canned oscillators are pretty good. But look carefully at the output load capacitance specs. If this is a canned oscillator designed to put out a digital train, and it's putting out a sine wave (what I see on the scope!), you must have a lot of capacitive load on the output.

If it's not a canned oscillator, it looks like your oscillator is not going into deep saturation after startup (that looks like a sine wave on the scope), so more gain would help with startup. (Not necessarily reliable startup! You will find with too much gain that you start mode- hopping or stick to some mode you didn't want to excite.)

Tim.

Reply to
Tim Shoppa

So , your repairing?

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Reply to
Jamie

Looked like it actually was encouraged to start as the first application of power was in the process of being removed.

Try delaying the enable signal. You don't indicate how it is being connected - assume currently a short to appropriate logic.

Check it's spec for minimum/maximum dv/dt on the supply pin. Check it's application supply voltage range.

RL

Reply to
legg

That is the geekiest video I ever saw on youtube. ;-)

I see that you are powering up a bench supply to start the module. Often these bench supplies have a soft start feature. Your module may not like the supply to be ramped slowly. Try leaving the supply on and just connecting the DC to the module, that is, hit it hard.

Reply to
miso

Startup problems are as common as dirt.

Most oscillators work in a large-signal regime where the active devices are nonlinear. Thus the average gain, impedance level, and feedback are all different when the oscillator is running, compared with the same circuit in a quiescent state.

The feedback loop may well be stable at small-signal conditions and only oscillatory at large signal conditions. If the power supply transient is quicker than the bias time constant of the oscillator, it'll find itself in a large-signal condition at power-up, which will get the oscillator going and mask the startup problem. It's really important to test oscillators under a wide variety of turn-on conditions, including very slow ramps.

It's also quite possible for oscillators to have too much feedback, in which case you get all sorts of distortion and self-modulation rather than a nice CW sine wave. ALC oscillators adjust their loop gain based on oscillation amplitude, and thus tend to avoid both kinds of problem (as well as being much quieter).

I gather that your power supply is producing different turn-on transients, depending on how long it has been switched off. From the symptoms, it appears that when it's been off long enough for the filter caps to have discharged, the turn-on is slower than when you turn it off and then on again, and the difference is enough to expose the startup problem.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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