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waveform problems on oscillator circuit

waveform problems on oscillator circuit

waveform problems on oscillator circuit

Part Number: SN74AHC1G04
Other Parts Discussed in Thread: SN74LVC1GU04 Hi: all, sorry I post two as there is something wrong in the first one but i did not find the delete button.
Here is the new one.
i'm working on an old board on which a pierce oscillator circuit is mounted:

Quartz Crystal is TQX6800/A Nominal frequency 9,0316 MHzC42 C0805C479C1GACTUC43 C0805C120J1GACR74 R75 R66 smd resistor.The problem i've been having these days is that a normal waveform on the output of SN74HC1G04 shows some distorsions as you can see below (purple trace on right figure).

Most of times the problem is solved by replacing the inverter IC with one in another reel. Sometimes it is solved by adding capacitance in parallel to C42.Could you help me What could be due to the malfunction?Let me know.

Apogeeweb Semiconductor Electronic

RE: waveform problems on oscillator circuit

The "right" image, which is presumably the one on the bottom looks more like your oscilloscope is aliasing, since most digital scopes have limited resolution when you change the time scale.

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RE: waveform problems on oscillator circuit

R74 and R66 are effectively in parallel resulting in a lower resistance value. The crystal may be trying to break into an overtone mode if not loaded properly.
You might check out this article series from EDN on this type of oscillator. I quickly found part 2 of this 4-part series.

RE: waveform problems on oscillator circuit

That's a really odd design... why is R74 there? At that frequency, R75 should likely be a cap, not a resistor.

Dan - Owner

RE: waveform problems on oscillator circuit

it looks like the crystal circuit has more than one frequency resonance, and the total circuit is actually oscillating at two frequencies simultaneously. have you looked at the wave form with an AC coupled spectrum analyzer.

you might have to add L-C circuits to suppress the unwanted oscillation

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