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SN74AHC1G04: waveform distorsion oscillator circuit

Albert Rossi
Prodigy 20 points
Part Number: SN74AHC1G04
Other Parts Discussed in Thread: SN74LVC1GU04

Hi,

i'm working on an old board on which a pierce oscillator circuit is mounted:

Quartz Crystal is TQX6800/A Nominal frequency 9,0316 MHz

C42 C0805C479C1GACTU

C43 C0805C120J1GAC

R74 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.

  • 0
    TI__Guru* 96591 points

    It looks to me like you're getting a harmonic that's coming off the oscillator circuit.

    My first recommendation would be to switch to an unbuffered inverter - the gain will be reduced and it should help with this. SN74LVC1GU04 is a good example of a device. Any of the "U04" devices will be an unbuffered inverter, which means it will have only a single stage of inversion (2 MOSFETs) and this drastically reduces gain (which is additive in subsequent amplifier stages). These are usually what are used in this type of oscillator.