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SN74HC14: Frequency shift in LC oscillator application

Arvind Wagh
Prodigy 40 points
Part Number: SN74HC14
Other Parts Discussed in Thread: TLV7011, TLV7012

Sir/Mam,

In our product we were using 74HC14 NXP make for LC oscillator application where inductance value L changes to produce variation in frequency.

The frequency produced are from 629 Hz to 1.1 KHz which is measured by MCU.

Now we replaced NXP make with TI SN74HC14DR and checked the frequency on oscilloscope which is comes 783.63Hz to 1.342 KHz for same inductance variation.

There is shift of frequency band which gives wrong count to MCU.

Please let us know why frequency have been shifted and what we should do to solve this problem.

I have attached schematic ,datasheet and equivalent circuit for your reference.

SN74HC14DR.pdf

74HC_HCT14-nxp.pdf

  • +1
    Guru 272400 points

    The output frequency is affected by the voltages at which the inverter switches. The Schmitt-trigger inputs of logic devices have a wide variation of voltage thresholds (see the VT+ and VT− specifications in the electrical characteristics table). The thresholds are likely to be consistent for a specific temperature, but it appears that NXP and TI have designs with a different default hysteresis. Also, any redesign of the chip might change these parameters (while still staying inside the datasheet limits).

    To get a controlled output frequency, you must have strictly defined switching thresholds, i.e., you must use a comparator like the TLV7011/TLV7012.

  • 0
    Prodigy 40 points

    Sir,

    I understand the reason ,thank you very much for nice explanations.

    If i want to use TI SN74HC14DR then need to change parameter in MCU.