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SN74S124D Frequency Response

Khaled M. Dadesh
Prodigy 65 points


Using 4-layer PCB, I am not able to get the 85MHz frequency response. I am able to generate up 4MHz of a pure square wave. So any suggestions. 

  • 0
    TI__Mastermind 46480 points

    Hi Khaled

    This part is rated at 85mhz typical into a very light load.  Load conditions and temp may effect its max frequency.  The chart on page 3 (switching characteristics) shows conditions used to test the frequency.( freq is set to 4V and rng is set to1V).  The guarranteed frequency across temp and process is 60Mhz.

    You May be able to vary the capacitor to get a higher frequency as shown on the chart on the following page. I am not very familiar with this part since it was born 30 years ago.  

     

  • 0
    Prodigy 10 points

    I suspect your problem is parasitic capacitance from the  power or ground plane and possibly the layout itself near the "CX" pins.  Also, the RNG pin voltage greatly affects the oscillator frequency.  The RNG voltage needs to be between 1.5 and 4 volts.  The higher the voltage the lower the oscillation.

    One of the things you need to watch out for is parasitic capacitance around the "CX" pins.  In a circuit I used, I made sure no power or ground planes existed under the area around the chip or components and in particular those associated with the "CX" pins.  My frequency was between 20 and 30 MHz.  I was able to get the thing to oscillate well above 30 MHz.