SN74S124: Formula for External Capacitance value based on Output Frequency

Part Number: SN74S124


I wish to know what should be the exact external capacitance value for 27 MHz as the output frequency from the VCO. If possible, could you share the formula for the same. I viewed the graph mentioned in the datasheet but if formula is available it would be better.

Also, since this is old part number, could you suggest any new Voltage Controlled Oscillators (VCOs) with output frequency 27 MHz and Voh and Vol within 2V.

Thanks & Regards

Shuresh Vishag C

  • For newer solutions, we don't really have very many VCOs in the logic portfolio, but the clocking/timing group has quite a few oscillators. I'm not very familiar with their portfolio though.  I would recommend to open a second post with a question similar to "Looking for 27 MHz oscillator" and use a clocking part number for the post like LMK62E2 (to get the post routed to the correct people). They may not have a solution, but it can't hurt to ask.

    As for the exact frequency of operation for the SN74S124 - I'm afraid what's available is all in the datasheet for this device. I would recommend to estimate a starting value based on the datasheet plots, then build a prototype with an EVM and test the operating frequency with your selected values.

    The plot in figure 1 becomes nonlinear above 10 MHz -- this was most likely data taken on the bench when the datasheet was developed in the early 1980s or late 1970s. If there was an equation used, I'm afraid it's lost to time.

    I used a free web based graph reading tool to get some values -- it looks like Cext should be between 3 and 3.9 pF to get a 27 MHz base frequency.

    Above image is from WebPlotDigitizer - Copyright 2010-2020 Ankit Rohatgi (

  • Hi Emrys

    Thanks for sharing the range of Cext value. 

    Will request separately to the Clock and Timing group.

    Thanks & Regards

    Shuresh Vishag C