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I have been designing following circuit as a hobby project. It should generate an output frequency of between 500kHz - 1500 kHz signal at the OUTH and OUTL pins. The 'dead-time' generator logic ensures that OUTH or OUTL are never high at same time.

74HCT4060 Oscillator

My problem is that I don't know if can I drive the 74HCT4060's RC oscillator at such high frequency that I can achieve the required output signal frequency range. A minimum ideal output frequency range is ~800-1200 kHz and I need to be able to tune the oscillator frequency via trimmer pot fairly accurately.

So I'd like to get some advice on:

  • What by-pass capacitors should I use for the ICs?
  • Are the C1 10pF timing capapacitor and R1 100 ohm enough to keep the oscillator running while trimming the potentiometer? What hazards should I try avoid here?
  • How can I add coarse+fine grain control trimmer potentiometers such that I get a more accurate frequency setting?

Caveats: I don't have access to an oscilloscope. I know need one, and badly. (The local university has closed my only way to access one in their hacker space due to covid19.)

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    \$\begingroup\$ 1) can I drive the 74HCT4060's RC oscillator at such high frequency Check the datasheet. For HCT logic 4 MHz should be doable. Do realize that on a breadboard, at 4 MHz, the parasitic capacitances (of the contact strips) might start to become important. 2) The 4060's first output of the ripple counter is after divide-by-4 (not sure that's correct, sure though that divide-by-2 isn't available). So your RC osc. needs to be at a quite high frequency. Consider using a HCT4040 (it has all outputs) and some inverters to make the RC osc. \$\endgroup\$ Aug 27, 2021 at 15:15
  • \$\begingroup\$ How can I add coarse+fine grain control trimmer potentiometers such that I get a more accurate frequency setting? Easy: 10 turn pot/trimmer or two in series: 100k in series with 5 k. Then the 5k (fine) will be about 20x less "sensitive" than the 100 k (coarse). Note that you can then set the frequency more accurately to a certain value. It will still drift meaning, slowly change frequency due to temperature changes mostly. RC oscillators aren't very stable, their frequency is unpredictable.... \$\endgroup\$ Aug 27, 2021 at 15:16
  • \$\begingroup\$ @Bimpelrekkie: Yea, I know bread boards are terrible for high freq stuff and what I know they begin to fail at >1MHz. Thanks for mentioning the RC oscillator drift, it might be a problem I'll try tackle later. \$\endgroup\$
    – JATothrim
    Aug 30, 2021 at 14:23
  • \$\begingroup\$ Thanks for mentioning the RC oscillator drift, it might be a problem I'll try tackle later. Tip: don't go into the "compensating for RC oscillator drift" route. If RC isn't good enough, I would advise to go for a solution where the timing is based on a crystal. For example a microcontroller. \$\endgroup\$ Aug 30, 2021 at 14:52
  • \$\begingroup\$ if I would build v2 circuit with SMD parts in future, I'll probably use xtal as clock source and µC. The RC oscillator is okay for now since I'm sticking to 74xx PDIP chips in this project. \$\endgroup\$
    – JATothrim
    Aug 30, 2021 at 15:16

1 Answer 1

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According to the datasheet, 10pF is too small.

You need a minimum of 10k and a minimum of 50pF:

enter image description here

Ct > 50 pF, up to any practical value and 10 kΩ < Rt < 1 MΩ.In order to avoid start-up problems, Rt ≥ 1 kΩ.

I think there's a typo there. "Rt ≥ 1 kΩ" should probably be "R2 ≥ 1 kΩ."

Given the minimum requirements, I get a maximum frequency of \$\frac{1}{2.5 \times R_t \times C_t} = \frac{1}{2.5 \times 10k \times 50pF} = 800 kHz\$

On the face of it, you can't get your desired range out of the 74HCT4060. It maxes out at 800kHz, and that's only the lower limit of your range.

It might do better than what the equations say, but the examples are all lower than 100kHz:

enter image description here

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  • \$\begingroup\$ Now I'm wondering how in heck it was still oscillating with 10pF C1 and RV1 @ ~50kOhm... I'll change the 74HCT4060 to 74HC4040 to solve the RC oscilator freq range problem. And for the inverter(s) I probably can use the extra NOR gate(s) that I have on the 74HC02? \$\endgroup\$
    – JATothrim
    Aug 30, 2021 at 14:56

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