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I want to build a 400kHz square wave oscillator with NAND GATE using the CD4011BE circuit.
Unfortunately, it does not work as expected when I use R2 and C values according this schematic:

enter image description here

Vcc = 5V
R1  = 1Mohms
R2  = 1100ohms
C   = 1nF  

Expected: Fosc = 1 /(2.2*1100*1E-9) = 413,223kHz

The signal on the 2nd NAND Q output stay at 5V with small noise oscillation (seen on scope).
But when i replace R2 by a 10Kohm resistor, there is a real square wave signal around 45kHz.
So, what is the problem with the 1.1kohm ?

Thanks for help !

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  • \$\begingroup\$ Did you measure the actual resistance of the resistor? Typically, resistors have some error in its true resistance value. Basically, your 1.1kOhm resistor may not actually equal exactly 1.1k... It could be 1.05k or 1.15k or something... That goes for your capacitor as well. Check the actual values of your components. \$\endgroup\$ – KingDuken May 15 '18 at 0:36
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    \$\begingroup\$ From looking at DC bias , why do you think it fails? \$\endgroup\$ – Sunnyskyguy EE75 May 15 '18 at 0:55
  • \$\begingroup\$ Do you think the ESD input diodes interfere? \$\endgroup\$ – Sunnyskyguy EE75 May 15 '18 at 1:10
  • \$\begingroup\$ I can adjust R2 with a 220ohms potentiometer added to 1000ohms resistor. I don't know what you are talking about ESD input diodes...?!!! \$\endgroup\$ – Teddol May 15 '18 at 2:04
  • \$\begingroup\$ Please, help... \$\endgroup\$ – Teddol May 15 '18 at 14:44
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The CD4011 has very little drive capability, especially with a 5V supply. CD74HCU04 would be better.

Also R1 will have to be much lower, maybe 10K or 20K, because of input and stray capacitance.

Try 100pF, 74HCU04, R2 = 11K, R3 = 20K. Probably R2 will have to be higher, maybe 15K, because of the loading from R3 but that should get you in the neighborhood.

Unbuffered inverters are better because they are less likely to have unwanted oscillation modes.

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I guess the 1.1k resistor is dragging the voltage low therefor preventing the oscillation from triggering the 2nd gate. By increasing the resistor to 10k the pull down is reduced so it started to oscillate.

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