Electrical – nand gate astable vibrator do not work at 400kHz

astablemultivibratornand

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 !

Best Answer

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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