Inverting a modulated signal (OOK).

Hello,
I'm modulating the message signal with OOK (on-off keying) modulation. Multisim software is used to model the schematic [attached]. At the resistor R1, the modulated signal is obtained. I wanted to invert the modulated single, for that I used a simple NOT gate (U3), however at the NOT's gate output, I got the same signal as input means without inverting [attached]. So my question is, why isn't the NOT gate inverting? Am I doing something wrong?

As hit and trail, I added two more NOT gates (U4 and U5) but this time I got an inverted signal, although further improvement is needed to achieve a 180-degree phase shift. So my second question is: Why is it an inverter this time? and how to get exactly a 180-degree phase shift between two signals?

Thanks for your time.

You don't give a part number for the NAND gate, so we don't know if it has hysteretic inputs. And, usually there is a resistor in the feedback path for more control and stability of the oscillating frequency.

Beyond all of that, the oscillator probably is producing a symmetrical output, which looks the same on a scope inverted or not because of how a scope trigger works. The extra inverters are merely a delay line, giving a time-shifted (not inverted) image.

What is the purpose of R1?

ak

AnalogKid said:

You don't give a part number for the NAND gate, so we don't know if it has hysteretic inputs. And, usually there is a resistor in the feedback path for more control and stability of the oscillating frequency.

Beyond all of that, the oscillator probably is producing a symmetrical output, which looks the same on a scope inverted or not because of how a scope trigger works. The extra inverters are merely a delay line, giving a time-shifted (not inverted) image.

What is the purpose of R1?

ak

Click to expand...

AnalogKid Thanks for your response. Based on your suggestion, I tried another schematic [attached], in which the resistor is used as feedback, I have mentioned the source from where I got this schematic, however, this schematic also did not work although I followed all the design steps for calculating the R and C.

The formula for calculating oscillating freq (Fc), R and C: Fc=1/2.2RC

Could you please review it? where I am making mistake?

Link: https://www.allaboutcircuits.com/technical-articles/how-to-design-schmitt-trigger-oscillators/